Ravenheat E43 Fault Code: Causes, Fixes & Repair Costs
What does the Ravenheat E43 fault code mean?
The E43 code on a Ravenheat boiler sits within the E42–E44 fault group, which collectively point to a 'different temperature points' issue — in other words, the boiler's control board is receiving conflicting or out-of-range temperature readings from one or more of its NTC thermistors (temperature sensors). In practice, E43 is closely associated with flame detection or ignition failure, where the boiler loses its flame signal during operation, sometimes repeatedly. The two main NTC sensors — one measuring the flow (outgoing hot water) temperature and one measuring the return (cooler incoming water) temperature — constantly feed data to the PCB. If either sensor sends erratic, implausible, or out-of-range values, the PCB triggers E43 as a safety lockout. Note that Ravenheat uses slightly different code sets across its RSF, CSI, and LS model ranges, so always check your specific model's documentation if the behaviour seems unusual.
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 degraded NTC thermistor Common
After years of use, NTC thermistors drift out of calibration or fail altogether — often accelerated by limescale coating the sensor tip, which insulates it from the water and causes it to read higher than the actual temperature. A failed sensor can read open-circuit (infinity) or short-circuit (near zero), both of which will immediately trigger E43.
- Damaged or chafed sensor wiring Common
The wiring looms running from the flow and return sensors to the PCB can chafe against sharp metal edges inside the boiler cabinet, especially near the heat exchanger. Intermittent breaks cause the PCB to receive unstable signals, producing the conflicting temperature readings that trigger this code — sometimes only when the boiler vibrates during firing.
- Limescale or magnetite build-up in the heat exchanger Common
Heavy sludge or scale deposits reduce water flow and heat transfer efficiency. This can cause localised hot spots around the sensor pockets, making the thermistor read a temperature that doesn't reflect the bulk water temperature and pushing readings outside the expected differential range.
- Flame detection or ignition electrode fault Sometimes
E43 can also be triggered by a loss of flame signal rather than a pure temperature issue. A dirty, cracked, or mispositioned flame-sensing electrode may fail to confirm the burner has lit, causing the boiler to lock out. Wind-induced flame disturbance via an exposed flue can produce the same intermittent signal loss.
- PCB fault Rare
If the control board itself has a failed input channel or a dry solder joint on the sensor terminals, it may misread perfectly good sensor signals and log an E43. This is less common and is usually a diagnosis of exclusion once sensors and wiring have been checked.
How to fix it
- Reset the boiler (up to 2–3 times) DIY safe
Locate the on/off or reset button on the boiler fascia and perform a manual reset. A one-off reset can sometimes clear a transient E43 caused by a brief wind gust down the flue or a momentary signal glitch. If the fault returns after one or two resets, further investigation is needed — do not keep cycling the reset repeatedly.
- Check your gas supply DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If there is no gas supply elsewhere, contact your gas network provider rather than a boiler engineer. If the gas supply is fine, the fault lies within the boiler itself.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when the system is cold. Very low pressure can cause the sensors to behave erratically. If pressure is below 0.8 bar, top up via the filling loop until it reaches 1.2–1.5 bar, then attempt a reset. If you are unsure how to use the filling loop, consult your boiler manual.
- Inspect the flue terminal for obvious blockages DIY safe
If you can safely see the external flue terminal from the ground (do not use a ladder), check that it is not blocked by leaves, a bird nest, or debris. A restricted flue causes back-pressure that can extinguish the flame and trigger flame-loss faults. Do not attempt to dismantle or clean the flue yourself.
- Have a Gas Safe engineer test the NTC thermistors Gas Safe engineer
An engineer will use a multimeter to check the resistance of both the flow and return NTC sensors — a healthy sensor reads close to 10 kΩ at 25°C. Readings well outside this range confirm a failed sensor. Ravenheat NTC thermistors for RSF, CSI, and LS models are inexpensive parts, but correct fitting, sealing, and calibration verification must be carried out by a qualified engineer.
- Have the engineer inspect and clean the flame-sensing electrode Gas Safe engineer
The flame sensor electrode can accumulate carbon deposits over time, reducing its ability to detect the burner flame. An engineer will inspect the electrode for cracks or heavy soiling, clean the tip carefully, and check the electrode gap and positioning. If damaged, the electrode should be replaced.
- Have the engineer check sensor wiring for damage Gas Safe engineer
The engineer will trace the wiring looms from both NTC sensors to the PCB terminals, testing for continuity and inspecting for chafing, heat damage, or corroded connectors. Damaged wiring sections should be repaired or replaced rather than taped over.
- Consider a power flush if sludge or scale is suspected Gas Safe engineer
If the central heating system has not been serviced in several years, magnetite and limescale build-up may be hampering heat transfer and causing localised overheating around the sensors. A Gas Safe engineer or heating engineer can carry out a power flush and, if appropriate, install a magnetic filter to protect the system going forward.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If all checks above have been completed and E43 continues to return, the PCB or gas valve may need further investigation. These are complex, safety-critical components. For Ravenheat boilers more than 10 years old, a Gas Safe engineer can also advise honestly on whether repair or full boiler replacement offers better long-term value.
Parts you may need
- Ravenheat NTC thermistor (flow or return sensor) · from £15
- Flame detection / ignition electrode · from £25
- Sensor wiring loom · from £30
- Ravenheat PCB (control board) · from £220
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
Is the Ravenheat E43 dangerous — should I stop using the boiler?
E43 is a safety lockout, meaning the boiler has shut itself down to prevent a potentially unsafe condition. You should not attempt to override the lockout or keep resetting indefinitely. While the fault itself is usually a sensor or electrode issue rather than a gas leak, a boiler locked out on E43 should be assessed by a Gas Safe engineer before being returned to normal use. If you smell gas at any point, turn off the gas at the meter, ventilate the property, and call the National Gas Emergency Service on 0800 111 999.
How much does it cost to fix a Ravenheat E43 in the UK?
For the most common cause — a faulty NTC thermistor — you can typically expect to pay £100–£200 including parts and labour. Replacing a flame electrode tends to sit in the same ballpark. If the fault has caused damage to the PCB, repair costs can rise to £300–£500 or more, in which case replacement of an older boiler may be the more economical choice. Hourly rates for Gas Safe engineers vary from around £50–£70 outside London to considerably more in the capital or through large national firms.
Can I replace the NTC thermistor myself to fix E43?
Physically, an NTC thermistor is a small and inexpensive part, but fitting it involves draining down the relevant circuit, correctly sealing the sensor pocket to prevent leaks, and verifying the repair with a live heat cycle. While it does not directly involve gas pipework, it is work inside a gas appliance and should be carried out by a Gas Safe registered engineer. Attempting it without the right expertise risks water leaks or an incorrect repair that masks an underlying problem.
Why does my Ravenheat keep showing E43 after I reset it?
A fault that returns quickly after resetting almost always points to a persistent hardware problem rather than a one-off glitch. The most likely culprits are a thermistor that has definitively failed, wiring with an intermittent break that reconnects briefly after handling, or a flame electrode that is too dirty or damaged to reliably sense the burner. In these cases, resetting will only provide a temporary restart and the boiler will lock out again once the sensor gives another bad reading. A Gas Safe engineer can pinpoint the exact component at fault and carry out a lasting repair.