Thermometers · SureTemp family

Welch Allyn SureTemp 767: troubleshooting and error codes

The ServiceForge knowledge base holds 25 documented symptoms for the Welch Allyn SureTemp 767, with 54 ranked causes, 65 diagnostic checks with test points and expected values, and 13 error codes. Each symptom below shows its causes and the first check; the rest is one connection away. Connect ServiceForge to your AI

Symptoms

Temperature reading too high in normal mode patient safety

Thermometer displays elevated temperature readings

Ranked causes

1. Instrument calibration error (common)
2. Improper probe placement (common)
3. Probe malfunction or characterization error (occasional)
4. System calibration failure (rare)
5. Improper technique during measurement (common)
6. Probe inserted before display ready (common)

First check

Verify display shows OrL before measurement

Test point: Display
Expected: Display shows OrL

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

Normal/Monitor mode switching problems patient safety

Unit switches between Normal and Monitor mode unexpectedly or incorrectly

Ranked causes

1. Ambient temperature above 33.9°C (93.0°F) (common)
2. Probe still cooling from previous use (occasional)
3. Auto-switch to Monitor mode after 60 seconds in Normal mode (common)
4. Defective mode button (occasional)
5. Defective probe (occasional)

First check

Verify ambient temperature

Test point: Surrounding environment
Expected: Below 33.9°C (93.0°F)
Tools: Thermometer

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

Monitor mode temperature reading too low patient safety

Temperature reading in Monitor mode is lower than expected

Ranked causes

1. Probe malfunction or drift (common)
2. Instrument malfunction or calibration drift (occasional)
3. Improper probe placement (common)
4. Temperature not stabilized (common)

First check

Test probe with calibration tester

Test point: Probe accuracy
Expected: Reading within tolerance per M9600 Calibration Tester
Tools: M9600 Calibration Tester

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

No operation

Unit powered but not functioning or responding to inputs

Ranked causes

1. Short circuit in power circuit (common)
2. Power circuit component malfunction (occasional)
3. Oscillator failure (occasional)
4. LCD PCB loose (occasional)

First check

Measure resistance at power input

Test point: Contacts J7 and J8
Expected: At least 3 M ohm
Tools: Ohm meter

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

Calibration: Probe type detection

Software determines probe type (Rectal, Calibration Key, or none) based on connector pin states at J1-3 (CAL), J1-5 (GND), and J1-6 (RCTL)

Ranked causes

1. Rectal probe connection (common)
2. Calibration Key connection (occasional)
3. No probe connected (occasional)

First check

Measure voltage at J1-3 (CAL) relative to J1-5 (GND)

Test point: J1-3 (CAL) and J1-5 (GND)
Expected: 0 V (connected for Rectal probe, floating for other modes)
Tools: DMM

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

Display problems

LCD display unclear, crooked, dim, or not showing expected output

Ranked causes

1. Dirty or crooked elastomeric LCD conductor (zebra) strips (common)
2. Broken or shorted flex cable (occasional)
3. Cracked LCD (rare)

First check

Inspect elastomeric strips and contacts for contamination

Test point: LCD elastomeric strips and glass contacts
Expected: Clean, properly aligned, no debris

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 patient safety

Temperature reading in Normal mode is lower than expected

Ranked causes

1. Probe drift or calibration offset (common)
2. Defective probe (occasional)
3. Instrument malfunction or calibration drift (occasional)

First check

Re-characterize probe by removal and reinstallation

Test point: Probe connection and recognition
Expected: Probe recharacterized, reading normalized

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

Probe wrong type displayed

Incorrect probe type shown on display or probe not recognized

Ranked causes

1. Missing A/D power to probe connector (occasional)
2. Incorrect probe wiring (common)
3. Defective probe (occasional)

First check

Check probe connector power pins without probe installed

Test point: Probe connector J1-3 and J1-6
Expected: Both pins pulled high during function active
Tools: DMM or logic probe

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

Unit does not power on

No power supplied to thermometer unit

Ranked causes

1. Power supply not plugged in or turned off (common)
2. Power supply voltage out of range (occasional)
3. Power contacts not making contact (occasional)

First check

Verify power supply is plugged in and powered on

Test point: Power supply
Expected: Power supply plugged in and on

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

Cal key does not activate thermometer

Calibration key not recognized or not activating thermometer

Ranked causes

1. Probe switch not activated (common)
2. Defective calibration key (occasional)

First check

Connect cal key and activate probe switch

Test point: Display and probe switch activation
Expected: Display reads CAL for two seconds then blanks; probe switch activated

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

Calibration: Model 9600 Calibration Tester procedure

Testing entire thermometer system (instrument and probe) using Model 9600 Calibration Tester

Ranked causes

1. Complete thermometer system requires calibration verification (common)
2. Probe requires individual testing (common)

First check

Verify 9600 Calibration Tester warm-up status

Test point: 9600 temperature display
Expected: Stable at one of two available temperature settings

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

Cannot enter Biotech mode

Unit cannot switch to Biotech mode operation

Ranked causes

1. Probe not connected or probe shaft sequence not completed (common)
2. Failed component or broken trace in mode button or probe switch (occasional)

First check

Verify probe is connected to instrument

Test point: Probe connector
Expected: Probe fully inserted and connected

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

Monitor mode temperature reading too high patient safety

Temperature reading in Monitor mode is higher than expected

Ranked causes

1. Probe malfunction or drift (common)
2. Instrument malfunction or calibration drift (occasional)

First check

Test probe and instrument with calibration tester

Test point: System calibration and probe accuracy
Expected: System within tolerance per M9600 Calibration Tester
Tools: M9600 Calibration Tester

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

Cal key displays OrL, rEC, or ALy

Calibration key shows error or invalid display codes

Ranked causes

1. Defective calibration key (common)

First check

Observe cal key display codes

Test point: Display with cal key connected
Expected: CAL display; not OrL, rEC, or ALy
Tools: Observation

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

Calibration Key test limitations

Calibration Key procedure does not test the probe; probe testing requires separate Model 9600 Calibration Tester equipment

Ranked causes

1. Probe accuracy verification requires Model 9600 Calibration Tester (common)

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

Calibration: Calibration Key procedure

Testing thermometer accuracy using Calibration Key inserted into probe connector receptacle

Ranked causes

1. Thermometer requires calibration verification (common)

First check

Display shows CAL indication

Test point: Display
Expected: CAL for two seconds, then blank

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

Calibration: Instrument and probe verification

Procedure to verify and adjust calibration of thermometer system

Ranked causes

1. Periodic calibration required (common)

First check

Quick calibration check with Calibration Key

Test point: Calibration Key interface
Expected: Calibration reading within specification

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

Calibration: Power-up reset and self-tests

Microprocessor executes power-up reset sequence and self-test suite upon power application

Ranked causes

1. Power supply startup and reset initialization (common)

First check

Monitor reset signal timing

Test point: U1-12 (reset line)
Expected: Active low reset after approximately 200 milliseconds from power application

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

Calibration: Self-tests on power-up

Microprocessor performs RAM, ROM, instruction set, self-calibration, probe warmer, probe, and ambient temperature tests; failures display specific error codes

Ranked causes

1. Self-test sequence initiated (common)

First check

Verify RAM test passes

Test point: Display
Expected: No error code displayed after RAM test

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

F/C switching problems

Mode button or function/celsius switching not responding correctly

Ranked causes

1. Defective mode switch or broken trace (common)

First check

Test mode button switch signal

Test point: U1-26
Expected: Low only when Mode button is pressed
Tools: DMM or oscilloscope

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

Microcontroller clock generation

4.9152 MHz crystal (X1) with capacitors C26 and C27 generates oscillator; microcontroller divides internally by 2 to run at ~2.5 MHz

Ranked causes

1. Clock oscillator fault (rare)

First check

Measure crystal oscillator frequency at X1

Test point: X1 crystal
Expected: 4.9152 MHz, +/- 50 ppm
Tools: Oscilloscope, frequency counter

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

Microprocessor power-up reset

Upon power-up, microprocessor receives reset signal from U1-12 (Econo reset chip U4) after ~200 ms delay, then launches self-tests

Ranked causes

1. Normal power-up reset sequence (common)

First check

Measure voltage at U1-12 reset line

Test point: U1-12
Expected: Low (active) for ~200 ms after power applied, then returns high
Tools: Oscilloscope

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

No backlight

Backlight function not working (optional feature)

Ranked causes

1. Broken trace or defective backlight component (occasional)

First check

Measure backlight control signal at U1-42

Test point: U1-42
Expected: Low (Gnd) when LCD is on
Tools: DMM or oscilloscope

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

Power supply output voltage regulation high voltage

8VAC input converted to regulated 5V DC by full-wave bridge rectifier and low-dropout regulator; output range 4.75–5.25 V

Ranked causes

1. Power supply regulation fault (rare)

First check

Measure 8VAC input voltage

Test point: Input to D7–D10 bridge rectifier
Expected: 8 VAC nominal
Tools: DMM (AC setting)

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

Probe input noise filtering

Bypass capacitors C19 and C20 (0.01 µF each) filter spurious noise on J1-6 (PROBE_1) and J1-3 (PROBE_0) microprocessor input lines

Ranked causes

1. Noise on probe input lines (occasional)

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 probe heater malfunction	warning
`E0.2`	Probe A/D pulse width test failure	Self-test detects incorrect A/D conversion pulse width	warning
`E0.3`	Adaptive probe gain out of range	Self-test detects probe gain too high or too low	warning
`E1.1`	Ambient temperature high	Self-test detects ambient temperature exceeds safe limit	warning
`E1.2`	Ambient temperature low	Self-test detects ambient temperature below safe limit	warning
`E2.1`	Dead battery	Self-test detects battery voltage insufficient or battery failed	error
`E3.1`	RAM read/write test failure	Self-test detects RAM memory corruption or malfunction	critical
`E3.2`	ROM checksum test failure	Self-test detects ROM data integrity compromised	critical
`E3.3`	CPU instruction test failure	Self-test detects CPU cannot execute instructions properly	critical
`E4.0`	PTB resistor A/D pulse width test failure	Self-test detects incorrect PTB resistor analog-to-digital conversion	error
`E4.1`	RatioCal resistor A/D pulse width test failure	Self-test detects incorrect RatioCal resistor analog-to-digital conversion	error
`E5.0`	Heater circuit test failure	Self-test detects heater circuit malfunction	error
`E5.1`	Heater overheated	Self-test detects heater temperature exceeds safe limit	critical

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.