Before assuming the thermostat is at fault, run through this checklist:

• Check the thermostat setpoint: Is the setpoint above the current room temperature? If not, the thermostat is working correctly — it simply doesn't see a need for heat.
• Check the time programme: Is the current time within a heating period? Programmable thermostats will not call for heat outside their scheduled times.
• Check the boiler: Is the boiler powered and showing no fault codes on its display? A boiler in lockout (red or yellow fault LED) will not respond to the thermostat regardless of the call signal.
• Check the wiring: On wired thermostats, inspect the connections at both the thermostat and the boiler for loose or corroded terminals.
• Test with a bridge: At the boiler's heating circuit terminals, bridge the two thermostat wires (or COM and HEATING terminals). If the boiler fires, the thermostat is faulty. If not, the fault is in the boiler or wiring.

If the temperature shown on the thermostat doesn't match what a separate thermometer reads in the room, the cause is almost always environmental rather than a faulty sensor:

• Direct sunlight: A thermostat in direct sun can read 3–5°C above actual room temperature. This causes under-heating. Reposition or shade the thermostat.
• Too close to a radiator: The thermostat registers the local heat from the radiator and switches off too early. The thermostat should be at least 1 metre from any radiator and not in the line of warm air rising from it.
• Draughts: Cold draughts from doors or windows make the thermostat read low, causing the heating to run longer than needed. Seal the draught first before recalibrating.
• Calibration offset: Some programmable and smart thermostats allow a temperature offset to be set in the advanced settings menu. Consult your manual.

If the heating continues running after the thermostat setpoint is reached and the thermostat shows the correct temperature, there is likely a relay fault inside the thermostat:

• Disconnect the thermostat from the circuit (remove batteries or switch off mains power). If the boiler continues to run, the boiler has its own internal fault — call an engineer.
• If the boiler stops when the thermostat is disconnected, the thermostat's relay is stuck closed. The thermostat needs replacing.
• On smart thermostats, check whether the app is showing a conflicting schedule or a boost that is still active.

• Battery-powered thermostats: Replace batteries. Even if the battery indicator hasn't appeared, old batteries can cause display issues before showing a low-battery warning.
• Mains-powered thermostats: Check the fuse for the heating circuit in your consumer unit (fuse box). Also check the wiring at the thermostat's base plate for loose neutral or live connections.
• Smart thermostats: Try a factory reset. Many smart thermostats have a pinhole reset button. After reset, re-pair via the app. If the display remains blank after reset, the device itself may be faulty.

Short cycling — where the boiler fires for a few minutes, switches off, and then fires again almost immediately — stresses the boiler and is a sign of a misconfigured or badly placed thermostat:

• The most common cause is thermostat placement too close to a heat source (radiator, sunlight, hot water cylinder). The thermostat reaches setpoint quickly, shuts the boiler off, the temperature drops rapidly, and the cycle repeats. Solution: relocate the thermostat.
• Anticipator settings on older bi-metal thermostats may be incorrectly set. This is rare on modern digital thermostats.
• If the boiler itself is short cycling (visible in the boiler's operation history on smart boiler systems), this may indicate a water pressure issue or a bypass valve problem — consult an engineer.

• Most smart thermostats require a 2.4 GHz Wi-Fi network. If your router operates on 5 GHz only (or mixed mode where the device can't find 2.4 GHz), the thermostat will fail to connect. Check your router settings.
• Distance and walls reduce signal. Move the router or add a Wi-Fi extender if signal strength is consistently poor at the thermostat location.
• If the thermostat connects but then drops intermittently, check for router firmware updates and ensure the thermostat's MAC address is not being filtered by your network security settings.

Call a qualified heating engineer if:

• The boiler shows a fault code that doesn't clear after reset.
• There is any smell of gas near the boiler.
• The wiring at the boiler's printed circuit board looks burnt or damaged.
• The heating system has not been serviced in over 2 years.

For the UK, all gas boiler work must be carried out by a Gas Safe registered engineer.