Error 151

Safety first

1) Do not work on the boiler with the gas turned on or without isolating mains power if you are not a qualified engineer. There are live voltages and gas components inside. If you are unsure, call a Gas Safe engineer.

2) If you smell gas, evacuate and contact the gas emergency number immediately.

Initial homeowner checks (safe, simple steps)

1) Note exactly when 151 appears: at initial start, during ignition, or after running for a while. That detail helps diagnosis.

2) Try a single reset: switch the boiler off at the fused spur or mains for 30 seconds and switch back on, then attempt to restart. Do not repeatedly reset many times if the error persists.

3) Check the flue outlet and air intake for obvious blockages (bird nests, debris, snow/ice). A blocked flue or obstructed fan intake can upset fan loading.

4) Check condensate pipe (if fitted) for freezing/ice if the fault occurs in cold weather — thaw if frozen and then reset.

5) Check boiler system pressure on the analogue/digital gauge — very low or abnormal pressure can produce other faults that complicate operation. Re-pressurise only if you know how and it is safe to do so.

Diagnostic steps an engineer or competent person will perform (do not attempt unless qualified)

1) Visually inspect fan and fan housing for debris or mechanical obstruction. Remove obstruction and test.

2) Check all wiring and connectors between the fan and PCB for secure connections, signs of corrosion, burn marks or broken wires. Re-seat connectors.

3) Use a meter to check for 230 VAC supply to the fan during start-up. If 230 VAC is present to the fan but it does not run, the fan motor is likely faulty and should be replaced.

4) If fan will not run under PWM control, an engineer may disconnect the PWM control lead (or remove PWM input) to allow the fan to run full speed on mains; if the fan runs at full speed with PWM removed, the PCB/PWM output is likely faulting. If fan still does not run on full mains, replace the fan.

5) Check the tachometer (tacho) signal from the fan to the PCB: the control unit expects a pulse signal indicating RPM. If the tacho signal is absent or erratic, the PCB will register speed out of range. Measure the tacho output (oscilloscope or suitable meter) to confirm presence and frequency. Replace fan if tacho output is missing and fan is otherwise OK.

6) Verify fan resistances and wiring continuity as per manufacturer service data. Replace fan if resistances out of spec.

7) Check for related faults that can cause timing/sequence issues: flow switch operation (some iR models will show 151 during ignition if the flow switch is open), pump starting and hydraulic flow. Confirm the flow switch closes correctly during pump run and that pump impeller runs freely.

8) Inspect the flue and heat-exchanger for recirculation of products or restriction which can load the fan and produce wrong RPM readings. Clean as required.

9) Check PCB/LMU operation and parameters via service menu (an engineer): confirm parameter P9 and boiler selection are correct and that the controller didn’t remain in production/test mode. Confirm the controller is issuing correct PWM and reading tacho feedback. If the fan and wiring test good but PWM/tacho behaviour is incorrect, replace the PCB/LMU.

10) After component replacement or repair, clear the fault and burn a controlled restart, watching for stable fan RPM within tolerance for the required monitoring period.

When to call a professional

- If the simple homeowner checks (reset, clear flue, check pressure/condensate) do not clear the fault call a Gas Safe/qualified heating engineer. Work on the fan electrical supply, PCB, gas components, or internal wiring must be carried out by a qualified technician.

- If the fault recurs, or the engineer finds the PWM/tacho/PCB ambiguous, insist on systematic measurements (230V supply checks, PWM presence, tacho pulses) and replacement of the failed part (fan or PCB) only after those checks.

Notes

- Fault 151 is commonly caused by either the PCB or the fan; many service records show the PCB as the frequent failure point, but the sequence above is intended to prove which part is defective before replacement.

- Do not repeatedly reset the boiler to force it to run; that can cause unsafe operation and mask the true fault.