Error E20

Safety precautions:

1) If you are not competent with electrical or gas appliances, stop and call a Gas Safe registered engineer. Do not attempt internal repairs on the boiler if you are not qualified. 2) Before opening any covers isolate the mains electrical supply to the boiler (switch off at the fuse box) and, if you will isolate or drain any water or disturb gas fittings, ensure the gas supply is turned off and obtain professional help. 3) Never bypass safety devices or run the boiler with known sensor faults; that can create unsafe conditions.

Initial homeowner checks (safe, no internal access required):

1) Note the fault code and any other lights or messages. Take a photo for the engineer. 2) Try a simple reset: follow the boiler manual reset procedure (selector to R or hold reset button as instructed) and wait 2–3 minutes for the boiler to restart. 3) Check the boiler pressure gauge — it should normally be around 1.0–1.5 bar when cold. Low pressure can cause faults; repressurise only if you know how and the system is safe. 4) Look for obvious signs of water leaks, dampness around the boiler, or corrosion on external pipework and connectors. 5) Ensure central thermostat/timer controls are calling for heat (so the boiler should be firing) and that any recent power cuts or storms haven’t coincided with the fault.

If the E20 persists and you are comfortable with basic electrical testing (and have isolated the mains):

1) Visual inspection inside the outer cover (only if you are competent and the mains is OFF): check the wiring from the sensor connector to the PCB for loose plugs, pin corrosion, chafed or damaged cables, and any visible water damage on the PCB. Do not touch components when mains is live. 2) Locate the flow thermistor/sensor (usually clipped to the flow pipe or heat exchanger) and check the connector is fully seated. 3) With the sensor disconnected from the wiring loom and the boiler electrical supply isolated, measure the sensor resistance with a digital multimeter set to ohms. Typical NTC behaviour: resistance is higher at low temp and falls as temperature rises. Many Baxi flow sensors read roughly 8–12 kΩ at room temperature (approx. 20–25°C), but check your boiler manual or part spec if available. Results: an open circuit (infinite/OL) or a dead short (0 Ω) indicates a faulty sensor; a reading far outside the expected room-temperature range or one that does not change when the sensor is heated/cooled indicates failure.

4) If the sensor reading looks reasonable, check continuity of the wiring from the sensor connector to the PCB and measure at the PCB connector to confirm the signal reaches the control board. A faulty connector or broken wire can mimic a sensor failure.

Fix steps (general guidance):

1) If the sensor is clearly failed on the multimeter, it normally needs replacing with the correct Baxi part for your model. Replacements should be fitted exactly in the same position and clipped securely to the flow pipe for correct temperature reading. 2) If wiring or connectors are corroded or damaged, isolate supplies and replace or repair the wiring/connector using correct crimp/soldering and insulation techniques; ensure secure, moisture-free connections. 3) After replacement or repair, restore power, reset the boiler, and observe for normal operation and disappearance of E20. 4) If the new sensor is immediately reporting implausible resistance values or the E20 returns, suspect PCB or wiring faults; stop and call a professional.

When to call a professional:

1) If the simple reset and external checks do not clear the fault. 2) If you are not confident or competent to isolate mains/gas, open the boiler, or use a multimeter. 3) If the sensor needs replacement, wiring requires repair, or there is evidence of water ingress or PCB damage. 4) Any time a gas or sealed combustion component is involved, a Gas Safe registered engineer must do the work. Provide the engineer with the E20 code, any measurements you took, photos, and the circumstances when the fault started — this speeds diagnosis and repair.

Do not attempt to bypass the sensor or reset the boiler repeatedly without addressing the root cause. The boiler lockout is a safety action; use qualified help for repairs inside the appliance.