E9 on a Worcester Bosch means the safety temperature circuit has tripped. The boiler got hotter than it should have and the overheat protection shut it down.

This is not a code to reset and forget. Something made the boiler overheat, and until that is found it will keep happening.

What E9 actually means

Every boiler has an overheat cut-out as a last line of defence. When the flow temperature climbs beyond the safe limit, that circuit opens and the boiler locks out.

The fault is almost never the cut-out itself. The cut-out is doing its job. The question is what allowed the temperature to run away in the first place, and that is nearly always about water not moving through the boiler fast enough.

What you can safely check yourself

There is very little here that is safe or useful for a homeowner to do.

You can check that the system pressure is in the normal range and that radiator valves have not been closed off, because poor circulation is the usual underlying cause. Beyond that, this one needs an engineer.

When to stop, and why

What causes it beyond the simple checks

A failed or seized pump, air locked circulation, sludge or scale restricting the heat exchanger, a closed or partially closed valve, a faulty flow thermistor giving a false reading, or a wiring fault in the safety circuit. On hard water areas with an older combi, scale in the heat exchanger is a frequent finding.

How Vantage can help

We are Gas Safe registered under 956889 and cover Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day. Call us on 01462 506374. See also boiler repair and boiler servicing.

Frequently asked questions

Is E9 dangerous?

The lockout is the boiler being safe. The underlying cause is what needs attention, and it should not be run until that is sorted.

Why does E9 come back after a reset?

Because resetting clears the lockout, not the cause. If circulation is poor, it will overheat again on the next cycle.

Could it just be a faulty sensor?

It can be, and a faulty thermistor is on the list. But it cannot be assumed without testing, which is why this is engineer work.

Related fault codes

Other Worcester Bosch codes: EA · A1 · E2 · C6 · all Worcester Bosch codes. The same family of fault on other makes: Vaillant F20 · Baxi E110. See also power flushing and boiler making a banging noise.

In Worcester Bosch’s own words

Worcester Bosch use different code sets across their ranges and generations. The definition above is taken from the Greenstar CDi Classic service booklet. If your boiler is a different model and the code behaves differently, check the manual for your appliance or ring us and we will tell you what it means on your unit.

When to call an engineer

If the checks above have not cleared it, the next steps are engineer work and the boiler should not be reset repeatedly in the meantime. Call Vantage Plumbing and Heating on 01462 506374. We are Gas Safe registered under 956889, the line is answered by a person at any hour of any day, and you get a price before anyone starts.

Need help today?

Call and speak to a person, any hour, any day of the year. Get a fixed price quote or call 01462 506374.