If your Ideal Logic or Ideal Logic+ boiler displays a Flow Thermistor Fault or Return Thermistor Fault, the appliance may struggle to monitor water temperature accurately. This can lead to heating problems, boiler lockouts, poor hot water performance, or prevent the boiler from operating altogether.

Fortunately, thermistors are relatively inexpensive components and can be quickly tested with a digital multimeter before replacing any parts.

At Certificates4UK Ltd, we’ve diagnosed hundreds of Ideal boiler faults across West Molesey, Esher, Walton-on-Thames, Weybridge, Thames Ditton, Kingston upon Thames, and surrounding Surrey locations. In this guide, we’ll explain what the fault means, how to test the thermistor safely, and when replacement is necessary.

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What Is a Boiler Thermistor?

A thermistor is a temperature-sensitive resistor that continuously measures the temperature of the water flowing through your boiler.

The Printed Circuit Board (PCB) monitors the thermistor’s resistance and adjusts the burner accordingly to maintain safe and efficient operation.

If the resistance falls outside the expected range, the boiler may display a thermistor fault and enter lockout mode to protect the heating system.

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Common Symptoms of a Faulty Thermistor

A failing thermistor doesn’t always cause a complete boiler breakdown. You may notice:

• Boiler displaying a Flow Thermistor Fault or Return Thermistor Fault
• Heating switching on and off unexpectedly
• Inconsistent hot water temperatures
• Boiler overheating
• Boiler locking out
• Slow heating response
• Incorrect flow temperatures
• Boiler failing to ignite

Many of these symptoms can also be caused by circulation problems or PCB faults, so accurate diagnosis is essential.

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Tools Required

Before testing the thermistor, you’ll need:

• Digital multimeter
• Insulated test probes
• Manufacturer’s resistance values
• Appropriate PPE

Always isolate the electrical supply before removing any boiler covers.

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Step-by-Step Thermistor Test

Step 1 – Isolate the Boiler

Turn off the electrical supply to the boiler and confirm it is fully isolated before beginning any work.

Safety should always be your priority.

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Step 2 – Locate the Thermistor

Identify whether you’re testing the:

• Flow thermistor
• Return thermistor

Both are located on the primary heating circuit.

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Step 3 – Disconnect the Wiring

Carefully disconnect the electrical connector from the thermistor.

This isolates the sensor from the PCB and allows an accurate resistance reading.

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Step 4 – Set Your Multimeter

Set your multimeter to measure Resistance (Ω).

Place one probe on each thermistor terminal.

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Step 5 – Compare the Reading

Compare your reading with the manufacturer’s expected resistance values.

Expected Resistance

Temperature	Expected Resistance
25°C	9,700 – 10,300 Ω
60°C	2,400 – 2,600 Ω
85°C	1,000 – 1,100 Ω

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How to Interpret the Results

Multimeter Reading	Diagnosis
Within specification	Thermistor likely OK
O/L (Open Circuit)	Replace thermistor
Very low resistance	Replace thermistor
Significantly outside specification	Replace thermistor
Reading unstable	Inspect wiring and connector

A simple resistance test can often confirm the fault within minutes.

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Why Thermistors Fail

Thermistors are exposed to constant heating and cooling cycles.

Over time, they can fail due to:

• Heat stress
• Internal electrical failure
• Moisture ingress
• Corrosion
• Damaged wiring
• Loose electrical connectors

Regular boiler servicing can identify deteriorating sensors before they cause unexpected breakdowns.

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Could It Be Something Else?

Although thermistors are a common cause of this fault, similar symptoms may also be caused by:

• Faulty PCB
• Poor circulation
• Pump failure
• Airlocks
• Sludge within the heating system
• Wiring faults
• Damaged connectors

Replacing a thermistor without proper testing may not solve the problem.

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Engineer’s Tip

Never replace the PCB before testing the thermistor.

A two-minute resistance test with a multimeter can save both time and unnecessary expense.

Professional diagnosis always beats guessing.

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Fault Summary

Fault	Likely Cause	Recommended Action
Flow Thermistor Fault	Failed flow sensor	Test resistance
Return Thermistor Fault	Failed return sensor	Test resistance
Boiler overheating	Incorrect temperature reading	Inspect thermistor
Lockout after heating	Sensor or PCB issue	Diagnose systematically

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Why Choose Certificates4UK Ltd?

Since 2011, Certificates4UK Ltd has repaired and serviced boilers throughout Surrey.

We regularly diagnose faults on:

• Ideal Logic
• Ideal Logic+
• Ideal Vogue
• Worcester Bosch
• Vaillant
• Baxi
• Alpha
• Viessmann

Our experienced Gas Safe engineers work across West Molesey, Walton-on-Thames, Esher, Hersham, Thames Ditton, Cobham, Weybridge, Kingston upon Thames, and surrounding Surrey areas, providing professional diagnostics and reliable repairs.

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Frequently Asked Questions

Can I reset the fault?

A reset may temporarily clear the error, but if the thermistor has failed, the fault will usually return.

Is replacing a thermistor expensive?

Thermistors are generally one of the more affordable boiler components, but they should always be diagnosed correctly before replacement.

Can homeowners test a thermistor?

Testing requires removing the boiler casing and working with electrical components. This should only be carried out by a qualified Gas Safe registered engineer.

What does O/L on the multimeter mean?

O/L (Open Line or Open Circuit) indicates there is no electrical continuity through the thermistor, meaning it has failed and should be replaced.

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Why Accurate Diagnosis Matters

Many temperature-related boiler faults have similar symptoms. By testing the thermistor first, engineers can often avoid unnecessary replacement of expensive components such as the PCB or pump.

A systematic diagnostic approach saves time, reduces repair costs, and increases the likelihood of a first-time fix.