Andrews E78 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E78 fault code mean?
The E78 fault code on Andrews water heaters (Baxi GA Range) indicates that the boiler's control board has detected the electrical current reaching the gas valve has dropped below the minimum acceptable threshold. As a safety precaution, the appliance locks out to prevent unreliable or unsafe combustion. This is a gas-circuit fault — not a temperature or pressure sensor issue — and almost always requires a Gas Safe registered engineer to diagnose and resolve.
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 worn gas valve Common
The gas valve regulates the flow of gas into the burner. Over time the internal solenoid coils can degrade, causing the valve to draw less current than required. The PCB detects this drop and triggers E78 as a safety lockout. A faulty valve is the single most common root cause of this code.
- Loose or damaged wiring to the gas valve Common
The electrical harness connecting the PCB to the gas valve can suffer from loose terminals, corrosion, or heat-damaged insulation. A poor connection increases resistance and reduces the current the valve receives, mimicking a valve failure. Engineers will inspect connectors and wiring continuity before condemning the valve itself.
- Failed ignition electrode or ionisation/rectifying probe Sometimes
If the ignition electrode or the rectifying probe that confirms a flame is present has deteriorated or become contaminated, the control system may misinterpret combustion conditions and generate an E78 lockout. Electrodes can be inspected and replaced relatively quickly during an engineer visit.
- PCB (printed circuit board) fault Rare
If the control board itself is unable to supply the correct drive signal or accurately measure the current to the gas valve, it can log E78 incorrectly. PCB faults are less common but should be considered after the gas valve and wiring have been ruled out.
How to fix it
- Attempt a single boiler reset DIY safe
Refer to your Andrews appliance manual for the reset procedure — typically holding the reset button for a few seconds until the display clears. One reset attempt is reasonable. If the E78 code returns immediately or comes back after a short while, do not keep resetting; repeated lockouts with the same fault code can indicate a worsening problem and further resets will not resolve the underlying fault.
- Check that the gas supply is on DIY safe
Confirm that the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). Check that other gas appliances in the property — hob or gas fire — are working normally. If no gas appliances are working, contact your gas supplier rather than an engineer.
- Call a Gas Safe registered engineer to inspect the gas valve and wiring Gas Safe engineer
An engineer will remove the boiler casing, visually inspect the gas valve for signs of wear or corrosion, and test the electrical harness and connectors for continuity and integrity. They will use a manometer and flue gas analyser to carry out a full combustion check. All work on gas components is a legal requirement for Gas Safe registered engineers only — do not attempt to access the valve or internal wiring yourself.
- Allow the engineer to test or replace the ignition electrode and rectifying probe Gas Safe engineer
If wiring and the valve appear sound, the engineer will inspect the ignition electrode gap and the rectifying probe for contamination or cracking. These components are inexpensive and quick to replace, and doing so rules them out before a more costly valve replacement is authorised.
- Engineer to test and replace the gas valve if confirmed faulty Gas Safe engineer
If testing confirms the gas valve solenoid is drawing insufficient current or is otherwise defective, the engineer will replace the valve with an approved replacement part. The boiler will then be re-commissioned with a full combustion analysis to confirm safe and efficient operation before being returned to service.
- PCB replacement if all other causes are eliminated Gas Safe engineer
Should the gas valve, wiring, and probes all test within specification, the fault lies with the PCB generating an incorrect reading or failing to supply adequate drive current. PCB replacement must be carried out by a qualified engineer and is followed by full system re-commissioning.
Parts you may need
- Gas valve (Andrews/Baxi GA Range compatible) · from £95
- Ignition electrode · from £18
- Rectifying/ionisation probe · from £22
- PCB (control board) · from £175
- Gas valve wiring harness · from £30
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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Andrews E78 myself?
Not beyond a single reset and confirming the gas supply is on. The E78 fault points to a problem within the gas circuit — specifically the current supplied to the gas valve — which means any further investigation or repair involves gas components and internal boiler wiring. In the UK, this work must legally be carried out by a Gas Safe registered engineer. Attempting it yourself is both dangerous and illegal.
How much does it cost to fix an Andrews E78 fault?
For most households the repair falls in the range of £150–£350, depending on which component is at fault. A wiring or probe fix sits at the lower end; a gas valve replacement typically costs £180–£300 including parts and labour. PCB replacement is less common but can cost £250–£400 or more depending on the model. Prices in London and the South East tend to run 15–25% higher than the national average.
Is E78 on an Andrews boiler the same as a gas valve fault?
E78 specifically means the PCB has detected that the current reaching the gas valve is below the minimum safe threshold. In most cases this does point to the gas valve itself being worn or faulty, but it can also be caused by a damaged wiring loom, a failed rectifying probe, or — less commonly — a PCB fault. An engineer will work through these possibilities systematically before replacing any major component.
How can I avoid the E78 fault coming back?
Annual servicing by a Gas Safe registered engineer is the most effective preventive measure. During a service, the engineer will clean the electrodes and probes, inspect wiring connections, and test the gas valve — catching early signs of wear before they cause a lockout. Andrews water heaters used in commercial settings may benefit from more frequent servicing depending on usage levels.