Worcester Bosch E9 220 fault codewhat it means and how to fix it
The safety temperature sensor on the heat exchanger has failed electrically — 220 is a short circuit, 221 an open circuit. Nothing is overheating; the boiler has stopped because it can no longer prove that.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch E9 220
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What E9 220 means
The Compact family carries a dedicated safety sensor on the main heat exchanger, separate from the flow sensor, and E9 219 is that sensor reporting a genuine overheat. E9 220 and E9 221 are the same sensor failing to report at all.
Worcester's table reads "E9 220 — safety sensor short circuit — maximum temperature sensor on the main heat exchanger has failed" and "E9 221 — safety sensor open circuit — maximum temperature sensor on the main heat exchanger has not been recognised". Both are locking errors cleared with the reset button, and both stop the boiler for the same reason: the control will not run a burner whose safety limit it cannot measure. So the temperature that matters here is not high. What has happened is that a thermistor has gone to a resistance the board treats as impossible, either near zero — a short, often water in the connector — or effectively infinite, meaning the sensor or its lead is disconnected.
The remedy in the D1 return-sensor rows of the same table is a fair guide to what the engineer does: check the harness and connector, test the sensor against the published resistance curve, replace as needed. The Greenstar i table publishes E2 350 and E2 351 for the equivalent short and open circuit on the flow sensor but does not list 220 or 221, and the CDi Classic display shows E9 alone for any safety-circuit fault, so which range you have decides whether these numbers can appear.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Water in the sensor connector | A weeping joint above the heat exchanger drips onto the sensor plug, and a wet connector reads as a short circuit and gives 220. | Common |
| Sensor lead disconnected or broken | A plug knocked off during a service, or a lead chafed through where it passes the casing, reads as open circuit and gives 221. | Common |
| Failed thermistor | The sensor element itself drifts out of range with age and heat cycling. It is a cheap part, confirmed by comparing its resistance with Worcester's table. | Occasional |
| Control board input fault | If the sensor and lead test correctly at the plug and the fault remains, the board's sensor input has failed and the board is replaced. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£200
Time on site
40 minutes
Sensor or connector £90–£160 fitted. If a leak has caused it, add the cost of the leak repair.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Worcester Bosch Greenstar flow temperature sensor (flow NTC)
87145000870£15–£40Authorised Worcester parts distributor
Price checked 28 September 2026
Worcester Bosch Greenstar control board (PCB)
£250–£450Authorised Worcester parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Worcester Bosch fault codes
- CCUsually simple to fixThe outdoor temperature sensor for weather compensation is not being read — open circuit, short circuit or disconnected. The boiler keeps heating but loses its weather-adjusted control.Related: sensors and thermistors
- A5Usually simple to fixThe cylinder or heat bank temperature sensor has failed — open or short circuit. Heating carries on; the boiler just no longer knows how hot the stored water is, so hot water control suffers.Related: sensors and thermistors
- EA 227Stop — call an engineerThe boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Related: same brand
- CE 207Get it checkedSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Related: same brand
- E9 219Stop — call an engineerThe boiler has overheated and the safety thermostat has tripped. Water is not carrying heat away from the exchanger, and it should not be run until that is fixed.Related: same brand
- C6 224Get it checkedThe fan is not reaching the speed the boiler asked for, so combustion cannot be confirmed as safe and the burner has been prevented from firing.Related: same brand
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch E9 220
Is 220 more serious than 221?
No. They are the two ways the same sensor can fail — shorted or disconnected — and the fix is the same. Worcester list both as locking errors with the reset button. Quote whichever you see.
Why does the boiler stop if nothing is too hot?
Because it cannot tell. The safety sensor is the board's proof that the heat exchanger is within limits, and a boiler that lost that proof and kept firing would be running blind. Stopping is the correct response.
Is this the same as E9 219?
Same sensor, different message. 219 is the sensor working and reading too high — a real overheat with real causes in the pump and system. 220 and 221 are the sensor not working at all.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.