Overview
This error (A3 317) indicates the boiler’s flue gas temperature sensor circuit is shorted. The flue gas temperature sensor (an NTC thermistor or similar) monitors the temperature of the flue gases to protect the boiler from overheating or unsafe flue conditions. A short circuit means the sensor wiring or contacts are electrically bridged (low resistance), or the sensor itself has failed internally, so the control unit is not getting a valid temperature signal. Severity: This is a safety-related fault because the boiler relies on the flue sensor to detect overheating or blocked flue conditions. When the control detects a short it will usually lock out or prevent safe operation to protect the appliance and occupants. While some simple checks are possible for a competent homeowner (visual inspection and reseating connectors), diagnosis beyond that and any repairs involving gas, flue components or internal electrics must be carried out by a Gas Safe registered engineer. Do not run the boiler if you suspect a flue or wiring fault; isolate power and gas if you smell gas or see damage. DIY or professional: You can safely perform basic visual checks (power off) and try a single reset after those checks, but you should call a Gas Safe professional for testing, sensor replacement, wiring repair or control-board work. The boiler may need a new sensor, wiring repair or replacement of a burner control module depending on the fault details found by an engineer.
Possible Cause: Short circuit in the flue gas temperature sensor contacts.
Troubleshooting Steps
Safety precautions:
1. If you smell gas, evacuate the property, do not operate electrical switches or the boiler, and call the gas emergency number immediately.
2. Turn off the boiler at the mains isolator and switch off the gas supply before opening the boiler cover for any inspection.
3. Allow the boiler to cool before touching components. Work inside a boiler can be hazardous; if you are not confident, stop and call a Gas Safe engineer.
4. Do not remove or disturb the flue terminal, or attempt repairs on the flue itself — this must be done by a qualified engineer.
Initial checks a homeowner can do (power off where indicated):
1. Note down the exact error code and any other codes displayed. Take a photo of the display for the engineer.
2. Try a single reset: with the boiler powered, press and hold the reset button for about 5 seconds as per the manual. Wait 2–3 minutes for the boiler to attempt restart. If the fault clears and the boiler runs normally, monitor for recurrence. Do not repeatedly reset the boiler.
3. With the boiler isolated from mains power and gas, remove the outer case only if you are comfortable doing so and it does not invalidate any warranty. Visually inspect wiring to the flue gas sensor and the connector at the control board for loose plugs, corrosion, water ingress, burn marks or obvious damage.
4. Check for signs of water in or around the boiler or wiring looms that could cause shorting (condensate leaks, damp). If you find water ingress, keep the boiler off and call a qualified engineer.
Specific diagnostic steps (only if competent and with power/gas off):
1. Locate the flue gas temperature sensor wiring harness. On the GB162 this sensor is typically mounted in or near the flue path and connects to the main control PCB. If unsure, do not probe further — call an engineer.
2. Carefully unplug and re-seat the sensor connector both at the sensor and at the PCB. Look for bent pins, corrosion, or damaged insulation. Repair only minor insulation nicks with proper sleeving; do not attempt soldering or extensive wiring repairs if you are not qualified.
3. If you have a multimeter and basic skills: with power off, measure resistance between the sensor wires. You should not read near 0 ohms (which indicates a short) and you should not read infinite (open circuit). Different boilers/sensors have different nominal resistances, so absolute values vary — the key is identifying a clear short (very low resistance) or open circuit. If you detect a definite short or open, stop and call a Gas Safe engineer.
4. After re-seating connectors and confirming no visible damage, restore power and try one reset. If the fault returns immediately or within a short time, the problem persists.
When to call a professional and what to tell them:
1. Call a Gas Safe registered engineer if the error persists after basic checks, if you found damaged wiring, water ingress, or if you are not comfortable performing the checks. This fault can require sensor replacement, wiring repairs, or electronic control replacement — all tasks for a qualified engineer.
2. When contacting the engineer provide: boiler model (Worcester Bosch GB162), exact error code (A3 317), description of any visual findings (burnt connector, damp, detached plug), and whether the boiler reset allowed a temporary restart.
3. Do not attempt to operate the boiler if the engineer advises it must remain off. Follow their guidance on isolating the appliance until repaired.
Final notes:
- A single reset is acceptable after doing the basic visual checks, but repeated resets are not recommended. If the fault returns, leave the boiler off and call a Gas Safe engineer.
- Do not attempt to replace the flue sensor, alter flue components, or work on gas connections yourself. These are controlled, safety-critical tasks requiring a qualified engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.