681 DHW supply temperature abnormality

What the control is telling you

The measured bypass ratio, calculated from the inlet, heat exchanger and outlet thermistors, doesn't match the expected ratio — often from a misaligned bypass valve or a bad temperature reading.

Applies to: I-Series Plus Combi boilers — 150K BTU and smaller and 175K BTU and higher

Likely causes

• Bypass valve/servo misalignment
• Faulty or disconnected inlet thermistor
• Faulty or disconnected heat exchanger thermistor
• Faulty or disconnected outlet thermistor
• Failed PC Board

Test and repair steps

1. Warning: Turn off electrical supply before taking resistance readings.
2. Check the inlet thermistor in performance data 08 against a thermometer at the same spot; over 5° difference means continue, -40 means unplugged/bad connection (see code 341).
3. Check the heat exchanger thermistor in performance data 10 the same way (see code 331).
4. Check the outgoing water thermistor in performance data 07 the same way (see code 161).
5. Replace any inlet, outlet or heat exchanger thermistor that reads abnormally.
6. Inspect the bypass valve and servo for misalignment and replace as needed (see code 661).
7. If all thermistor values are correct and the code comes back, replace the PC Board.

Models that show this code

8 models in I-Series Plus 150K and smaller, I-Series Plus 175K and higher

Other codes on the same units

• 631 DHW recirculation pump – insufficient flow
• 651 Water flow control valve fault
• 661 Bypass valve fault
• 670 Three-way valve fault
• 700 PC Board fault
• 710 Gas solenoid circuit – valve position mismatch
• 720 Flame rod fault
• 763 0-10V external control input over-voltage