Verified boiler fault code
Worcester Bosch U1 Fault Code (Greenstar FS CDi ErP Regular)
Boiler flow temperature is too high; the boiler will shut down until it has cooled down.
Safety disclaimer
Fault code summary
- Manufacturer
- Worcester Bosch
- Fault code
- U1
- Fault type
- Overheating, Central heating
- Severity
- high
- Reset
- Not specified
- Applicable models
- Greenstar FS 30CDi ErP Regular, Greenstar FS 42CDi ErP Regular
Quick answer
Boiler flow temperature is too high; the boiler will shut down until it has cooled down.
What does this fault code mean?
Boiler flow temperature is too high; the boiler will shut down until it has cooled down.
Common causes
- Boiler flow temperature is too high.
Symptoms
- The boiler shuts down until it has cooled.
What can the homeowner safely check?
This information was not provided in the verified manufacturer source.
Boiler reset information
Reset information was not provided in the verified manufacturer source. Do not repeatedly reset an unresolved fault.
When should I call an engineer?
Contact a suitably qualified heating engineer when the fault persists, the boiler locks out, or the manufacturer instructions do not resolve it. For gas appliance work in the UK, use a Gas Safe registered engineer.
Engineer diagnosis
- Investigate the high flow temperature and the heating circulation.
Applicable boiler models
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Code plug not detected.
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Fan speed too low.
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Condensate pump fault.
Read fault code - E2 — Greenstar FS CDi ErP Regular
CH flow NTC sensor defective.
Read fault code - E9 — Greenstar FS CDi ErP Regular
Safety temperature limiter in the CH flow has tripped.
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Flame not detected.
Read fault code
Manufacturer documentation
Greenstar FS 30CDi & 42CDi ErP Regular user instructionsU1 | Boiler flow temperature is too high, the boiler will shut down until it has cooled down
Last verified
2026-09-12
Frequently asked questions
What does Worcester Bosch U1 mean?
Boiler flow temperature is too high; the boiler will shut down until it has cooled down.