Worcester Bosch A3 317 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch A3 317 fault code mean?
The A3 317 code indicates that the boiler's control system has detected a short circuit in the flue gas temperature sensor circuit. This sensor keeps a continuous watch on the temperature of exhaust gases as they leave the heat exchanger and travel through the flue. When the electronics register an abnormal resistance consistent with a short circuit — rather than the expected sensor reading — the boiler shuts itself down as a precaution and displays A3 317. Because the flue gas sensor is a safety-critical component, the boiler will not restart until the underlying fault has been resolved. This code sits within Worcester Bosch's sensor fault family, which includes codes such as E2, D1, and A7; however A3 317 points specifically to the flue gas sensor contacts rather than flow or return temperature sensors.
General guidance only — not a substitute for professional advice. 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
- Short circuit in the flue gas sensor contacts Common
The most directly stated cause for this code. A short circuit at the sensor's electrical contacts causes the PCB to receive an out-of-range signal, triggering the lockout. This can occur when contacts become corroded, contaminated, or physically damaged, allowing current to flow along an unintended path.
- Damaged or degraded sensor wiring Common
The cable run between the flue gas sensor and the PCB passes through a warm environment inside the boiler casing. Over time, insulation can harden, crack, or become pinched against metal edges, leading to a short between conductors. Rodent damage in boilers installed in lofts or outbuildings is an occasional culprit.
- Failed flue gas temperature sensor Sometimes
The sensor element itself can degrade or fail outright, producing electrical characteristics that the PCB interprets as a short circuit. A sensor that has been exposed to sustained high flue temperatures over many years is particularly prone to this.
- Blocked or restricted flue Sometimes
A partially obstructed flue can cause exhaust gases to back up, subjecting the sensor to abnormally high temperatures. This thermal stress can accelerate sensor degradation and, in some cases, trigger the A3 317 fault indirectly.
- PCB fault Rare
If the PCB's input circuitry for the flue gas sensor channel develops a fault, it may misinterpret a perfectly healthy sensor as a short circuit. PCB issues are less frequent but must be considered when sensor and wiring checks come back clear.
How to fix it
- Check for obvious external issues first DIY safe
Before calling an engineer, confirm that your gas supply is working (check other gas appliances such as a hob), that the boiler's system pressure sits between 1.0 and 1.5 bar on the pressure gauge, and that the condensate pipe (typically a white plastic pipe exiting through an outside wall) is not frozen solid. A frozen condensate pipe can trigger multiple fault codes; if it is iced up, you can carefully thaw it with warm — not boiling — water poured over the external section.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (or rotate the dial to the reset position, depending on your model) for around three seconds. If the boiler fires up and runs normally, monitor it over the next few hours. If A3 317 returns, do not keep resetting — repeated resets without fixing the root cause can mask a worsening fault. Limit yourself to two or three reset attempts in total.
- Inspect the flue terminal externally DIY safe
From outside your property, visually check that the flue outlet is not blocked by debris, bird nesting material, or ice. Do not attempt to reach or dismantle any part of the flue — this is a visual check only. If you can see an obvious blockage, report it to your engineer.
- Engineer: inspect the sensor wiring and contacts Gas Safe engineer
A Gas Safe registered engineer will remove the boiler casing and systematically check the wiring loom between the flue gas sensor and the PCB. They will look for signs of chafing, melting, corrosion at connectors, and any pinch points. Resistance across the sensor contacts will be measured with a multimeter and compared against the manufacturer's specification.
- Engineer: test and replace the flue gas temperature sensor if required Gas Safe engineer
If the sensor's resistance reading falls outside the acceptable range, or if the contacts are visibly burnt or corroded, the sensor will be replaced. Using a genuine Worcester Bosch replacement part is strongly recommended to maintain warranty validity and ensure correct calibration.
- Engineer: inspect and clear the flue system Gas Safe engineer
The engineer will verify that the flue is unobstructed end to end, that joints are sealed, and that exhaust gases are venting correctly. An obstructed flue is a carbon monoxide risk and must be resolved regardless of whether it is the primary cause of A3 317.
- Engineer: assess the PCB if sensor and wiring checks are clear Gas Safe engineer
If no fault is found in the sensor or its wiring, the engineer will investigate the PCB. A damaged input channel on the PCB can mimic a sensor short circuit. PCB replacement is a more involved repair and may prompt a conversation about the overall age and condition of the boiler.
- Reset the boiler after repair and verify normal operation Gas Safe engineer
Once the engineer has resolved the root cause, they will reset the boiler, run it through a full heating and hot water cycle, and confirm that A3 317 does not reappear. They should also check flue gas combustion readings to confirm safe operation before leaving.
Parts you may need
- Flue gas temperature sensor (Worcester Bosch OEM) · from £45
- Sensor wiring loom / harness · from £35
- PCB (printed circuit 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 £150–£350, depending on the underlying cause.
Frequently asked questions
Is the A3 317 fault code dangerous — can I keep using the boiler?
You should not attempt to override or continuously reset the boiler to keep it running. The flue gas sensor is a safety device; if it is genuinely malfunctioning, the boiler could be operating with unmeasured — and potentially dangerously high — flue gas temperatures. Leave the boiler locked out and arrange for a Gas Safe engineer to attend promptly, especially if you have no alternative heating source.
How much does it typically cost to fix a Worcester Bosch A3 317 fault?
For most households, the repair involves diagnosing the fault, replacing the flue gas temperature sensor or repairing wiring, and resetting the boiler. This typically comes to £150–£350 including parts and labour. If the PCB also needs replacing, costs can rise to £400–£600 or beyond, which is why engineers always test the sensor and wiring first before recommending the more expensive component. Getting two or three quotes is worthwhile for any repair above £250.
Can I reset a Worcester Bosch A3 317 fault myself?
You can attempt a reset once or twice to see whether the fault clears — sometimes a transient electrical glitch can trigger a code. Press and hold the reset button for three seconds. However, if A3 317 returns after resetting, repeated resets will not fix the underlying problem and could delay getting the boiler properly diagnosed. Limit yourself to two or three attempts and then call a Gas Safe engineer.
Will annual servicing help prevent the A3 317 fault?
Yes. During a boiler service, the engineer checks sensor readings, inspects wiring for signs of wear, and clears any debris from the flue. Catching a degrading sensor or a chafed wire before it causes a lockout is far less disruptive and usually cheaper than an emergency call-out. Annual servicing typically costs £90–£140 and is one of the most cost-effective ways to avoid unexpected breakdowns.