Worcester Greenstar 12i System fault Heat exchanger overheat: lockout on overheat, no bangs or gurgles, just cuts out, pressure…
Published 3 June 2026 · Generated by BoilerTag's diagnostic AI from an engineer's real-world submission
One of 13 real-world fault checklists on BoilerTag, across 5 boiler makes.
Most likely cause
The Worcester Greenstar 12i System displaying a "Heat exchanger overheat" fault code indicates the boiler's safety systems have activated due to excessively high temperatures within the heat exchanger. This is a critical safety lockout designed to prevent damage to the appliance. Common causes include poor water circulation, a blocked heat exchanger (often due to limescale or sludge), airlocks, or a faulty pump. The intermittent nature suggests a developing issue rather than a sudden component failure. The pressure reading of 1.5 bar is within the normal operating range, so low pressure is unlikely to be the primary cause.
The engineer's report
A gas engineer submitted this case through BoilerTag's free fault diagnostic tool, working on a Worcester Greenstar 12i System displaying fault code Heat exchanger overheat. They described the behaviour they were seeing on site as follows:
“lockout on overheat, no bangs or gurgles, just cuts out, pressure gauge reads 1.5 bar, no heating or hw & fault code displays on screen. been intermittent for a couple of weeks”
Before requesting the AI diagnostic, the engineer had already worked through the following checks:
“checked system pressure, looked at pump for blockages, bled all rads, checked wiring to safety stats & pump harness”
BoilerTag's diagnostic AI then cross-checked manufacturer documentation and common trade knowledge to produce the prioritised checklist below.
Diagnostic checklist
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Commonly failed parts
- System pump (if seized or faulty)
- Flow/Return temperature sensors
- Diverter valve motor/cartridge (if applicable)
- Heat exchanger (if heavily scaled or blocked beyond cleaning)
More Worcester fault codes
Sources
The AI cross-checked the following manufacturer and trade sources while producing this checklist.
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