Boiler displays error 21, temperature reading is erratic or fixed at extreme value, boiler may short cycle.

Safety: Isolate power and allow the boiler to cool before touching sensors on the pipework or heat exchanger.

• Visual inspection: Locate the flow NTC sensor on the primary flow pipe or heat exchanger. Check that it is firmly clipped in place and that the cable is intact.
• Check connector: Reseat the NTC plug on the PCB and at the sensor end to ensure good contact.
• Measure resistance: Using a multimeter, measure the NTC resistance at room temperature and compare with Immergas specification. If the sensor is open circuit or shows a very low resistance (short), replace it.
• Inspect wiring: If the sensor tests OK, check the wiring harness for damage or chafing. Repair or replace as needed.
• PCB check: If sensor and wiring are good but error 21 persists, the PCB input may be faulty and require replacement.
• Re-test: After replacing components, clear the error and verify that the displayed temperature responds realistically to boiler operation.