Error E33

Safety first

1) If you are not competent with electrical or boiler work, call a Gas Safe registered engineer. Never work on gas parts unless qualified.

2) Before opening the boiler, isolate electrical supply at the fuse or isolator and switch off the gas at the meter or appliance isolation valve if you will access internal gas-carrying parts. Allow boiler to cool and avoid scald risk.

3) Take photos of any wiring/connector positions before you disconnect anything.

Initial homeowner checks (safe, low-skill)

1) Press the boiler Reset button once. If the code clears and stays clear, monitor the boiler — intermittent faults can reappear.

2) Check for obvious signs: any visible water leaks around the boiler, corrosion or damp around sensor wiring or connectors, rodent damage to cables, or loose push-on connectors on the wiring loom.

3) Make sure the boiler has correct power and the fault is not caused by a general electrical supply issue.

If you are comfortable doing basic electrical checks (only with power isolated) — safe diagnostic steps

1) Isolate electrical supply to the boiler. Confirm it is dead with a correctly rated tester.

2) Remove the boiler front panel to access the control box and sensor wiring.

3) Locate the DHW sensor — typically a small probe or two-pin connector on the hot water/plate heat exchanger or the wiring loom labeled DHW/Hot Water.

4) Visually inspect the connector and wiring for corrosion, water ingress, melted insulation or broken wires. Reseat any loose connectors and ensure they click into place.

5) With power still isolated, use a multimeter on the ohms range to test the sensor wiring:

- Measure resistance between the two sensor wires. A normal thermistor will show a finite resistance (not zero, not infinite) that varies with temperature. A very low reading (near 0 ohms) indicates a short. A very high or infinite reading indicates an open circuit. (Do not rely on specific resistance values unless you have the sensor spec.)

- Measure resistance from each sensor wire to earth (boiler chassis). A very low reading means the sensor or cable is shorting to ground.

6) If the wiring at the sensor end shows normal resistance but the wiring at the control PCB shows a short, disconnect the sensor connector at the PCB and repeat the resistance checks at both ends to localise the fault. If the sensor is OK at the connector but the wiring to the PCB is shorted or damaged, the harness needs repair or replacement.

Possible fixes

1) If the fault is clearly a loose/corroded connector or contamination, clean and dry the connector, reseat it securely, restore power and test.

2) If the sensor measures as shorted (very low resistance) or open (infinite), replace the DHW sensor. Replacements should match the original sensor type and sealing requirements. Replacing the sensor typically requires isolating the boiler water, draining below the sensor or isolating pipework, and changing the probe and O-ring — if you are not competent, have an engineer do this.

3) If wiring is damaged (shorted to earth or between cores), the damaged section must be replaced or the entire harness repaired. Repairing wiring inside the boiler or replacing internal harnesses is best left to a Gas Safe engineer.

4) If the sensor and wiring are good but the control PCB still reports E33, the PCB or connector on the board may be faulty and will need professional diagnosis and replacement.

Re-commissioning after repair

1) After replacing or repairing, reassemble, restore electrical supply and gas (if isolated), refill and repressurise the heating system if you drained it, bleed any trapped air, and clear the fault via the boiler reset.

2) Verify normal DHW operation and that the E33 code does not return. Monitor for any leaks or recurring faults.

When to call a professional

1) Call a Gas Safe–registered engineer if you are unsure at any stage, if the issue persists after the simple checks, for sensor replacement that requires draining the boiler, for wiring repairs inside the appliance, or if you suspect a control PCB fault.

2) Do not attempt to run or bypass the boiler’s safety controls. Faults involving sensors or the control board should be repaired by a qualified engineer to ensure safe and correct operation.