Error E20

Safety first

1) If you are not comfortable working near boilers, electrical parts, or gas appliances, stop and call a Gas Safe registered engineer. Do not attempt gas work yourself. 2) Before doing any checks you can safely perform: switch the boiler off at the control panel and the isolator / fused spur. Allow the boiler to cool. Turn off the electrical supply at the consumer unit if you will be removing covers. 3) Wear gloves and eye protection when inspecting the unit.

Initial homeowner checks you can do (safe, non-invasive)

1) Note the exact error and any behaviour (red lockout light, no heating, intermittent operation). 2) Try a simple reset: on many Baxi models turn the selector switch to R and hold for 5 seconds (or press the dedicated reset button until lights change). If the code clears and the boiler runs normally, monitor it; persistent reappearance means a fault remains. 3) Check the system pressure on the boiler gauge – cold pressure typically around 1.0–1.5 bar. Low pressure can cause other faults; if very low, top up following the manufacturer’s instructions and then retry. 4) Visually inspect the boiler case and pipes for obvious signs of damage, leaks, water on or around electrical connectors, and check for loose plugs/cables around the flow sensor area if they are accessible without removing covers. 5) Check for recent power surges or wet/condensed areas around the boiler which might indicate water ingress affecting sensors or PCB.

Basic diagnostic steps a competent DIYer with a multimeter can consider (only if comfortable and after isolating power)

1) Isolate electrical supply to the boiler at the fuse or isolator and confirm it is de-energised. 2) Locate the central heating (flow) NTC sensor — typically clipped to the flow pipe or heat exchanger. 3) Disconnect the sensor plug at the connector (do not pull on wires). 4) Measure the sensor resistance with a digital multimeter across the two sensor pins at room temperature. Many NTC sensors used on boilers are in the low kilo-ohm range at ambient temperature (for example around 10 kΩ at 25°C on many models), but values vary by model — consult the boiler service manual or parts list for exact nominal values. A reading of open-circuit (OL) or a wildly incorrect value indicates a faulty sensor. 5) Warm the sensor gently (with warm water or by hand) while watching the meter: resistance should fall as temperature rises (NTC = negative temperature coefficient). If resistance does not change or is intermittent when moving the lead, the sensor or wiring is faulty.

Wiring and control checks

1) With power still isolated, inspect the sensor wiring harness and connector for corrosion, damaged insulation, loose pins or signs of burning. Wiggle-test the connector gently while watching the meter to detect intermittent faults (do not do this with power connected). 2) If the sensor reads correctly but the boiler still shows E20, check continuity of the wires from the sensor connector back to the PCB connector (trace wiring visually and measure continuity). 3) If wiring is good and sensor behaves correctly, suspect the control PCB input stage; visual signs such as burn marks, swollen components or water damage on the PCB suggest a professional repair or replacement is needed.

Corrective actions and restart

1) If the sensor tests faulty: replace it with the exact part specified for your Baxi 600 combi. Refit and reconnect securely, restore power and reset the boiler. 2) If wiring/connectors are corroded or damaged: replace or repair the wiring and connectors using proper crimped or soldered joints and appropriate insulation. 3) After any repair, restore power, reset the boiler and observe operation until heating and any lockout clears. Monitor for recurrence and any other error codes.

When to call a professional

1) Call a Gas Safe registered engineer if the reset fails, the sensor resistance is outside manufacturer specifications, you find PCB damage, wiring faults you cannot safely repair, or if you are unsure at any step. 2) Do not attempt to replace or work on gas valves, burners, or sealed PCB components yourself — these require gas-safe certification and manufacturer knowledge. 3) If the fault is intermittent and not replicated during checks, inform the engineer of the intermittent behavior and any conditions (temperature, vibration, recent power issues).

Final notes

Document the fault, reset attempts and any measurements you took; this helps the engineer diagnose faster. A competent engineer will confirm sensor values against the manufacturer's table, check wiring continuity and insulation, and verify the PCB input before replacing components. Always use a Gas Safe registered engineer for any mains, gas or sealed-boiler-component work.