P0271
PowertrainCylinder #4 Injector Circuit High
The ECU has spotted the voltage in the cylinder 4 injector circuit sitting higher than it should be. That usually points to a wiring fault, a dodgy connector, or an injector that's not behaving electrically. For you it normally shows up as a misfire on that one cylinder, a lumpy idle, and an engine light, and the job is finding out whether it's the injector or the wiring feeding it before you spend a penny on parts.
ⓘ Information only. This page provides general educational information about fault code P0271. 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 P0271 mean?
P0271 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder #4 Injector Circuit High.
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:
- • Engine warning light on, often the first thing you notice
- • Lumpy, uneven idle that's most obvious sitting at the lights
- • Flat spot or hesitation when you put your foot down
- • A noticeable misfire that you can feel as a stumble, worst when cold
- • Fuel economy creeping up because cylinder 4 isn't pulling its weight
- • Longer cranking than usual before it catches, and sometimes a whiff of raw fuel out the back
Possible causes
Causes commonly associated with P0271, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Wiring or connector fault at the cylinder 4 injector, by far the most common cause. Heat and vibration chafe the harness or corrode the pins, and the circuit reads high
- 2. Corroded or loose injector connector, the little spade terminals tarnish or back out of the plug and you lose a clean connection
- 3. Chafed or damaged harness insulation, often where the loom rubs on a bracket or sits near a hot exhaust component
- 4. A short to voltage in the harness, where a wire has worn through and touched a 12V supply
- 5. Faulty injector with an internal open or high-resistance fault on the windings
- 6. Poor connection or corrosion at the ECM connector itself, less common but worth checking before condemning the module
- 7. Failed ECM driver for that injector, rare, and only after everything upstream checks out clean
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. Pull all stored codes and write down anything alongside the P0271. A P0304 misfire on the same cylinder is a strong clue, and seeing P0271 with codes for other injectors points you toward a shared earth or supply problem rather than one bad injector
- 2. Get to the cylinder 4 injector connector and look at it properly. Unplug it, check for green corrosion, burnt or pushed-back terminals, and waggle the loom to find any spot where the insulation has rubbed through
- 3. Measure the injector resistance across the two pins with a multimeter. Most petrol injectors land somewhere around 12 to 17 ohms, but check your workshop figure because diesels and some direct-injection units read very differently
- 4. Back-probe the connector with the ignition on and the injector still plugged in, watching for voltage sitting higher than the rest of the circuit, which confirms the high reading the ECU is flagging
- 5. Swap the suspect injector's connector reading against a known-good cylinder if you can. If the wiring and resistance both look fine, the injector is the likely culprit
- 6. Clear the code and drive it through a few warm and cold cycles to confirm the fix held before you call it done
Common questions about P0271
How long can I keep driving like this before something gets damaged? +
You can usually limp it to a garage, but don't make a habit of long trips with it. A dead or weak cylinder 4 dumps unburnt fuel into the exhaust, and that's exactly what cooks a catalytic converter over time, turning a wiring repair into a much bigger bill. If it's running really roughly, stalling, or has dropped into limp mode, get it recovered rather than thrashing it down the motorway.
Is it the injector that's gone, or is it just the wiring and plug? +
Far more often than not it's the wiring or the connector, not the injector itself. The plug terminals corrode, the loom rubs through near a bracket, or a pin backs out, and the circuit reads high. Injectors do fail electrically, but they're the less common answer here. That's why the connector inspection and a resistance check come before you order a part. Plenty of people throw an injector at this and the light comes straight back.
How long does the repair actually take? +
A connector clean-up or a small wiring repair is often well under an hour once the fault is found, the diagnosis is what eats the time. Replacing the injector is usually a half-day job on most engines, longer on anything where you've got to strip the inlet manifold or fight a buried loom to reach cylinder 4. Diesel common-rail injectors take more time and care because the fuel system needs depressurising first.
Are the cheaper aftermarket injectors any good, or should I stick with the proper part? +
For a straightforward petrol injector, a quality aftermarket unit from a reputable brand like Bosch or Delphi is fine and a fair bit cheaper than main dealer prices. Where I'd spend the extra is on diesel common-rail injectors, the cheap remans and eBay specials have a poor reputation for spray pattern and longevity, and a bad one can flag fresh codes within weeks. If you do fit a new injector, match it to the same part the others are running so cylinder 4 behaves like its neighbours.
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 →