BoilerFaultCodesThe 6 most common faults
Your boiler has sensed a blockage or a pump failure that is stopping water from moving through the system properly.
Why it happens: This fault occurs when water is unable to flow freely through the boiler, leading to a sudden rise in internal pressure. The most common reason is a failed circulating pump that can no longer push water around your radiators, or a large pocket of air trapped within the pipework that is acting as a physical blockage.
Check that your radiator valves are openBleed your radiators to remove trapped airEnsure the system pressure gauge is within the correct range (usually 1.0 to 1.5 bar)
Your boiler has detected that the water pressure in the central heating system is too low to operate safely.
Why it happens: This fault occurs when the water level within your heating system drops below the minimum required for the boiler to heat up safely. The most common reason is a slow leak from a radiator valve or joint, though it often happens after you have bled air out of your radiators without refilling the system afterwards.
Locate the filling loop, which is usually a flexible silver hose beneath the boiler.Slowly open the small valves at either end of the hose until you hear water flowing.Watch the pressure gauge and close the valves tightly once the needle reaches between 1.0 and 1.5 bar.
Your boiler's main internal computer has encountered a processing error and cannot safely operate.
Why it happens: This fault occurs when the boiler's main control board experiences a critical electronic failure, meaning it can no longer communicate with the internal sensors or components. The most common reason is a component failure on the circuitry itself, often caused by age, a power surge, or moisture ingress affecting the delicate electronics.
Press the reset button on the control panel onceTurn the main power switch to the boiler off, wait 30 seconds, and turn it back onEnsure the boiler has not been subject to a recent power surge or lightning strike
Your boiler has stopped working because it cannot correctly measure the temperature of the water returning to the unit.
Why it happens: This fault occurs when the boiler's control board loses communication with the sensor that monitors the temperature of the water coming back from your radiators. The most common reason is that the internal components of the sensor have failed or the electrical connection has become loose or corroded. Because the boiler cannot safely monitor the water temperature, it shuts down to prevent potential overheating.
Full guide for PP1Your boiler has detected a problem with a sensor that monitors the temperature of the exhaust gases, and it has likely stopped working as a safety precaution.
Why it happens: This fault occurs when the boiler's internal computer loses communication with the sensor that measures the temperature of the exhaust gases. The most common reason is that the sensor has developed an electrical fault or the wiring connecting it to the main control board has become loose or damaged. To ensure the appliance doesn't overheat or become unsafe, the boiler will automatically stop firing until the part is replaced.
Full guide for PP2Your boiler has stopped working because an internal sensor that monitors the temperature of the heating coil is no longer sending a correct reading.
Why it happens: This fault occurs when the boiler's control board loses communication with the sensor monitoring the internal heating coil. The most common reason is that the sensor has developed an internal electrical failure or the wiring connecting it to the main board has become loose or damaged over time.
Full guide for PP3on the Grant Vortex
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Grant shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue