Boiler shows code 4 and will not fire, may occur intermittently

Safety first: Turn off power before accessing internal wiring. Allow the boiler to cool before touching any pipework or sensors.

Diagnostic steps:

• Visual inspection: Locate the Intergas flow NTC sensor on the heating flow pipe. Check that the sensor is firmly clipped to the pipe and that its connector is fully seated on the harness.
• Check wiring: Follow the NTC cable back to the PCB. Look for damage, pinched insulation, or signs of overheating. Reseat the connector on the PCB.
• Resistance test (technician): With power off and sensor disconnected, measure the resistance of the NTC at room temperature and compare to Intergas resistance tables. An open circuit (infinite) or near-zero reading indicates a faulty sensor.
• Check for shorts: Inspect for moisture or corrosion at the connector that could cause a short.

Remedy: Replace the flow NTC sensor if readings are out of specification. If a known-good sensor still gives code 4, the PCB temperature input may be defective and the Intergas main PCB will need replacement by a qualified technician.