Dunkirk Error E40: Return water temperature
Blocking (soft-lockout, auto-reset) error: the return water temperature sensor reads return temperature higher than 200F (e.g. low flow / wrong heat-exchanger-pump orientation). Burner shuts off and refires automatically when temp drops below limit; persistent recovery escalates to Lockout A-06. Tree checks the 2-brown-wire return-sensor harness, sensor resistance (950-33,000 ohm), and Heat Exchanger Pump orientation (arrow down). Display 'E40 Return Temp'. We diagnose and repair it across Greater Vancouver — same-day and after-hours.
Blocking (soft-lockout, auto-reset) error: the return water temperature sensor reads return temperature higher than 200F (e.g. low flow / wrong heat-exchanger-pump orientation). Burner shuts off and refires automatically when temp drops below limit; persistent recovery escalates to Lockout A-06. Tree checks the 2-brown-wire return-sensor harness, sensor resistance (950-33,000 ohm), and Heat Exchanger Pump orientation (arrow down). Display 'E40 Return Temp'.
Dunkirk fault codes can vary by model and series. Use this as a guide — the same code can have more than one cause. Our Technical Safety BC–licensed technicians read the code, confirm it against your unit's manual, and diagnose the true cause before any repair.
Likely causes of Dunkirk error E40
- Return water sensor not connected or wrong harness (2 brown wires)
- Failed return water sensor (out-of-range resistance)
- Heat-exchanger pump installed in wrong orientation (arrow not pointing down) reducing flow
- Low system flow raising return temperature
- Damaged sensor wiring
What you can safely check
- Confirm the circulator pump is running and the system has good flow
- Check system pressure is normal (about 12-15 psi cold)
Beyond these checks, do not open the unit or touch gas/electrical parts — BC law requires a licensed gas fitter. Call us for a proper diagnosis.
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Parts often behind Dunkirk error E40
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Dunkirk Error E40 Repair Across Metro Vancouver
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