Andrews A08 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews A08 fault code mean?
The A08 code indicates a problem with the return NTC thermistor — the temperature sensor that monitors the water coming back into the boiler after it has circulated around the heating system. The boiler's PCB continuously compares readings from the flow and return thermistors; if the temperature differential falls outside acceptable limits, or if the return sensor sends a signal that is clearly out of range, the boiler treats this as a safety risk and locks out. Note: although A08 is sometimes labelled as an 'Andrews' code, it is formally associated with Vokera boilers, which share engineering roots and fault-code conventions with certain Andrews commercial heating products. If your display shows A08, this page applies to your boiler.
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 return NTC thermistor Common
The return thermistor is a resistive sensor — at around 20°C it typically reads approximately 12 kΩ, dropping as temperature rises. Over time, limescale, magnetite sludge, or water ingress can damage the sensor element, causing it to read incorrectly or go open-circuit. When the PCB receives a reading that is wildly out of range or contradicts the flow sensor, it triggers A08 and locks the boiler out.
- Loose, corroded, or water-damaged wiring and connections Common
Boilers vibrate during normal operation, and over months or years this can work plug-in connectors and spade terminals loose. Small internal leaks from the heat exchanger, pump seals, or pipework joints can also drip onto wiring looms, causing corrosion or intermittent short circuits. Either scenario interrupts the signal between the thermistor and the PCB, producing the same A08 lockout even if the thermistor itself is perfectly healthy.
- Blocked or failing circulation pump Sometimes
If the pump is partially blocked with sludge or beginning to seize, water moves through the system more slowly than it should. The water then has more time to lose heat before it reaches the return thermistor, creating an unusually large temperature differential between flow and return. The PCB interprets this as an unsafe condition and raises A08. A pump fault may also present alongside other codes, so engineers will check pump speed and pressure alongside thermistor resistance.
- Faulty PCB Rare
The PCB is responsible for reading and interpreting the resistance signal from the thermistor. If the PCB's analogue input circuitry has failed — due to a power surge, moisture ingress, or component age — it may misread a perfectly good thermistor and log A08. PCB failure is less common than sensor or wiring faults but should be considered if thermistor resistance and wiring both test within specification.
How to fix it
- Check your gas supply is working DIY safe
Before assuming a boiler fault, confirm that other gas appliances in your home — such as a gas hob or gas fire — are working normally. If you have no gas at all, contact your gas supplier rather than investigating the boiler further.
- Reset the boiler once or twice DIY safe
Press and hold the reset button (refer to your boiler's manual for its location) for the required duration, typically 3–5 seconds. Wait for the boiler to attempt a restart. If it relights and runs without returning to A08 within a short time, monitor it closely. Do not reset more than two or three times in quick succession — repeated lockouts indicate a genuine fault that needs professional attention.
- Check and top up system pressure if low DIY safe
Look at the pressure gauge on the boiler. For most Vokera models the normal cold operating range is 1.0–1.5 bar. If pressure is below 0.8 bar, use the filling loop (the braided or plastic valve connecting the cold mains to the heating circuit) to slowly top up until the gauge reads around 1.2 bar, then close the filling loop valve. Low pressure does not directly cause A08 but can contribute to poor circulation and thermistor differential faults.
- Call a Gas Safe registered engineer to test the return NTC thermistor Gas Safe engineer
An engineer will disconnect the return thermistor and measure its resistance with a calibrated multimeter at a known water temperature, comparing the reading against the manufacturer's resistance-temperature table (approximately 12 kΩ at 20°C). A reading that is open-circuit, short-circuit, or significantly off-curve confirms the thermistor has failed and needs replacing. This is the most common and least expensive fix for A08.
- Engineer to inspect wiring loom and connections Gas Safe engineer
With the thermistor disconnected, the engineer will check the wiring harness from the thermistor plug back to the PCB for continuity, correct resistance, and signs of water damage or corrosion. Loose connectors will be re-seated or replaced; corroded terminals or damaged cable sections will be repaired or renewed.
- Engineer to assess the circulation pump Gas Safe engineer
The engineer will check that the pump is running at the correct speed and producing adequate flow. A pump that is noisy, slow, or seized will be removed for inspection. If blockage is the issue, it can sometimes be cleared and the system dosed with a quality central heating inhibitor to prevent recurrence; a magnetic filter may also be fitted to catch residual sludge. If the pump is beyond cleaning, replacement is the recommended course of action.
- Engineer to test and, if necessary, replace the PCB Gas Safe engineer
If thermistor resistance, wiring continuity, and pump performance all test within specification, the PCB's analogue input circuit is the likely culprit. The engineer will carry out voltage and signal checks on the PCB. If it is confirmed faulty, a replacement PCB will be sourced for the specific Vokera model and fitted, after which the boiler will be commissioned and the fault code cleared.
Parts you may need
- Return NTC thermistor (Vokera compatible) · from £20
- Wiring loom / thermistor harness · from £35
- Circulation pump (e.g. Grundfos UP 15-50 or equivalent) · from £85
- Replacement PCB (model-specific) · from £180
- Central heating inhibitor (1 litre) · from £12
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £55
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix an A08 fault myself?
Not safely, no. While you can check your gas supply, reset the boiler once or twice, and top up system pressure yourself, all internal boiler work — including testing and replacing the thermistor, inspecting the wiring, servicing the pump, or touching the PCB — must be done by a Gas Safe registered engineer. Working on a gas appliance without the correct registration is both dangerous and illegal in the UK.
How much does it cost to fix an A08 fault?
If the return NTC thermistor itself is the problem, the total cost including parts and an engineer's labour is typically around £100–£150 — the thermistor part costs roughly £20, and the rest is labour. If the pump needs cleaning or replacement, expect £150–£350 depending on whether a new pump is required. PCB replacement is less common but can push costs to £350–£500 or more depending on the model, which is why it tends to be the last thing an engineer investigates rather than the first.
Why does my Vokera boiler keep showing A08 after I reset it?
A single successful reset followed by normal operation can occasionally be a one-off glitch, but if A08 returns within a short time or comes back repeatedly, the underlying fault — most likely a failing thermistor, a loose wiring connection, or a struggling pump — has not been resolved. Do not keep resetting the boiler more than two or three times; each lockout is there to protect the boiler and your home. Book a Gas Safe engineer to diagnose the root cause.
Is A08 an Andrews boiler code or a Vokera code?
Strictly speaking, A08 is a Vokera fault code. Andrews Water Heaters is a separate brand focused on commercial hot water products, and does not use the same fault-code series. However, because Vokera and certain Andrews commercial heating products share engineering heritage, the A08 code is sometimes searched for in connection with Andrews boilers. If you see A08 on your boiler display, your appliance is almost certainly a Vokera model, and all of the diagnostic information on this page applies.