Error E20

Safety precautions:

1) If you smell gas, evacuate and call your gas emergency number immediately; do not attempt any work. 2) Before touching the boiler, switch the electrical supply off at the isolator and allow the boiler to cool. 3) Do not attempt gas-carrying or sealed combustion component repairs unless you are a Gas Safe registered engineer. 4) Wear gloves/eye protection if you access internal components and be aware that parts may be hot.

Initial homeowner checks you can do (no tools or basic tools only):

1) Try a reset: use the boiler reset procedure (on many Baxi models turn the selector to R for about 5 seconds or hold the reset button for 10 seconds), wait 2–3 minutes and see if the error clears. 2) Check boiler pressure: read the pressure gauge; correct pressure is generally around 1–1.5 bar when cold. If very low, top up the system to the correct pressure following the boiler manual. 3) Look for obvious issues: any visible water leaks, corrosion, damp or signs of water ingress around the boiler, damaged sensor leads or loose connector clips. 4) Ensure the heating demand is present (thermostat set above room temp) and bleed radiators if radiators are cold or system feels air-locked. 5) Note if the fault is intermittent or follows a power cut or storm (which could indicate electrical damage).

Specific diagnostic steps (requires basic electrical tools and competence):

1) Isolate the boiler electrically and remove the service panel to access the sensor and wiring (only if you are competent and have isolated power). 2) Visually inspect the NTC thermistor and connector at the flow pipe for corrosion, broken wires, or a loose plug. Re-seat any plug connections. 3) If you have a multimeter set to resistance (ohms), carefully disconnect the sensor from the wiring loom and measure its resistance at known room temperature. Compare the reading with the manufacturer specification in the manual. Many common NTC flow sensors read in the order of ~10 kΩ at ~25°C (values vary by part—use the boiler manual or the replacement part spec). An open circuit (infinite resistance) or wildly out-of-spec value indicates a faulty sensor. 4) Check continuity between the sensor connector and the PCB connector (wiring continuity) and look for intermittent changes when gently flexing the cable (intermittent wiring faults are common). 5) If the sensor is confirmed faulty, replace it with the correct Baxi part number. Ensure the replacement matches the original and is installed in the same position (usually clipped into the flow pipe housing). 6) After replacement or wiring repair, reassemble, restore power, reset the boiler and run the system to confirm normal operation and that E20 does not return.

When to call a professional:

1) If the reset and basic checks do not clear the fault, or if you are not comfortable isolating electrical supply, opening the boiler, or using a multimeter, call a Gas Safe registered heating engineer. 2) If the sensor reads within spec but the E20 persists, the boiler PCB or internal electronics may be at fault—this requires specialist diagnostics. 3) Any repair that affects gas plumbing, sealed components or safety devices must be done by a qualified engineer. 4) If you replace the sensor and the fault returns, stop using the appliance and arrange a technician; repeated lockouts indicate a deeper fault.

After any repair, have the engineer run the boiler through heating and hot water cycles, confirm correct operation and clear the fault history. If the problem was caused by debris, low pressure or system issues, consider routine system maintenance (inhibitor, magnetic filter, flush) to reduce recurrence.