Ravenheat E42 Fault Code: Causes, Fixes & Repair Costs
What does the Ravenheat E42 fault code mean?
E42 on a Ravenheat boiler belongs to the E42–E44 fault group, which Ravenheat's documentation describes as a 'different temperature' error — the control board is comparing readings from its temperature sensors (typically the flow and return NTC thermistors) and finding a difference or pattern that should not be possible if water were circulating properly and both sensors were healthy. Rather than keep firing on readings it cannot reconcile, the board stops the boiler and displays the code. In practice E42 comes down to one of two things: a sensor (or its wiring) drifting out of specification, or a genuine circulation problem — a struggling pump, air in the system or a blockage — making the real temperatures diverge abnormally.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or drifting NTC temperature sensor Common
An ageing flow or return thermistor can drift so its reading no longer tracks the true water temperature. When the board cross-checks it against the other sensor, the disagreement triggers the E42-family fault. Sensors are tested against a resistance/temperature table and replaced if out of tolerance.
- Poor circulation exaggerating the flow/return difference Common
A tired pump, a partially seized pump rotor, sludge or a closed valve slows circulation, so water leaves hot and crawls back cold. The abnormal spread between sensors is real, and the sensor comparison flags it. The clue is a boiler that also heats radiators slowly or noisily.
- Air in the system or heat exchanger Sometimes
Trapped air creates hot spots and erratic sensor readings, especially just after refilling or repressurising. Bleeding radiators and re-checking pressure can settle the readings.
- Sensor wiring or connector fault Sometimes
A chafed lead or corroded connector adds resistance to the sensor circuit, skewing one reading against the other. Engineers check the loom before condemning sensors.
How to fix it
- Check the system pressure gauge DIY safe
Low pressure worsens circulation and sensor disagreement. If the gauge is below 1 bar, top up slowly to around 1.2 bar using the filling loop, then reset once.
- Bleed the radiators DIY safe
If radiators are cold at the top or the system has recently been refilled, bleed the air out with a radiator key, re-check pressure afterwards, and try one reset.
- Reset once and observe DIY safe
Reset the boiler a single time and watch it run. If E42 returns within minutes, stop resetting — a drifting sensor or circulation fault will not clear itself.
- Have an engineer test the sensors and circulation Gas Safe engineer
A Gas Safe registered engineer will measure both NTC sensors against the specification table, inspect the wiring, and check the pump and system flow. The usual outcome is a replacement sensor or attention to the pump/system, after which the readings reconcile and the code clears.
Parts you may need
- Flow/return NTC thermistor · from £20
- Circulation pump · from £120
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£300, depending on the underlying cause.
Frequently asked questions
What does E42 mean on a Ravenheat boiler?
It is part of Ravenheat's E42–E44 'different temperature' group: the board has compared its temperature sensor readings and found a difference it cannot reconcile. That points either at a faulty/drifting sensor (or its wiring) or at a genuine circulation problem such as a struggling pump or air in the system.
Can I fix E42 myself?
You can do the safe basics: top the pressure up to around 1.2 bar if it is low, bleed the radiators, and reset once. If the code returns, the diagnosis — testing sensors against their resistance table and checking the pump — needs a Gas Safe registered engineer.
How is E42 different from E43 and E44?
They are siblings in the same comparison-fault family; the specific code reflects which check tripped. The practical diagnosis is the same across all three: verify the sensors and their wiring, then verify circulation.