Overview
E9 210 on a Worcester Bosch GB162 means the boiler has detected an over-temperature condition on the flue gas temperature sensor and the sensor circuit is indicating an 'open' (no signal) or a temperature reading above the safe limit. The boiler’s safety logic has locked the unit out to prevent operation while the control thinks flue temperatures are too high or the sensor has failed. This is a protective shutdown — the appliance will not operate until the fault is cleared. There are a few common reasons this code appears: the flue gas sensor itself can be faulty or its wiring/connector can be open; the safety temperature limiter has tripped because the primary flow has overheated (often due to poor circulation from a seized/blocked pump, blocked heat exchanger, or heavy limescale/sludge); a blocked or obstructed flue causing flue gases to remain too hot; or, less commonly, a PCB/wiring fault producing an incorrect reading. Severity is medium-to-high: the boiler has shut down to prevent damage or unsafe temperatures, so the issue needs prompt attention. Some simple checks are safe for a homeowner, but diagnosis and internal repairs must be done by a Gas Safe registered engineer — working on gas appliances or internal electronics without certification is dangerous and illegal.
Possible Cause: The temperature measured by the flue gas sensor is too high and is open.
Troubleshooting Steps
Safety precautions:
1) If you smell gas or suspect a leak, leave the building immediately and call the gas emergency number. Do not attempt to light the boiler.
2) If you will perform any external checks, switch the boiler off at its front panel and at the electrical isolator. Do not open the boiler casing or attempt internal electrical or gas work. Any internal inspection or repair must be carried out by a Gas Safe registered engineer.
3) Avoid repeated automatic resets: repeatedly resetting a fault without diagnosis can mask a developing fault and cause further damage.
Initial homeowner checks (safe, do these first):
1) Record the exact fault code and any additional cause codes or messages shown. Note when it happened and what the boiler was doing.
2) Try a single controlled reset: switch boiler off, wait 30 seconds, switch back on and see if the code returns. If the fault returns immediately or within a short time, stop further resets.
3) Check the flue terminal outside for obvious blockages (bird nest, debris, snow/ice). If the flue looks blocked, do not operate the boiler and call an engineer.
4) Check for frozen or blocked condensate pipe (common in cold weather). If frozen, carefully thaw with warm (not boiling) water. If you suspect a condensate blockage, call an engineer.
5) Check the heating system pressure on the boiler gauge. Low pressure (typically below about 1.0 bar) can cause circulation issues; top up only per the boiler handbook if you know how and it’s safe to do so.
6) Listen for pump noise or rattling when the boiler tries to run (if it briefly runs). If radiators remain cold and the pump appears silent or noisy, circulation may be compromised.
7) Look for visible signs of overheating or leaks around the boiler or pipework (discolouration, steam, burn marks). If present, isolate the boiler and call an engineer immediately.
If the simple checks do not clear the fault, further diagnostic/fix steps for an engineer (do not attempt yourself):
1) The engineer will read the full fault log and cause codes, then safely remove the boiler cover to inspect sensor wiring and connectors for open circuits, corrosion or water damage.
2) They will test the flue gas temperature sensor (NTC) resistance values at ambient and when warmed, check its wiring continuity and the connector to the control board. If the sensor is open or out of range it will be replaced.
3) The safety temperature limiter (high limit) and any related safety NTCs will be checked for continuity and correct operation; faulty limiters may need replacement.
4) The engineer will confirm pump operation and flow: test the circulating pump for seizure, air lock, or blockage; they may free or replace the pump if it is faulty and will purge/bleed the system if needed.
5) The heat exchanger and system will be checked for limescale or sludge buildup; if scale or blockage is found they may recommend a system flush, descaling or component replacement (heat exchanger or parts) depending on severity.
6) The flue draught and terminal will be tested for restriction; the engineer may measure actual flue gas temperatures during operation to verify whether overheating is real or caused by sensor/PCB error.
7) The PCB and control electronics will be tested; if wiring and sensors check out but the fault persists, the control board or associated burner control module may be replaced.
Final notes and when to call a professional:
1) E9 210 is not a routine reset fault — if the fault remains after the basic homeowner checks, stop and contact a Gas Safe registered engineer. Internal electrical, sensor, pump and heat-exchanger work must be done by a qualified professional.
2) Tell the engineer you have E9 210 and provide any additional cause codes and the observations you made (flue blockage, frozen condensate, pump noise, pressure reading, etc.).
3) If your boiler is under warranty or a service contract, contact the installer/manufacturer before commissioning repairs. Avoid DIY internal repairs; unsafe or uncertified work can invalidate warranties and is illegal for gas work.
Call a Gas Safe registered heating engineer promptly to diagnose and repair the fault safely.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.