Pulse oximeters

Nellcor N-395: troubleshooting and error codes

The ServiceForge knowledge base holds 23 documented symptoms for the Nellcor N-395, with 46 ranked causes, 48 diagnostic checks with test points and expected values, and 20 error codes. Each symptom below shows its causes and the first check; the rest is one connection away. Connect ServiceForge to your AI

Symptoms

Unit does not power up on AC or DC caution

No power from either AC or DC source; Battery Low indicator may light or fail to light depending on condition

Ranked causes

1. Defective AC outlet or no AC power (common)
2. Open fuses in Power Entry Module (common)
3. Defective power supply (occasional)
4. Defective User Interface PCB or harness connection (occasional)

First check

Verify AC power supply and indicator

Test point: AC outlet and AC indicator light
Expected: AC indicator on

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

Alarm does not sound patient safety

Audible alarm function non-operational; alarms do not sound when triggered or condition occurs

Ranked causes

1. Alarm silenced or muted by user setting (common)
2. Speaker defective or disconnected (common)
3. User Interface PCB defective (occasional)

First check

Verify alarm sound setting is enabled

Test point: Monitor alarm silence/mute setting
Expected: Alarm sound setting enabled (not silenced)

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

Battery does not charge

Battery fails to accept or hold charge when connected to AC power; Battery Low indicator remains on or battery depletes during charging

Ranked causes

1. Battery age exceeds 2 years (common)
2. Battery unable to hold charge (internal degradation) (common)
3. Defective power supply (occasional)

First check

Check battery age

Test point: Battery manufacture date label
Expected: Battery less than 2 years old

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

Cooling fan does not run caution

Fan not operating; unit may overheat.

Ranked causes

1. Temperature below fan activation threshold (common)
2. Temperature sensor U3 failure (occasional)
3. Unit running on battery power (common)

First check

Verify AC power connection

Test point: Power cord and outlet
Expected: AC power connected and active

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

Display pixels do not light

LCD display or sections of display do not illuminate; pixels remain dark or inactive

Ranked causes

1. Poor connection between User Interface PCB and Display PCB (common)
2. Display PCB defective (occasional)
3. User Interface PCB defective (occasional)

First check

Check connection between User Interface PCB and Display PCB

Test point: PCB interconnect connectors
Expected: Connection secure and properly seated

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

Display values are missing or erratic patient safety

Displayed values on screen are incomplete, flickering, or unstable; readings jump or disappear intermittently

Ranked causes

1. Sensor extension cable defective or loose (common)
2. Pulse oximetry sensor defective (common)
3. Interface PCB defective (occasional)

First check

Inspect sensor connection and cable

Test point: Sensor extension cable connectors
Expected: Cable firmly connected, no damage or moisture

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

RS-232 nurse call not working

Nurse call signal on data port pin 11 not functioning when alarm occurs

Ranked causes

1. Improper connections on data port (common)
2. Incorrect output voltage on nurse call pin (occasional)
3. User interface PCB failure (occasional)

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

SpO2 or Pulse values change rapidly; Pulse Amplitude indicator erratic patient safety

Readings fluctuate significantly cycle to cycle; waveform bar jumps erratically; unstable measurements

Ranked causes

1. Sensor damp or reused excessively (common)
2. Electrosurgical unit (ESU) electromagnetic interference (occasional)
3. User Interface PCB defective (occasional)

First check

Inspect sensor for moisture and reuse condition

Test point: Pulse oximetry sensor
Expected: Sensor dry and within acceptable reuse limit

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

Alarm sounds for no apparent reason

Audible alarm triggered spontaneously without an alarm condition; nuisance alarms occurring repeatedly

Ranked causes

1. Moisture or spilled liquid on or in monitor (common)
2. User Interface PCB defective (occasional)

First check

Inspect monitor for moisture or liquid contamination

Test point: Monitor interior and PCBs
Expected: No visible moisture or liquid present

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

Low battery alarm triggered

Unit displays low battery alarm despite charging.

Ranked causes

1. Battery voltage below threshold (common)
2. Defective battery (occasional)

First check

Measure battery charging voltage

Test point: Battery charging circuit output
Expected: 6.8 VDC ±0.15 V, ±0.15 V
Tools: Multimeter

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

No data port printout received

Monitor not sending data to external printer via RS-232 data port

Ranked causes

1. Baud rate mismatch between monitor and printer (common)
2. User interface PCB failure (occasional)

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

Pulse Amplitude indicator shows pulse but digital displays show zeroes patient safety

Waveform indicator bar bounces or shows activity, but numeric SpO2 and Pulse Rate displays remain at zero

Ranked causes

1. Sensor damaged or failed (common)
2. User Interface PCB defective (occasional)

First check

Inspect sensor condition and reconnect

Test point: Pulse oximetry sensor
Expected: Sensor intact and properly seated

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

Unit does not power on high voltage

No output from power supply PCB; unit fails to run.

Ranked causes

1. Blown fuse in PEM (Hot or Neutral line) (common)
2. Short circuit in DC circuitry (occasional)

First check

Verify AC input voltage at PEM

Test point: PEM inlet
Expected: 120 VAC or 240 VAC depending on SW1 setting
Tools: Multimeter

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

Unit does not respond to buttons

Monitor powers on but does not respond to some or all button inputs; buttons non-functional

Ranked causes

1. Top Case assembly defective or misaligned (common)
2. Interface PCB defective (occasional)

First check

Test all buttons for response

Test point: All user interface buttons
Expected: All buttons respond to presses

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

Analog output signal levels

Three analog output channels (saturation, pulse rate, plethysmographic waveform) provide 0.0–1.0 VDC. 1.0 VDC = 100% SpO2, 250 bpm, or 254 pulse amplitude units. Output drops as parameter values decrease. If no data available, output is 1.0 VDC.

Ranked causes

1. Normal analog output operation during measurement (common)

First check

Measure saturation output during normal operation

Test point: Saturation analog output
Expected: 0.0–1.0 VDC linear with SpO2 (1.0 VDC = 100%)
Tools: DMM

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

Battery connected backwards caution

Reverse battery connection is protected by circuit components on User Interface PCB and Power Supply

Ranked causes

1. Battery polarity reversed during installation (rare)

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

Battery Low indicator on during DC operation with alarm sounding

Battery Low indicator illuminated during battery-powered operation accompanied by audible alarm

Ranked causes

1. Battery charge critically low (15 minutes or less remaining) (common)

First check

Monitor remaining battery time

Test point: Battery Low indicator and alarm status
Expected: Battery Low indicator on, alarm sounding, 15 min or less charge remaining

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

Battery short circuit detected caution

Fuse F3 opens to protect battery when output current exceeds 5.0 amps

Ranked causes

1. Battery output current exceeds 5.0 amps (short to ground) (rare)

First check

Measure battery output current

Test point: Battery output circuit or power supply output
Expected: Below 5.0 amps, < 5.0 amps
Tools: Ammeter or clamp meter

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

Battery temperature exceeds 50°C caution

Battery protection circuit opens charging circuit when battery temperature rises above 50°C

Ranked causes

1. Battery temperature exceeds 50°C (occasional)

First check

Verify battery temperature

Test point: Battery temperature sensor or battery casing
Expected: Below 50°C

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

Calibration: analog output signals

Automatic three-step calibration signal at power-on after POST completes. Signal sequence: 0.0 VDC for 60 sec, then 1.0 VDC for 60 sec, then stair-step 0.0–1.0 VDC in 0.1 VDC increments (1 sec each).

Ranked causes

1. Normal power-on calibration procedure (common)

First check

Verify first segment of calibration signal

Test point: Analog output (saturation, pulse rate, or plethysmographic)
Expected: 0.0 VDC held for 60 seconds
Tools: DMM or oscilloscope

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

Critical battery level error code displayed patient safety

Unit displays error code and will not monitor patient when powered on at critical battery voltage

Ranked causes

1. Battery voltage at or below 5.67 VDC critical level (common)

First check

Measure battery voltage

Test point: Battery terminals or internal battery connection
Expected: 5.67 ± 0.1 VDC (critical threshold), ±0.1 VDC

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

Low battery alarm declared

Unit displays low battery alarm and will power down after 15 minutes of continued operation

Ranked causes

1. Battery voltage dropped below 5.85 VDC (common)

First check

Measure battery voltage

Test point: Battery terminals or internal battery connection
Expected: 5.85 ± 0.1 VDC or higher, ±0.1 VDC

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

Unit does not operate when disconnected from AC power

Device non-functional on DC battery power; may be intermittent or complete loss of operation

Ranked causes

1. Battery is discharged (common)

First check

Check battery charge state and recharge

Test point: Battery and AC power
Expected: Battery fully charged after connection to AC

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

Error codes

Code	Meaning	Trigger	Severity
`1`	Power-on self-test (POST) failure	POST failure detected during startup	critical
`11`	Flash ROM corruption	Flash memory checksum failed	error
`12`	Excessive microprocessor resets	Too many CPU reset events	error
`4`	Battery dead or non-functional	Battery cannot supply power	error
`5`	Too many microprocessor resets within period of time	Excessive CPU reset events detected	error
`52`	Loss of monitor settings	Settings lost or corrupted	warning
`6`	Boot CRC error	Checksum error in boot code	error
`76`	Error accessing EPROM	EPROM read/write failure	error
`8`	Boot CRC error	Checksum error in boot code	error
`80`	Institutional default values lost, reset to factory defaults	Institutional settings loss	warning
`81`	Settings lost (non-default values lost)	Configured settings lost from memory	warning
`82`	Real-time clock lost	RTC data loss or failure	warning
`84`	Internal communications error	Communication between internal components failed	error
`DISALLOWED ON BATTERY`	Print or download operation attempted while on battery power	User attempted print/download during battery operation	info
`DISALLOWED ON LOW BATTERY`	Backlight activation attempted during low battery condition	User turned on backlight during low battery	info
`INVALID SILENCE DURATION`	Alarm silence duration power-on default set to OFF	User attempted to set silence duration to OFF	info
`INVALID SpO2 LIMIT`	SpO2 alarm limit set below 80	User attempted to set SpO2 limit power-on default below 80	info
`READING TRENDS`	Monitor gathering trend information for display	System collecting historical trend data	info
`SENSOR DISCONNECTED`	Pulse oximetry sensor or cable disconnected	Sensor unplugged from patient or cable unplugged from monitor	warning
`SENSOR OFF`	Pulse oximetry sensor removed from patient	Sensor no longer on patient	warning

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.