P0204
PowertrainCylinder 4 Injector Circuit Malfunction
The ECU drives each injector by switching its earth on and off thousands of times, and it watches that circuit for the expected electrical response. When the signal for cylinder 4 doesn't behave the way the ECU expects, whether that's an open coil, a broken wire, or no response from the injector driver, it logs P0204. For you as the owner, it means cylinder 4 isn't getting fuelled properly, so the engine runs rough and down on power until the circuit is sorted.
ⓘ Information only. This page provides general educational information about fault code P0204. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0204 mean?
P0204 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 4 Injector Circuit Malfunction.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Check engine light on, sometimes flashing if the misfire is bad enough
- • Lumpy, shaky idle that you can feel through the steering wheel and seat
- • A clear stumble or flat spot when you pull away or put your foot down
- • Noticeable loss of power, especially uphill or when overtaking
- • Worse fuel economy because cylinder 4 is wasting fuel or not burning it cleanly
- • Smell of raw petrol from the back of the car, particularly on a cold start
Possible causes
Causes commonly associated with P0204, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Corroded or loose injector connector on cylinder 4. The cheapest and most common cause, especially on cars that have had injectors disturbed for other work
- 2. Chafed or broken wiring in the injector loom, often where it rubs against the engine or a bracket as the engine rocks
- 3. Failed injector solenoid coil, an open or shorted winding inside the injector itself
- 4. Bad earth or supply feed to that injector circuit causing high resistance
- 5. Injector driver inside the ECU has failed, which is rare but does happen on older units
- 6. Water or oil getting into the connector and corroding the pins, common on engines where the injectors sit in a valley that collects muck
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read the codes and the freeze frame, then note whether you've also got a P0304 misfire on the same cylinder. That tells you the fuelling fault is actually affecting how the engine runs
- 2. Pull the cylinder 4 injector connector and inspect it properly. Look for green corrosion, bent or spread pins, oil contamination, and a connector that isn't clicking home. This finds a fair share of these faults on its own
- 3. Measure the injector coil resistance with a multimeter across the two pins and compare it to the other three injectors. They should all read within a fraction of an ohm of each other. A wildly high or open reading points to a dead injector
- 4. Do a wiggle test on the loom near the injector while the engine idles. If the running changes when you move a section of wire, you've found a chafe or a broken strand
- 5. Use a noid light or a scope on the connector to confirm the ECU is actually sending a pulse. No pulse with good wiring points back to the driver in the ECU
- 6. Check continuity from the injector connector back to the ECU pin if everything else looks fine, to rule out a hidden break in the harness
Common questions about P0204
How do I work out whether it's the wiring, the connector, or the injector itself? +
Start at the connector because that's where most of these live. Unplug it, look for corrosion and oil, and make sure the pins are tight. Then put your meter across the injector pins and compare the resistance to the other three. If cylinder 4 reads roughly the same as the rest, the injector is probably fine and you're chasing a wiring or connector fault. If it reads open or way out, the injector coil has gone. A wiggle test on the loom while it idles will catch a chafed wire that the static tests miss.
Can I sort this myself or is it a garage job? +
The diagnosis is doable at home if you've got a multimeter and a bit of patience. Cleaning a corroded connector, repairing a chafed wire, or refitting a connector that's worked loose are all jobs you can do on the drive. Where it gets harder is the injector swap. On some engines it's a ten minute job, on others the intake manifold has to come off and the new injector may need coding to the ECU, which is when most people hand it to a garage.
If I clear the code does it stay gone or come straight back? +
If the actual fault is still there, it'll be back within a few minutes of running, often before you've left the driveway. An intermittent connector or a wire that only breaks contact when the engine warms up can let you clear it and drive for a bit before it returns. Clearing the code is only useful for confirming you've actually fixed something. If it pops straight back, you haven't found the problem yet.
What's the harm in just driving with it for a while? +
You can drive short distances, but cylinder 4 is either misfuelling or not firing, and raw petrol passing through the engine ends up cooking your catalytic converter. A cat is a far more expensive item than an injector or a bit of wiring. Long motorway runs and towing under this fault are asking for trouble. If the engine has dropped into limp mode you'll also be down on power, so get it looked at rather than leaving it for weeks.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →