Clogged fuel injector symptoms, and the faults that look the same

What it means

An injector is a small electrically controlled valve. On most engines a coil lifts a pintle off its seat and fuel sprays through a fine nozzle. Deposits can narrow that nozzle or disturb the spray pattern, so the cylinder gets less fuel and it mixes poorly with the air.

The engine computer can hide a small, even loss across all the injectors with fuel trim, but one injector flowing less than the others is a misfire on that cylinder that no trim can fix.

Common causes

• A misfire on one cylinder, worst when cold or at idle
• A rough idle
• An engine that feels uneven even though the fuel trim looks normal
• A misfire that comes and goes, which can also be a connector or wiring
• A dead cylinder and lost power if the injector sticks closed

What to do now

1. If the check engine light is flashing and the engine shakes, reduce load and stop somewhere safe.
2. Have the fault codes and freeze frame data recorded before anything is cleared.
3. Ask for the ignition to be ruled out first, for example by swapping the coil to another cylinder and seeing whether the misfire moves with it.
4. Ask which test showed that the injector itself has failed, before one is replaced.
5. If you use a fuel additive, use one approved for your car and fuel. It cannot fix an electrical or mechanical fault.

Injector, or something that looks like one?

• The misfire moves when the coil is swapped: the coil, not the injector.
• A misfire on one cylinder, worst cold or at idle: possibly a restricted injector, but also a plug, compression or an air leak.
• A dead cylinder, a steady misfire, the engine down on power: an injector stuck closed, or ignition or compression.
• A misfire that comes and goes: a loose connector or wiring as much as the injector.

What people often get wrong

That the code said injector, so the injector is bad. A code points at a cylinder or a system. Coils, plugs, compression and air leaks give the same misfire, and "the code said injector" is not an answer.

When the car must go to a workshop

Injector testing, balance tests and anything involving the fuel rail are workshop jobs. The fuel system holds pressure, and on direct injection that pressure is extremely high. What you can do is note when the misfire happens and keep the codes before anyone clears them.

Read next

• Engine misfiring: rough running and what it means
• Rough idle: why the engine runs unevenly when stopped
• P0300: misfire on more than one cylinder

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.