Respiratory humidifiers

Fisher & Paykel Healthcare MR850: troubleshooting and error codes

The ServiceForge knowledge base holds 40 documented symptoms for the Fisher & Paykel Healthcare MR850, with 95 ranked causes, 117 diagnostic checks with test points and expected values, and 40 error codes. Each symptom below shows its causes and the first check; the rest is one connection away. Connect ServiceForge to your AI

Symptoms

Heater-wire connector alarm flashes with audible alarm patient safety

Heater-wire connector indicator flashes with accompanying audible alarm; indicates connection or adapter fault

Ranked causes

1. Heater-wire adaptor not properly connected (common)
2. Heater-wire adaptor physical damage (occasional)
3. Incorrect heater-wire adaptor polarity or connection (occasional)
4. Breathing circuit failure (occasional)
5. Heater-wire adaptor failure (occasional)
6. Electromagnetic interference (EMI) (rare)
7. Humidifier PCB fault (occasional)

First check

Verify heater-wire adaptor connection

Test point: Heater-wire adaptor connectors on humidifier and breathing circuit
Expected: Adaptor fully seated and connected

5 more checks with pass and fail actions, 7 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Temperature indicator flashes with audible alarm and low temperature displayed patient safety

Temperature indicator flashing with audible alarm; temperature displayed below 35.5 °C

Ranked causes

1. Gas flow disconnected from humidifier (common)
2. Gas flow rate out of specification (common)
3. Drafts around breathing circuit (occasional)
4. Gas flow reversed through humidifier (occasional)
5. Excessive condensate pooling in breathing circuit (occasional)
6. Insufficient water in chamber or delivery tube occluded (occasional)
7. Humidifier or probe faulty (occasional)

7 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Humidifier will not turn on with on/off button

Unit does not power on when button pressed; no indicators lit

Ranked causes

1. Humidifier not plugged into mains supply (common)
2. Mains supply out of specification (common)
3. Fuse open circuit (common)
4. Mains power cord continuity broken (occasional)
5. Transformer winding open circuit (occasional)
6. Humidifier faulty (rare)

6 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Humidifier will not turn on with on/off button, no indicators lit

Unit does not power on when on/off button pressed; no display indicators illuminated.

Ranked causes

1. Humidifier not plugged into mains (common)
2. Mains supply out of specification (occasional)
3. Fuse open or blown (common)
4. Mains power cord open or broken (occasional)
5. Transformer primary or secondary windings open circuit (rare)
6. PCB faulty (occasional)

First check

Verify mains plug connection

Test point: Mains outlet
Expected: Humidifier plugged in securely

7 more checks with pass and fail actions, 6 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Airway probe alarm flashes with audible alarm patient safety

Airway probe indicator flashes with accompanying audible alarm; indicates probe detection or signal issue

Ranked causes

1. Airway probe insertion or breathing circuit setup error (common)
2. Circuit connected incorrectly to ventilator (reversed flow) (common)
4. Excessive condensate or ambient conditions (occasional)
5. Airway probe accuracy degradation (occasional)
6. Humidifier PCB fault (rare)

First check

Verify airway probe insertion and circuit assembly

Test point: Airway probe in breathing circuit
Expected: Probe correctly inserted; circuit assembled per manual; water in chamber

4 more checks with pass and fail actions, 6 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Temperature indicator flashes with low temperature (<35.5°C) patient safety

Temperature indicator flashes with audible alarm while displaying temperature below 35.5°C; humidifier unable to maintain setpoint

Ranked causes

1. Gas flow disconnected from humidifier (common)
2. Gas flow rate out of specification (common)
3. Drafts around breathing circuit from fans or air conditioning (occasional)
4. Circuit connected incorrectly to ventilator (reversed flow) (common)
5. Excessive condensate in breathing circuit (occasional)

First check

Verify gas flow connection to humidifier

Test point: Gas inlet connection on humidifier
Expected: Gas flow connected and active

4 more checks with pass and fail actions, 5 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Low temperature displayed, no audible alarm patient safety

Humidifier displays low temperature without triggering an alarm sound.

Ranked causes

1. Insufficient warm-up time or gas flow (common)
2. Humidifier cannot maintain temperature (occasional)
3. Insufficient water in chamber or obstructed delivery (common)
4. Humidifier or probe faulty (occasional)

First check

Verify warm-up time and gas flow

Test point: Breathing circuit
Expected: Sufficient gas flow present, warm-up period allowed

3 more checks with pass and fail actions, 4 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Temperature probe connector indicator flashes with audible alarm patient safety

Temperature probe connector indicator flashes with accompanying audible alarm; indicates connector or probe communication issue

Ranked causes

2. Circuit connected incorrectly to ventilator (reversed flow) (common)
3. Humidifier calibration drift (occasional)
4. Temperature probe failure (occasional)
5. Humidifier PCB fault (rare)

First check

Verify probe connector seating

Test point: Temperature probe connector in humidifier
Expected: Connector fully seated and locked

3 more checks with pass and fail actions, 5 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Calibration probe resistance verification

Verify correct operation of MR850 calibration probe (part 900MR870) by measuring resistance values of probe plugs

Ranked causes

1. Blue plug out of tolerance (unknown)
2. Grey plug out of tolerance (unknown)
3. Measurement instrument insufficient accuracy (unknown)

First check

Measure blue plug chamber temperature resistor

Test point: Blue plug chamber temperature resistor
Expected: Nominal value ±0.3%
Tools: Ohmmeter ±0.2% accuracy or better

3 more checks with pass and fail actions, 3 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Chamber probe alarm flashes with audible alarm patient safety

Chamber probe indicator flashes with accompanying audible alarm; indicates probe detection or signal issue

Ranked causes

1. Probe insertion or breathing circuit setup error (common)
3. Incorrect chamber type (occasional)
6. Temperature probe or humidifier fault (rare)

First check

Verify chamber probe insertion and circuit setup

Test point: Chamber probe in breathing circuit
Expected: Probe correctly inserted; circuit assembled per manual

5 more checks with pass and fail actions, 6 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Low temperature displayed without audible alarm patient safety

Low temperature shown on display; no audible alarm sounding

Ranked causes

1. Insufficient warm-up time or gas flow (common)
2. Humidifier cannot maintain temperature (occasional)
3. Humidifier or probe faulty (occasional)

3 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

See manual indicator lit, not flashing

'See manual' indicator on display is lit but not flashing.

Ranked causes

1. Mains supply out of specification (occasional)
2. 'PtS' displayed (unknown)
3. PCB faulty (occasional)

First check

Verify mains supply voltage and frequency

Test point: Mains inlet
Expected: Within specification

2 more checks with pass and fail actions, 3 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Unit fails to reach temperature in humidifier control check

Unit cannot reach specified temperature during calibration or control check procedure.

Ranked causes

1. Low chamber outlet temperature; heater-plate element faulty (occasional)
2. Low airway temperature; breathing circuit faulty (occasional)
3. PCB faulty (occasional)

First check

Check chamber outlet temperature

Test point: Chamber outlet
Expected: Temperature within specification

2 more checks with pass and fail actions, 3 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Water-out indicator flashes with audible alarm patient safety

Water-out indicator flashes accompanied by audible alarm; may indicate low water, flow obstruction, or sensor fault

Ranked causes

2. Water level in MR290 chamber exceeds marked line (occasional)
5. Temperature probe faulty (occasional)
6. Humidifier performance degradation (rare)

First check

Verify water level in chamber

Test point: Chamber water level
Expected: Water present at adequate level; bag full; tube not kinked

5 more checks with pass and fail actions, 6 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Humidifier performance verification

Check humidifier operation and calibration after maintenance or if operating abnormally. Service menu accessed by holding power button while applying mains power.

Ranked causes

1. Routine maintenance or recent servicing completed (common)
2. Humidifier problem or recent service without verification (occasional)

First check

Inspect humidifier and accessories for physical damage before testing

Test point: Visual inspection of unit, heater-wire adaptor, probe
Expected: No signs of physical damage

6 more checks with pass and fail actions, 5 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Humidifier warm-up and control

Verify heater control systems and temperature stability after warm-up period

Ranked causes

1. Chamber or airway temperature out of tolerance after warm-up (unknown)
2. Alarms occurring during warm-up (unknown)

First check

Verify calibration and display checks completed

Test point: MR850 display
Expected: Humidifier passes sections 8.1.2 and 8.1.3 checks

6 more checks with pass and fail actions, 2 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

High displayed temperature without temperature alarm patient safety

Temperature displayed as high; no temperature alarm indicator lit

Ranked causes

1. Insufficient time for temperature stabilization (common)
2. Humidity compensation active (occasional)

2 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

High displayed temperature, no temperature alarm patient safety

Humidifier displays high temperature without triggering a temperature alarm.

Ranked causes

1. Recent flow change, temperature stabilizing (common)
2. Manual or automatic humidity compensation active (occasional)

First check

Allow temperature stabilization after flow change

Test point: Display temperature reading
Expected: Temperature stabilizes within 30 minutes

1 more check with pass and fail actions, 2 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

See manual indicator flashing with audible alarm patient safety

Manual indicator flashes and audible alarm sounds; error code displayed on front panel

Ranked causes

1. Production test mode accessed (occasional)
2. Humidifier faulty (occasional)

2 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

See manual indicator light permanently on without alarm

Front panel See manual indicator illuminated continuously; no audible alarm sounding

Ranked causes

1. Production test mode active (common)
2. Humidifier hardware fault (occasional)

2 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

See manual indicator light permanently on without audible alarm patient safety

Front panel 'See manual' indicator light remains on continuously; no audible alarm present

Ranked causes

1. Production test mode activated (common)
2. Humidifier PCB fault (occasional)

First check

Check for PtS displayed on front panel and production test button status

Test point: Front panel display and production test button
Expected: PtS not displayed; production test button not depressed

2 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Annual MR850 humidifier performance verification

Full performance test required annually to verify all humidifier functions operate within specification.

Ranked causes

1. Annual maintenance requires full performance verification (common)

First check

Check mains cable for damage

Test point: mains cable exterior
Expected: No damage, cuts, or insulation failure

2 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: MR850 heater wire adaptor high voltage

Annual inspection of heater wire adaptor for physical damage including exposed wires, strain relief damage, and casing cracks; functional check via diagnostic menu

Ranked causes

1. Scheduled annual maintenance (common)

First check

Setup heated circuit with adaptor

Test point: MR850 with temperature probe and heater wire adaptor
Expected: Circuit established and functional

4 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: MR850 physical inspection caution

Annual inspection of humidifier body, mains cable, heater-plate, and heater-wire adaptor for damage, corrosion, and proper connector coupling

Ranked causes

1. Scheduled annual maintenance (common)

First check

Perform full performance test

Test point: MR850 complete unit
Expected: All functions nominal per section 8

3 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: MR850 temperature probe

Six-monthly inspection of temperature probe for physical damage, deposits, and cable condition; verify connector coupling and perform temperature and flow accuracy checks

Ranked causes

1. Scheduled six-monthly maintenance (common)

First check

Temperature accuracy check

Test point: MR850 temperature probe
Expected: Temperature within specification per section 8.2

1 more check with pass and fail actions, 3 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: MR850 temperature probe accuracy

Temperature probe readings require verification against expected values; electrical connectors and physical integrity must be confirmed before use.

Ranked causes

1. Temperature probe requires periodic calibration verification (common)

First check

Visually inspect chamber probe glass thermistor

Test point: thermistor
Expected: No damage visible

4 more checks with pass and fail actions, 2 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Probe flow accuracy

Verify temperature/flow probe flow measurement is within range at 10±1 SLPM

Ranked causes

1. Probe flow measurement outside 5-15 LPM range (unknown)

First check

Perform humidifier calibration test

Test point: MR850 display
Expected: Humidifier reading correctly

6 more checks with pass and fail actions, 2 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Probe temperature accuracy

Verify temperature/flow probe reads within tolerance in water bath at 40°C

Ranked causes

1. Probe out of temperature accuracy tolerance (unknown)

First check

Perform humidifier calibration test

Test point: MR850 display
Expected: Humidifier reading temperature correctly

4 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Verification of calibration probe resistor accuracy

Check the absolute accuracy of the two Redel plugs containing 0.1% tolerance resistors in the MR850 calibration probe to ensure correct operation.

Ranked causes

1. Resistors out of specification tolerance (unknown)

First check

Measure resistance of each resistor in the calibration probe using precision ohmmeter

Test point: Connector pins (access after unscrewing cable collet)
Expected: ±0.3% of nominal value (0.1% tolerance resistors), +0.3%
Tools: Ohmmeter with ±0.2% accuracy or better

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Cleaning: MR850 humidifier housing and components caution

Humidifier exterior requires regular cleaning using approved disinfectants to maintain hygiene and function.

Ranked causes

1. Routine cleaning required to prevent contamination buildup (common)

First check

Disconnect humidifier from electrical outlet

Test point: power connection
Expected: Humidifier unplugged

2 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Cleaning: Temperature flow probe disinfection and sterilization caution

Airway temperature probes require disinfection or sterilization using approved solutions to meet FDA requirements and prevent contamination.

Ranked causes

1. Probe disinfection required for patient safety and FDA compliance (common)

First check

Physically clean probes to remove visible contaminants

Test point: probe surfaces
Expected: All visible contaminants removed

4 more checks with pass and fail actions, 4 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Heater wire adaptor diagnostic display anomaly

Display does not show expected S-E or S-- reading when scrolling to Cct in diagnostic menu

Ranked causes

1. Heater wire adaptor malfunction or incompatibility (rare)

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Heater-wire alarm not working patient safety

Heater-wire alarm function disabled or non-functional; no alarm when heater-wire circuit issue occurs

Ranked causes

1. Non-heater-wire mode activated in diagnostic menu (common)

First check

Verify heater-wire mode setting in diagnostic menu

Test point: Diagnostic menu heater-wire mode flag
Expected: Heater-wire mode enabled or heater-wire circuit connected

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Probe fails Probe Accuracy Check

Temperature or humidity probe does not pass accuracy check test.

Ranked causes

1. Probe faulty or out of calibration (common)

First check

Replace probe with new unit

Test point: Probe connector
Expected: Accuracy check passes

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Probe fault condition alarm

Humidifier alarms when probe fault detected during service mode

Ranked causes

1. Defective or unseated probe (common)

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Safety electrical testing ground point identification caution

Electrical safety testing requires use of correct ground test point to avoid equipment damage.

Ranked causes

1. Incorrect ground point during electrical safety testing causes permanent damage (common)

First check

Locate ground test point on heater plate

Test point: heater plate front underside edge
Expected: Anodizing layer removed, bare metal visible at specified location

1 more check with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Temperature / Flow Probe alarm with airway or chamber indicators flashing patient safety

Temperature / Flow Probe alarm occurs simultaneously with chamber or airway indicators flashing

Ranked causes

1. Temperature probe faulty (common)

First check

Complete probe accuracy check

Test point: Temperature probe
Expected: Probe accuracy within specification

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Temperature indicator flashes with audible alarm, high temperature displayed patient safety

Temperature indicator flashing on display with audible alarm and high temperature reading.

Ranked causes

1. Sudden gas flow rate change (common)

First check

Monitor displayed temperature after flow change

Test point: Display temperature reading
Expected: Temperature falls rapidly

1 more check with pass and fail actions, 2 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Temperature/Flow probe alarm with chamber or airway indicators flashing patient safety

Temperature/Flow probe alarm triggered simultaneously with chamber or airway indicator flashing

Ranked causes

1. Temperature probe faulty (common)

First check

Complete temperature probe accuracy check

Test point: Temperature probe output and linearity
Expected: Readings within calibration specification

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Unit fails calibration check

Unit does not pass calibration check procedure.

Ranked causes

1. Out of calibration or PCB faulty (unknown)

First check

Send unit for recalibration or replace PCBs

Test point: Humidifier
Expected: Calibration within specification

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Error codes

Code	Meaning	Trigger	Severity
`E00`	No fault	Normal operation	info
`E02`	Microprocessor stack overflow	Microprocessor stack memory exceeded	critical
`E03`	RAM fault	Memory test failure	critical
`E04`	ROM fault	Memory test failure	critical
`E05`	EEPROM version older than ROM version	EEPROM has not been updated to match ROM	error
`E06`	EEPROM version newer than ROM version (old software)	EEPROM version exceeds ROM version	error
`E07`	Model mismatch with software	Startup model verification failure	error
`E10`	Temperature circuit calibration out of range: Range Amp 0: 25.5 °C	Temperature sensor calibration data incorrect at 25.5 °C	error
`E11`	Temperature circuit calibration out of range: Range Amp 0: 65.0 °C	Temperature sensor calibration data incorrect at 65.0 °C	error
`E12`	Temperature circuit calibration out of range: Range Amp 1: 25.5 °C	Temperature sensor calibration data incorrect at 25.5 °C	error
`E13`	Temperature circuit calibration out of range: Range Amp 1: 34.5 °C	Temperature sensor calibration data incorrect at 34.5 °C	error
`E14`	Temperature circuit calibration out of range: Range Amp 2: 34.5 °C	Temperature sensor calibration data incorrect at 34.5 °C	error
`E15`	Temperature circuit calibration out of range: Range Amp 2: 44.6 °C	Temperature sensor calibration data incorrect at 44.6 °C	error
`E16`	Temperature circuit calibration out of range: Range Amp 3: 65.0 °C	Temperature sensor calibration data incorrect at 65.0 °C	error
`E20`	Heater wire circuit has malfunctioned; heater cannot be energized	Heater wire activation failure	error
`E21`	Heater-wire circuit control fault	Heater wire cannot be de-energized or monitoring circuit failed	error
`E21`	Heater wire circuit has malfunctioned; heater cannot be de-energized or monitoring failed	Heater wire deactivation or monitoring failure	error
`E23`	Heater-wire voltage measurement circuit faulty	Voltage measurement circuit malfunction detected	error
`E25`	Transistor Q17 not turning on	Q17 drive circuit malfunction	error
`E26`	Transistor Q16 not turning on	Q16 drive circuit malfunction	error
`E27`	Heater-wire relay short-circuited	Heater-wire relay shorted	error
`E28`	Mains voltage measurement is uncalibrated	Voltage measurement calibration not performed	error
`E29`	Heater plate thermistor is short circuited	Thermistor resistance test failure	error
`E2A`	Heater plate thermistor is open circuit	Thermistor resistance test failure	error
`E2C`	Heater plate element circuit has malfunctioned; element not turning on	Heater plate activation failure	error
`E2D`	Heater plate element is not turning off	Heater plate deactivation failure	error
`E30`	Power (on/off) button stuck on	Button continuity test failure during startup	error
`E31`	Mute button stuck on	Button continuity test failure during startup	error
`E32`	Mode button stuck on	Button continuity test failure during startup	error
`E40`	Unit not functional tested at time of manufacture	Manufacturing test record check	warning
`E41`	Failed the functional test at time of manufacture	Manufacturing test failure	error
`E42`	Was not stress tested during manufacture	Manufacturing test record check	warning
`E43`	Failed stress test during manufacture	Stress test failed during production	critical
`E44`	Not tested on functional tester 2	Manufacturing test record check	warning
`E45`	Failed production functional tester 2	Manufacturing functional tester 2 failure	error
`E4A`	EEPROM write error	EEPROM write operation failed	error
`E4B`	EEPROM write verify error occurred	EEPROM write verification failure	critical
`E4C`	EEPROM read error occurred	EEPROM read operation failure	critical
`E50`	Flow circuitry not functioning	Flow measurement circuit failure	error
`E51`	Flow circuitry shorted on	Flow measurement circuit short	error

Likely causes and actions for each code are in ServiceForge. Connect ServiceForge to your AI

Community

0 fixes logged by technicians through their AI, with outcomes, for this model.

ServiceForge knowledge base record. Field-unverified unless technicians confirmed it. A teaching summary of the record, not a service manual. Export 2026-10-14.