Which cylinder is number 1?

It depends on the engine, and guessing is how people replace the wrong injector. On most inline engines cylinder 1 is at the opposite end from the gearbox, which on a conventional front-engined, rear-wheel-drive car means the front of the engine. On a transverse engine in a front-wheel-drive car, that usually puts cylinder 1 at the offside — the driver's side in the UK — but several manufacturers number from the other end. On a V engine, cylinder 1 is on bank 1, and which bank is bank 1 varies between makers. Check the manufacturer's numbering for your specific engine before ordering parts.

Are P0201, P0202, P0203 and P0204 the same fault?

They are the same fault at different cylinders. P0201 is cylinder 1, P0202 is cylinder 2, P0203 is cylinder 3 and P0204 is cylinder 4, and the detection method, the causes, the symptoms and the repair are identical in every case. The only thing that changes is which injector, connector and section of loom you examine. If two or more are stored together, suspect the shared part of the circuit — the injector loom, a common earth or supply, or the control unit — rather than several injectors failing at once.

Will an injector circuit code fail an MOT?

Yes, and often twice. The illuminated engine management lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against it. A cylinder that is not firing also pushes hydrocarbon readings up sharply on a petrol car and produces smoke on a diesel, either of which can fail the emissions element on its own.

Can I swap injectors to find the faulty one?

Yes, and it is the most useful test available for this family of codes. Move the suspect injector to a different cylinder, clear the code and run the engine. If the code follows the injector to its new cylinder, the injector is faulty. If it stays with the original cylinder, the fault is in the connector, the wiring or the control unit's driver for that cylinder. One caveat: on many modern diesels and direct-injection petrols, injectors carry individual calibration codes that are programmed to a specific cylinder, so a swap needs recoding afterwards and is a job for a garage with the right tool.

Powertrain code · standard

P0201Injector circuit fault on cylinder 1

Standard definition: Injector Circuit Malfunction — Cylinder 1

Last updated 27 July 2026

Quick answer

The engine control unit cannot drive the fuel injector on cylinder 1 properly, or cannot see it responding. The cylinder is likely to be misfiring or not firing at all, and the same code exists as P0202, P0203 and P0204 for the other cylinders.

How the car works this out

The control unit switches each injector on and off itself, and while it does so it watches the electrical behaviour of the circuit — the current drawn, the voltage the coil produces as it collapses, and whether the circuit is open or shorted. It knows what a healthy injector looks like electrically. P0201 is set when cylinder 1's circuit does not behave: an open coil, a short to earth or to power, or no response at all. This is a circuit code, so it describes wiring and electrical faults rather than an injector that is dirty or worn but still connected.

What you might notice

Rough idle with a distinct uneven beat
Misfire, sometimes with the engine management lamp flashing
Loss of power and poor acceleration
Hard starting, or extended cranking
A smell of unburnt fuel from the exhaust
Vibration through the car at idle that smooths out as revs rise

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A corroded, cracked or unlatched injector connectorCommon
How to check: Unplug and inspect. Injector plugs sit in the hottest, most vibrated part of the engine, and the small locking clips become brittle and fail with age.
2Chafed or heat-damaged wiring in the injector loomCommon
How to check: Follow the injector loom looking for insulation that has hardened, split or melted against the cylinder head or the manifold.
3An injector coil that has failed open or shortCommon
How to check: Measure the resistance across the injector terminals and compare it against the other cylinders. One injector reading noticeably different from its three neighbours is the fault.
4A failed injector driver inside the engine control unit, which is uncommon but does happen after a shorted injector has taken it outLess common
How to check: Swap the suspect injector with one from another cylinder and see whether the code follows the injector or stays with the cylinder. If it stays, the fault is in the loom or the control unit.
5Water ingress into the loom or a badly made previous repairLess common

What to do about it

1Establish which cylinder is number 1 on your engine before touching anything. Getting this wrong is the single most common way people waste money on this family of codes.
2Swap the suspect injector with one from a neighbouring cylinder, clear the code, and see whether it moves. If the code moves with the injector, the injector is faulty; if it stays with the cylinder, look at the wiring.
3Check the connector and the loom before buying an injector. Injectors are not cheap, and connectors and wiring account for a large share of these codes.
4Stop driving if the engine management lamp is flashing. A flashing lamp is a misfire severe enough to send unburnt fuel into the catalytic converter, and a few miles of that can destroy it.
5Have the car coded if it takes a new injector. Many modern diesels and direct-injection petrols require the injector's correction code to be programmed in, and a new injector fitted without coding will run badly.

Can you keep driving?

Only to a garage

A cylinder that is not firing dumps raw fuel into the exhaust, and that is what destroys catalytic converters. If the lamp is flashing rather than steady, stop as soon as it is safe. Even with a steady lamp, running on three cylinders puts the engine badly out of balance and does the mountings and the catalytic converter no good at all.

Will it fail an MOT?

The engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, and it accounts for 371,859 recorded failures. A misfiring cylinder is also one of the most reliable ways to fail the emissions element outright, because the unburnt fuel from a dead cylinder sends hydrocarbon readings up sharply on a petrol car. If the misfire has already damaged the catalytic converter, that failure will remain after the injector is fixed, and a tester who finds the converter missing altogether records a separate major defect.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Which cylinder is number 1?: It depends on the engine, and guessing is how people replace the wrong injector. On most inline engines cylinder 1 is at the opposite end from the gearbox, which on a conventional front-engined, rear-wheel-drive car means the front of the engine. On a transverse engine in a front-wheel-drive car, that usually puts cylinder 1 at the offside — the driver's side in the UK — but several manufacturers number from the other end. On a V engine, cylinder 1 is on bank 1, and which bank is bank 1 varies between makers. Check the manufacturer's numbering for your specific engine before ordering parts.
Are P0201, P0202, P0203 and P0204 the same fault?: They are the same fault at different cylinders. P0201 is cylinder 1, P0202 is cylinder 2, P0203 is cylinder 3 and P0204 is cylinder 4, and the detection method, the causes, the symptoms and the repair are identical in every case. The only thing that changes is which injector, connector and section of loom you examine. If two or more are stored together, suspect the shared part of the circuit — the injector loom, a common earth or supply, or the control unit — rather than several injectors failing at once.
Will an injector circuit code fail an MOT?: Yes, and often twice. The illuminated engine management lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against it. A cylinder that is not firing also pushes hydrocarbon readings up sharply on a petrol car and produces smoke on a diesel, either of which can fail the emissions element on its own.
Can I swap injectors to find the faulty one?: Yes, and it is the most useful test available for this family of codes. Move the suspect injector to a different cylinder, clear the code and run the engine. If the code follows the injector to its new cylinder, the injector is faulty. If it stays with the original cylinder, the fault is in the connector, the wiring or the control unit's driver for that cylinder. One caveat: on many modern diesels and direct-injection petrols, injectors carry individual calibration codes that are programmed to a specific cylinder, so a swap needs recoding afterwards and is a job for a garage with the right tool.

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