Error D.33

Safety precautions:

- If you are not a qualified boiler engineer, do not open or work inside the boiler. Internal parts include live electrical connections and gas components. Contact a Gas Safe engineer for repairs.

- Before any inspection that requires access to internal components, isolate the electrical supply at the mains and, where relevant, the gas supply. If you only look at external terminals or the flue outside, isolation is not usually required but take care around live appliances.

- If the boiler shows a separate fault code indicating loss of flame or unsafe conditions (e.g. F.27, F.32, F.20 or F.64), stop using the boiler and call a professional immediately.

Initial homeowner checks you can do:

1) Note and photograph the display showing D.33 and any other accompanying F (fault) codes and the boiler model/serial. This information is helpful to the engineer.

2) Perform a simple reset: press the boiler reset button once and observe whether the D.33 display returns or whether any F codes appear. Do not repeatedly reset a boiler that keeps faulting.

3) Check the flue terminal and air intake outside the property for obvious blockages (bird nests, snow, debris, vegetation). Clear any obstruction safely from ground level if possible.

4) Make sure vents, airbrick(s) and combustion air openings to the room where the boiler sits are unobstructed.

5) Listen to the boiler when it starts: an unusually noisy, rough or non-spinning fan sound can indicate a fan problem.

6) Check for other symptoms: repeated lockouts, failure to ignite, or loss of heating/hot water. If you see F.32 (fan fault) or persistent lockouts, do not attempt internal repairs.

Specific diagnostic steps (for an engineer or to prepare for a call-out):

1) Enter the boiler diagnostic menu (consult the user/installer manual for exact button sequence for your model) and record related diagnostic values: D.33 (fan speed target) and any diagnostic channel that shows actual fan speed or RPM. Also record recent fault history.

2) Compare the target fan speed with the actual fan speed reading (if available). A large discrepancy suggests the fan is not reaching commanded speed.

3) Visually inspect the fan wiring connector and cable for signs of damage, corrosion or loose connections (ONLY if qualified and after isolating mains). Check the connector between the fan and the PCB and the cable for chafing.

4) Check the flue and condensate path for blockages that could increase back-pressure or stall the fan. Confirm the flue gas path is intact and there is no evidence of condensation build-up blocking the flue.

5) Measure voltage/control signal to the fan motor during a start attempt (qualified engineer task). If the control signal is correct but the fan does not reach speed, the fan motor or impeller is likely faulty. If the control signal is absent or incorrect, suspect the PCB or control electronics.

6) Check for related PCB errors or intermittent wiring faults. Replace/repair the fan assembly or the PCB only as a qualified engineer would recommend.

When to call a professional and what to tell them:

- Call a Gas Safe registered heating engineer if D.33 is persistent, if the boiler locks out, if you hear unusual fan noise, or if there is any accompanying fault code such as F.32, F.64, F.20, F.27 or repeated ignition failures.

- Provide the engineer with the boiler make/model/serial, photos of the display showing D.33 and any F codes, and a description of what you tried (reset, flue check, symptoms). This speeds up diagnosis.

Final notes:

- D.33 is primarily diagnostic information. It can point to a simple airflow obstruction or to a failing fan or control electronics. Because the fan and its controls are part of the combustion and safety chain, internal repairs must be performed by a Gas Safe registered engineer. Do not attempt to replace or test the fan motor, PCB or gas components yourself.