Error E20

Safety precautions:

1) Before any work turn the boiler off at the mains and isolate the gas supply if you will access gas-carrying parts. Do not work on gas fittings or the burner assembly unless you are Gas Safe registered. Avoid touching live terminals—only touch components when the power is off. If you are unsure, stop and call a qualified engineer.

Initial homeowner checks you can do safely:

1) Reset the boiler: set the selector to the reset position (R) and hold for the recommended time (often ~5 seconds) and see if the code clears. Note whether it returns immediately or after running. If E20 returns straight away, proceed.

2) Check system water pressure: confirm boiler pressure is in the normal range (commonly around 1.0–1.5 bar). Low pressure can cause faults; top up only if you know how and the system is working normally.

3) Visual inspection: with the boiler isolated and cover removed only if you are comfortable doing so, look for obvious signs—loose connectors, corrosion at sensor plugs, water ingress, a split cable or burn marks. Smell for damp or burnt electronics. Do not try to repair gas-related parts.

Specific diagnostic steps (requires basic electrical tools and a multimeter):

1) Locate the flow thermistor/NTC sensor (usually clipped to the flow pipe/heat exchanger). Consult the boiler manual or labeling to confirm the correct sensor.

2) With the boiler isolated from mains, disconnect the thermistor connector (or unplug the sensor lead). Measure the sensor resistance across its two terminals with a digital multimeter set to ohms. At room temperature an NTC will normally show several kilo-ohms; as temperature rises the resistance falls. Exact resistance values vary by model—check the boiler manual or parts specification for expected values. If the sensor reads infinite (open circuit) or short (near 0 Ω) it is faulty.

3) While still isolated, check continuity and insulation of the wiring from the sensor connector back to the PCB connector. Measure for continuity and check for intermittent faults by gently flexing the cable while watching the meter for changes.

4) If the sensor resistance is incorrect or open, replace the thermistor with the correct OEM or manufacturer-specified part. Fit the sensor correctly to the same location and re-seat connectors securely. If the sensor reads plausibly but the boiler still shows E20, and wiring is good, suspect the PCB or connector pins—do not attempt PCB repairs yourself unless qualified.

5) After replacing or securing wiring, reassemble the boiler, restore mains and gas, and reset the boiler. Run the heating and check stability: observe whether flow temperature readings behave sensibly and E20 does not reappear. If the fault is intermittent, test by operating the boiler and gently manipulating wiring/connectors to see if the error is triggered by movement.

When to call a professional:

1) If the fault persists after replacing the sensor and checking wiring, call a Gas Safe registered engineer—they can test the sensor against manufacturer charts, trace wiring back to the PCB, and diagnose PCB faults safely.

2) If you are not confident isolating mains, removing the boiler casing, or working with electrical tests, do not continue—contact a qualified engineer.

Notes and cautions:

1) Do not attempt to alter or repair gas-carrying components, the burner, or sealed parts of the appliance unless you are qualified. 2) Always compare measured resistance values to the manufacturer specification rather than relying on generic numbers. 3) Repeated resets without a proper fix can mask a developing failure; arrange professional repair rather than prolonged temporary resets.