Overview
E20 on a Baxi Platinum means the boiler has detected a fault with the central heating NTC (negative temperature coefficient) sensor — often called the flow or primary temperature thermistor. That sensor measures the water temperature in the primary circuit/heat exchanger and tells the control board how hot the system is. If the sensor is open, shorted, giving intermittent or implausible resistance readings, or if the control board cannot read it correctly, the boiler will lock out with E20 as a safety measure to avoid overheating or running with incorrect temperature control. Severity: this is not usually an immediate danger like a gas leak, but it will stop the boiler from operating properly (no heating or possibly no hot water depending on model) and should be resolved promptly. Common causes are a failed NTC thermistor, a bad connector or wiring between the sensor and PCB, water ingress or corrosion, or (less commonly) a fault on the boiler control board. Simple checks and a reset can be done by a competent homeowner, but diagnosing wiring, replacing sensors, or any work that involves the gas side or PCB should be carried out by a qualified Gas Safe engineer.
Possible Cause: Central heating NTC error
Troubleshooting Steps
Safety first
1) If you are not competent with boilers or electical testing, stop and call a Gas Safe registered engineer. Working on gas appliances can be dangerous. 2) Before you open the boiler: turn off the electrical supply at the isolator and switch the boiler off at its controls. If you will be doing internal work and are unsure, isolate the gas supply at the appliance valve. Allow the boiler to cool. 3) Use insulated tools and a multimeter only if you know how to use them safely; do not probe live parts unless trained.
Initial checks a homeowner can do
1) Try a reset: follow the boiler’s reset procedure (press and hold the reset or selector as per manual for several seconds). If E20 clears and the boiler runs normally for a while, monitor it — an intermittent sensor fault could reappear. 2) Check mains power and display for other codes. 3) Check boiler pressure (pressure gauge) and top up if the system is low (follow your manual). Low pressure alone won’t typically cause E20 but is good practice. 4) Look for obvious signs: water leaks, corrosion around pipework, damp inside the boiler casing, or frozen condensate pipes (external). 5) Note when the fault happens (startup, after running, intermittently) — this helps diagnostics.
Basic diagnostic steps (requires basic tools: screwdriver, multimeter, thermometer or warm water)
1) Remove the boiler outer case only if you are comfortable doing so and the electricity is isolated. Locate the central heating flow/primary NTC thermistor (consult the parts diagram for your model). It’s usually clipped to the flow pipe or on the heat exchanger. 2) Visually inspect the sensor and its connector: look for loose connectors, damaged wires, corrosion or water ingress. Reseat the connector and ensure it clicks or is tight. 3) Measure the sensor resistance with a multimeter (with power off). An NTC should show a finite resistance at room temperature and the resistance should decrease as you warm the sensor (for example, by holding it gently or using warm water). Typical NTCs are often around ~10kΩ at about 20–25°C on many boilers, but values vary by model — check the Baxi part specification if available. If the meter shows OL/open circuit or very erratic values, the sensor is faulty. 4) If the sensor shows implausible fixed low resistance (short) or open circuit, replace the sensor. If resistance changes when warmed/cooled, but readings are intermittent when installed, suspect a wiring/connector fault or intermittent sensor.
Wiring and PCB checks (leave live testing to professionals)
1) If the sensor reads OK out of the boiler but E20 persists, check continuity of the wires between the sensor connector and the PCB connector (power off). Look for broken strands, corrosion or chafing. 2) If you are confident with multimeter work and the manual, you can check the plug pins for corrosion and that the socket on the PCB is secure. Do not attempt to repair PCB traces or components — leave PCB work to an engineer.
Replacing the NTC thermistor
1) Obtain the correct Baxi-approved replacement sensor for your exact model. Using an incorrect part can cause further faults. 2) With electricity isolated, unclip/remove the old sensor, noting its orientation and how it is seated against the pipe/heat exchanger. 3) Fit the new sensor in the same position, ensure it is securely clipped/located so it can read the temperature correctly, reconnect the wiring harness firmly, reassemble the case. 4) Restore power, reset the boiler and run the system. Check for E20 reappearance and confirm normal operation of heating/hot water. Bleed radiators if needed and monitor system behaviour.
When to call a professional
1) Call a Gas Safe registered engineer if: you are unsure about any step; the sensor tests OK but E20 remains; there is evidence of wiring damage inside the boiler; the control PCB appears to be at fault; or you are not confident working on gas appliances. 2) Do not attempt PCB repairs, live signal tracing, or gas-valve work yourself. A qualified engineer will perform safe live diagnostics, replace sensors correctly, check wiring harnesses, and if necessary replace or repair the PCB.
Final notes
If after a sensor replacement and wiring check the boiler still shows E20, leave the boiler off and call a professional. Persistent or intermittent electrical faults can cause further lockouts and may point to a failing control board or moisture-related problem that requires specialist attention.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi Platinum.