Baxi Boiler E20 Error Code: Causes & Fixes

The E20 error code means your Baxi boiler has detected a problem with its central heating temperature sensor, also known as the NTC thermistor. This sensor sits on the flow pipe and tells the control board how hot the water is. 

When the reading is missing, out of range or inconsistent, the boiler locks out for safety and you lose heating and hot water. 

UK homeowners and landlords regularly see this code appear without warning. Some simple checks can be done at home, but most E20 faults need a Gas Safe engineer. 

Having a boiler cover plan in place means you are never left facing sudden repair costs when sensor or wiring issues arise, and it can also protect you from more expensive problems that develop if the fault is left unresolved.

What Does the Baxi Boiler E20 Error Code Mean?

The Baxi boiler E20 error code means a fault with the central heating NTC thermistor. This small sensor constantly monitors the temperature of the water leaving the heat exchanger. The control board relies on accurate readings to keep the boiler running safely and efficiently. 

When the sensor sends no signal, a short-circuit signal or a reading that is far outside the normal range, the boiler displays E20 and shuts down.

You will typically lose both central heating and hot water until the fault is cleared. A single reset may temporarily clear the display, but the code nearly always returns until the underlying sensor or wiring problem is fixed.

What Causes the Baxi Boiler E20 Error Code?

The E20 error code is almost always linked to the central heating temperature sensor or the wiring that connects it to the control board. The main causes are a failed NTC thermistor, damaged or loose wiring, corroded connections, and occasionally a problem with the printed circuit board.

Faulty Central Heating NTC Thermistor

The NTC thermistor is the temperature sensor clipped or fitted to the flow pipe inside the boiler. Over time the sensor can fail completely, drift out of its correct resistance range, or become intermittent. When this happens the control board receives no usable temperature information and locks the boiler out with the E20 code. This is by far the most frequent cause.

How to Fix

1. Switch the boiler off at the mains and allow it to cool.
2. Do not attempt to remove or test the sensor yourself, as it sits inside the sealed combustion area.
3. Call a Gas Safe registered engineer who will measure the sensor’s resistance with a multimeter and compare it against Baxi’s published values.
4. If the reading is open circuit, shorted or outside the expected range, the engineer will fit a genuine replacement thermistor.
5. Once the new sensor is fitted and the boiler is reset, the E20 code should clear and normal operation resume.

Loose or Damaged Wiring to the Sensor

The thin wires that run from the thermistor to the printed circuit board can work loose through vibration, become pinched, or suffer insulation damage. Any break or poor connection stops the temperature signal reaching the control board and triggers the E20 fault.

How to Fix

1. Power the boiler down completely.
2. A Gas Safe engineer will open the boiler casing and carefully inspect the wiring harness between the sensor and the PCB.
3. Any loose terminals will be re-secured, and damaged sections of cable will be repaired or replaced.
4. Connections will be cleaned and tested for continuity.
5. The engineer will then reset the boiler and confirm the E20 code has gone.

Corroded or Oxidised Connections

Moisture, condensation or minor leaks inside the boiler can cause corrosion on the sensor plug or the pins on the control board. Corroded contacts create high resistance or intermittent signals that the board interprets as a sensor fault, resulting in the E20 error code.

How to Fix

1. Isolate the electrical supply to the boiler.
2. The engineer will disconnect the sensor plug and examine both the plug and the socket for green or white corrosion.
3. Contacts will be carefully cleaned with an appropriate electrical cleaner.
4. If the corrosion has damaged the plug or pins beyond repair, the relevant connector or wiring section will be replaced.
5. After reassembly the boiler is powered up, reset, and checked to ensure the E20 code no longer appears.

Faulty Printed Circuit Board

In a smaller number of cases the thermistor and wiring are perfectly healthy, yet the control board itself cannot correctly read the signal. A failed input channel on the PCB, water damage to the board, or a power surge can produce the same E20 lockout.

How to Fix

1. The engineer will first confirm that the thermistor and wiring test within specification.
2. If both are good, the PCB will be tested for correct voltage and signal processing on the temperature sensor circuit.
3. When the board is confirmed faulty, a genuine Baxi replacement PCB is fitted and programmed if required.
4. The boiler is then commissioned and reset so the E20 code is cleared.
5. The engineer will also check for any underlying cause such as moisture ingress that could damage the new board.

How to Prevent the Baxi Boiler E20 Error Code

Preventing the Baxi boiler E20 error code centres on keeping the temperature sensor and its connections in good condition through regular maintenance. A few straightforward habits make a real difference.

1. Arrange an annual boiler service with a Gas Safe engineer. During the service the thermistor is checked, connections are inspected and any early signs of wear or corrosion are spotted.
2. Keep the system pressure between 1 and 1.5 bar when the boiler is cold. Correct pressure reduces stress on components and helps avoid overheating that can affect sensors.
3. Bleed radiators regularly so air does not collect and cause temperature fluctuations that strain the sensor.
4. Address any small leaks inside the boiler casing promptly, as moisture is a common cause of corroded connections.
5. Consider a boiler cover plan that includes annual servicing and prioritised call-outs. This keeps your Baxi boiler well maintained and means any future sensor or wiring fault is dealt with quickly without unexpected bills.

Protect Your Baxi Boiler with Cover from PlusHeat

Sensor faults such as the Baxi boiler E20 error code can leave a home without heating at short notice. 

A boiler cover plan from PlusHeat gives you 24/7 access to Gas Safe engineers, parts and labour included, and an annual service that helps catch temperature sensor problems early. Whether you own your home or manage rental properties, cover means one less worry when the boiler suddenly stops working.

FAQs

Can I replace the temperature sensor on a Baxi boiler myself?

No. The central heating thermistor sits inside the combustion chamber. Opening the sealed casing requires specialist tools and knowledge, and any incorrect work can create a safety risk. Always use a Gas Safe registered engineer.

How much does it usually cost to fix a Baxi E20 fault?

Most E20 repairs involve replacing the thermistor or repairing wiring and typically cost between £120 and £250 including labour, depending on the model and parts needed. A full PCB replacement costs more. Boiler cover can remove or greatly reduce this cost.

Will the E20 code clear itself after a while?

It is very unlikely. The fault is caused by a failed sensor or connection, so the code will keep returning until the component is repaired or replaced.

Is the E20 code related to the E50 code on Baxi boilers?

They are similar in nature but affect different sensors. E20 is the central heating flow temperature sensor. E50 relates to the domestic hot water temperature sensor. Both need professional diagnosis.

Does a Baxi boiler still heat water if the E20 code is showing?

No. The boiler locks out completely for safety, so neither central heating nor hot water will be available until the fault is cleared.