Error E85

Safety precautions:

1. If you smell gas, leave the property immediately, do not operate electrical switches, and call your gas emergency number. Do not attempt any checks.

2. Before opening any covers on the boiler or any external control units, switch the boiler off at the isolator and turn off mains power at the consumer unit if you will be touching wiring. If you are unsure, do not open electrical covers.

3. Do not touch the printed circuit board (PCB), live terminals, or the gas valve. Only a qualified engineer should work on live boiler internals.

4. Do not repeatedly reset the boiler more than a couple of times if the fault returns; this can mask a serious fault and may cause further damage.

Initial checks a homeowner can do (non-intrusive):

1. Note exactly when the E85 appeared and whether anything changed recently (new thermostat, wiring work, power cut, app update, or bad weather that could affect wireless signal).

2. Reset the boiler: use the reset button or selector on the control panel. On many Baxi units press and hold the reset for 5–10 seconds or turn the selector to the R (reset) position for 5 seconds, then return to normal. Wait to see if the code clears and the boiler restarts.

3. Check mains power: confirm the boiler has power (display/backlight on) and there are no tripped RCDs/MCBs.

4. If you use a wireless thermostat or smart control, check that its batteries are good, the thermostat shows it is calling for heat, and the thermostat/receiver are paired and powered. Try moving the thermostat closer to the boiler temporarily to rule out RF range/interference.

5. Check any wired room thermostat or programmer: make sure it is turned on and calling for heating; try raising the thermostat set temperature. If you have a separate programmer/timer, ensure it is set to demand heat now.

Specific diagnostic and safer fix steps:

1. Verify other error codes: check the display for any additional faults which may provide a clue (loss of flame, pressure faults, etc.). If multiple faults appear, report them to the engineer.

2. Repeat a controlled reset: after checking controls and batteries, switch the boiler off for 30 seconds at the isolator, then back on and allow one full restart cycle. If E85 returns immediately or repeatedly, proceed to the next checks.

3. Wireless thermostat troubleshooting: replace batteries in the thermostat, confirm the receiver at the boiler (if visible) has power, and re-pair according to the device manual. If you have a spare wired thermostat or a simple battery thermostat, try that as a temporary swap to test communication.

4. Wired thermostat visual check (only if comfortable and safe to do so): with the boiler and power isolated, remove the room thermostat cover and check the two control wires for secure connection and for visible damage. Then, with power restored and thermostat calling for heat, verify the boiler display responds. If you do open the thermostat, turn the mains off first.

5. Terminal/connection check at the boiler (only if you are competent with basic electrical isolation): isolate power, open the boiler outer cover following the manufacturer instructions, visually inspect the low-voltage control terminals and connectors for loose screws, corrosion, or water ingress. Do not touch live parts. If you see obvious loose connectors that can be tightened with the power isolated, do so carefully. Reassemble, restore power and retest.

6. Temporary isolation test (advanced, only if you know what you are doing): a simple continuity/jump test between the thermostat call-for-heat terminals at the boiler can confirm whether the boiler will fire without the room thermostat in the circuit. This involves shorting the two call terminals while the boiler is powered and asking the system to run; this should only be done by someone confident with electrical safety. If the boiler runs when the call is jumped, the problem is with the thermostat or its wiring. If the boiler still shows E85 or will not run, the fault is within the boiler or its receiver/PCB.

When to call a professional and what they will do:

1. Call a Gas Safe registered engineer if the steps above do not clear E85, if you are not comfortable performing any of the visual/wiring checks, or if there are signs of water ingress, burnt components, or internal damage. Also call an engineer if the fault returns after a reset.

2. The engineer will carry out safe electrical and communications diagnostics: check wiring continuity, inspect and test the PCB, test or replace RF receivers/modules, check connectors to the gas valve and external controls, and determine if PCB replacement or reprogramming is required.

3. Tell the engineer the exact fault code (E85), any other codes shown, what you tried (reset, battery changes, bridging test), and whether the fault is constant or intermittent.

Final notes:

1. E85 is commonly a communications link problem and can be resolved by simple measures (reset, batteries, re-pairing, tightening connections), but persistent faults need a trained technician.

2. Avoid trying to replace or solder PCB components, gas valves, or any live electrical parts yourself. Always use a Gas Safe registered engineer for repairs involving gas or live boiler internals.