Thermometers · SureTemp family

Welch Allyn SureTemp 678: troubleshooting and error codes

The ServiceForge knowledge base holds 20 documented symptoms for the Welch Allyn SureTemp 678, with 48 ranked causes, 28 diagnostic checks with test points and expected values, and 15 error codes. Each symptom below shows its causes and the first check; the rest is one connection away. Connect ServiceForge to your AI

Symptoms

Normal/Monitor mode switching problems

Unit switches between Normal and Monitor modes unexpectedly or fails to switch when intended

Ranked causes

1. Ambient temperature above 33.9°C (93.0°F) (common)
2. Probe cooling from previous measurement (occasional)
3. Mode button inoperative or low signal (occasional)
4. Defective probe (occasional)
5. Failed component or broken trace (rare)

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

Battery life problems

Battery drains quickly or does not maintain full charge during normal use

Ranked causes

1. Excessive alarms (common)
2. Excessive use in Monitor mode (common)
3. Dead cell in battery (occasional)
4. First set shelf life reduction due to long storage (occasional)

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

Display problems

LCD inactive, distorted, or showing incorrect information

Ranked causes

1. LCD frame loose or misaligned (common)
2. Dirty LCD elastomeric conductor strips (common)
3. Cracked LCD (occasional)
4. Microprocessor failure (rare)

First check

Inspect LCD frame assembly

Test point: LCD frame plastic hooks
Expected: All 4 hooks tight, frame flush against PCB

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

Monitor mode temperature reading too low

Temperature displayed in Monitor mode is lower than expected during measurement

Ranked causes

1. Probe malfunction (common)
2. Instrument malfunction (occasional)
3. Improper placement of probe (common)
4. Temperature not stable (common)

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

Normal mode temperature reading too high

Temperature displayed in Normal mode is higher than expected after stabilization

Ranked causes

1. Probe malfunction (common)
2. Instrument malfunction (occasional)
3. Improper technique - movement in mouth (common)
4. Improper technique - probe insertion timing (common)

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

Unit does not power on

No operation, display inactive, no response to button presses

Ranked causes

1. Dead, missing, or incorrectly installed batteries (common)
2. Broken battery wire (occasional)
3. Short circuit preventing operation (occasional)
4. Failed component (oscillator or microprocessor) (rare)

First check

Verify battery installation

Test point: Battery compartment
Expected: All batteries installed in correct polarity

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

Cannot enter Biotech Mode

Mode button press does not activate Biotech Mode on unit in wall holder

Ranked causes

1. Mode button not pressed or instrument not in wall holder (678 only) (common)
2. Probe not connected or probe shaft not inserted and removed (common)
3. Failed component or broken trace (rare)

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

No beeper sound

Horn does not sound during operation, errors, or timer cycles

Ranked causes

1. Defective horn speaker (common)
2. Broken connection between microprocessor and horn (occasional)
3. Defective microprocessor U1 (rare)

First check

Check horn control signal

Test point: U1-49
Expected: Square wave signal present during operation or error condition
Tools: Oscilloscope

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

Normal mode temperature reading too low

Temperature displayed in Normal mode is lower than expected after stabilization

Ranked causes

1. Probe malfunction (common)
2. Instrument malfunction (occasional)
3. Improper placement of probe (common)

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

Monitor mode temperature reading too high

Temperature displayed in Monitor mode is higher than expected during measurement

Ranked causes

1. Probe malfunction (common)
2. Instrument malfunction (occasional)

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

No Backlight

LCD backlight does not illuminate during operation

Ranked causes

1. Defective backlight LED D3 or resistor R23 (common)
2. No backlight control signal from microprocessor (occasional)

First check

Inspect backlight LED and resistor

Test point: LED D3 and resistor R23
Expected: LED intact with no visible cracks; resistor not burnt or open
Tools: Visual inspection, DMM for resistance measurement

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

No Timer function

Timer button S1 does not activate or timer mode does not operate

Ranked causes

1. Defective timer button S1 (common)
2. Broken PCB trace to timer button (occasional)

First check

Measure signal at U1-26 when timer button pressed

Test point: U1-26 (microprocessor /PTIMER input)
Expected: Logic low (ground/0 V) when button S1 depressed; logic high (VCC) when released
Tools: DMM or oscilloscope

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

Cal Key displays OrL, rEC, or ALy

Cal Key shows error text instead of normal CAL display

Ranked causes

1. Defective Cal Key (common)

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

Cal Key does not activate thermometer

Cal Key connected but does not activate calibration mode or display remains blank

Ranked causes

1. Probe switch not activated (common)

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

Calibration: Self-tests

Verify microprocessor self-checks including RAM, ROM, instruction set, calibration, and sensor tests

Ranked causes

1. Power-up reset sequence initiated after battery installation (common)

First check

Verify RAM test

Test point: Microprocessor memory
Expected: RAM test passes without error code

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

Erroneous temperature readings patient safety

Temperature readings display incorrect values during operation

Ranked causes

1. Battery voltage degraded below functional threshold (occasional)

First check

Perform self-calibration test

Test point: Electronics accuracy, hi cal, low cal
Expected: All self calibration tests pass without error codes
Tools: Thermometer self-test function

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

Low battery error condition

E2.1 error message displayed, thermometer operation halted

Ranked causes

1. Battery voltage fallen to approximately 3.0 volts or lower (common)

First check

Measure battery voltage

Test point: Battery terminals
Expected: approximately 3.0 volts or lower indicates error condition V, below 3.0V indicates dead battery
Tools: Voltmeter

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

Low battery warning

Battery icon flashes in display indicating batteries are getting low

Ranked causes

1. Battery voltage fallen to approximately 3.2 volts (common)

First check

Measure battery voltage

Test point: Battery terminals
Expected: approximately 3.2 volts or above for normal operation V, above 3.2V for normal operation
Tools: Voltmeter

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

Probe: wrong type displayed

Instrument displays incorrect probe type in normal or monitor mode operation

Ranked causes

1. Incorrect wiring of probe (common)

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

Recalled temperature is not correct

Unit displays temperature lower than expected when temperature is recalled from memory

Ranked causes

1. Unit switched to Monitor mode (common)

First check

Verify thermometer operating mode

Test point: Unit mode display or settings
Expected: Normal mode active (not Monitor mode)

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

Error codes

Code	Meaning	Trigger	Severity
`E0.1`	Probe heater accumulation test failure	Self-test detects error during power-on or operational self-test	error
`E0.2`	Probe A/D pulse width test failure	Self-test detects error during analog-to-digital conversion measurement	error
`E0.3`	Adaptive probe gain too high or too low	Self-test detects out-of-range probe signal conditioning gain during startup	warning
`E1.1`	Ambient temperature high test failure	Ambient probe temperature measured above 104.0°F during self-test or operation	warning
`E1.2`	Ambient temperature low test failure	Ambient probe temperature measured below 60.8°F during self-test or operation	warning
`E2.1`	Low battery error condition	Battery voltage falls to approximately 3.0 volts or lower	error
`E3.1`	RAM read/write test failure	Internal RAM memory test fails during self-test or power-on	error
`E3.2`	ROM checksum test failure	Internal ROM memory checksum validation fails during self-test	critical
`E3.3`	CPU instruction test failure	Microprocessor instruction execution self-test fails during startup	critical
`E4.0`	PTB resistor A/D pulse width test failure	Precision temperature bridge resistor analog-to-digital conversion fails	error
`E4.1`	RatioCal resistor A/D pulse width test failure	Ratio calibration resistor analog-to-digital conversion fails during self-test	error
`E5.0`	Heater circuit test failure	Probe heater power circuit test fails during self-test	error
`E5.1`	Heater overheated test failure	Probe heater temperature exceeds safe operating limit during self-test or operation	error
`E5.2`	Heater watchdog timeout test failure	Heater circuit control watchdog timer expires during probe heating cycle	error
`E6.0`	PTB resistor temperature test failure	Precision temperature bridge resistor temperature measurement fails during self-test	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.