Verified boiler fault code
Vaillant F.23 Fault Code (ecoTEC plus/pro (manual models))
Water shortage; the temperature difference between flow and return NTC is too large.
Safety disclaimer
Fault code summary
- Manufacturer
- Vaillant
- Fault code
- F.23
- Fault type
- Low pressure, Circulation, Temperature sensor
- Severity
- high
- Reset
- Not specified
- Applicable models
- ecoTEC plus 612, ecoTEC plus 615, ecoTEC plus 618, ecoTEC plus 624, ecoTEC plus 630, ecoTEC plus 637, ecoTEC plus 824, ecoTEC plus 831, ecoTEC plus 837, ecoTEC pro 28
Quick answer
Water shortage; the temperature difference between flow and return NTC is too large.
What does this fault code mean?
Water shortage; the temperature difference between flow and return NTC is too large.
Common causes
- Pump blocked or defective.
- Pump output too low.
- Flow and return NTCs interchanged.
Symptoms
This information was not provided in the verified manufacturer source.
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
- Assess pump delivery, flow/return sensor identity and the circulation circuit.
Applicable boiler models
Related fault codes
- F.22 — ecoTEC plus/pro (manual models)
Dry fire.
Read fault code - F.24 — ecoTEC plus/pro (manual models)
Water shortage; temperature rise too quick.
Read fault code - F.25 — ecoTEC plus/pro (manual models)
Interruption in the compact thermal module cable harness.
Read fault code - F.28 — ecoTEC plus/pro (manual models)
The appliance does not start: attempts to ignite during start failed.
Read fault code - F.29 — ecoTEC plus/pro (manual models)
The flame goes off during operation and subsequent ignition attempts fail.
Read fault code - F.32 — ecoTEC plus/pro (manual models)
Fan speed variation.
Read fault code - F.49 — ecoTEC plus/pro (manual models)
eBUS undervoltage.
Read fault code - F.61 — ecoTEC plus/pro (manual models)
Gas-valve control defective.
Read fault code
Manufacturer documentation
Instructions for installation and servicing ecoTECF.23 | Water shortage, temperature difference between flow and return NTC too large | Pump blocked or defective, pump output too low, flow and return NTC interchanged
Last verified
2026-09-12
Frequently asked questions
What does Vaillant F.23 mean?
Water shortage; the temperature difference between flow and return NTC is too large.