P0271 – Cylinder 4 Injector Circuit High

P0271 is an OBD-II trouble code that points to a problem with the fuel injector on cylinder 4, specifically an electrical “high” condition in the circuit. In simple terms, your engine computer is seeing abnormal voltage or resistance on that injector’s control wire. This can lead to rough running, misfires, poor fuel economy, and in some cases a no-start. If you’ve scanned your car and found P0271, you’re dealing with an issue that can damage the engine or catalytic converter if ignored, but it’s usually fixable with proper diagnosis.

What Does P0271 Mean?

P0271 stands for “Cylinder 4 Injector Circuit High.” Your ECU (engine control unit) constantly monitors the electrical signal going to each fuel injector. When it sees voltage or resistance outside the expected range on injector #4, it flags this code and usually turns on the check engine light.

“Circuit high” doesn’t always mean the injector itself is bad. It can also mean you have a wiring problem, poor connection at the injector plug, corrosion in a connector, or even an internal fault in the ECU. The key is to test, not guess, before throwing parts at it.

Quick Reference

• Code: P0271 – Cylinder 4 Injector Circuit High
• Type: Fuel injector electrical fault
• Primary concern: Misfire, rough running, possible engine damage if ignored
• Difficulty: Moderate – basic electrical testing required
• Urgency: Fix as soon as possible; avoid prolonged driving

Real-World Example / Field Notes

In the shop, I often see P0271 on higher-mileage trucks and SUVs that tow or see heavy use. One example was a 5.4L V8 with a rough idle and poor throttle response. Scan data showed a consistent misfire on cylinder 4 with P0271 stored. The owner had already replaced the injector based on a parts-store recommendation, but the problem stayed. A quick voltage drop test on the injector circuit revealed excessive resistance in the harness near the valve cover where the wiring had rubbed through. Repairing and insulating the harness cleared the code and restored smooth operation. That’s a good reminder that wiring issues are just as common as failed injectors.

Symptoms of P0271

• Check engine light on – P0271 will usually set the MIL and may store as a pending code first.
• Rough idle – The engine may shake or feel uneven, especially at stoplights.
• Misfire under load – You may feel hesitation, bucking, or lack of power when accelerating.
• Reduced fuel economy – Poor combustion or compensation from other cylinders wastes fuel.
• Hard starting – Extended cranking or needing to apply throttle to start can occur.
• Fuel smell or rich/lean condition – Depending on failure type, the mixture can go off-balance.
• Engine noise or vibration – A dead or weak cylinder can cause noticeable vibration.
• Possible limp mode – Some vehicles may limit power to protect the engine and emissions system.

Common Causes of P0271

Most Common Causes

• Faulty cylinder 4 injector – Internal coil failure, short, or open circuit inside the injector.
• Damaged injector wiring – Chafed, pinched, or broken wires causing high resistance or short to voltage.
• Corroded or loose injector connector – Poor contact at the plug raises circuit resistance.
• Poor ground or power supply – Shared grounds or power feeds for the injector bank can create issues.
• Previous engine work – Harness not routed correctly after repairs, leading to rubbing and damage.

Less Common Causes

• ECU/PCM internal fault – Failed injector driver circuit inside the engine computer.
• Aftermarket tuning or wiring mods – Remote start, piggyback tuners, or splices can introduce electrical noise or faults.
• Water intrusion – Moisture in connectors or harnesses, especially in vehicles used off-road.
• Rodent damage – Chewed injector wires causing intermittent or high-resistance connections.
• Poor battery or charging system – Low system voltage can aggravate marginal injector circuits and trigger codes.

Diagnosis: Step-by-Step Guide

For P0271, you’ll want a basic scan tool (preferably one that can read live data and Mode $06), a digital multimeter, and if possible a noid light or test light rated for injector circuits. Access to a wiring diagram for your specific vehicle is a big help.

1. Verify the code and record freeze frame data. Scan the vehicle, confirm P0271, and note engine load, RPM, and temperature when it set. Clear the code and see if it returns quickly.
2. Check for related codes. Look for misfire codes (P0304), other injector codes, or voltage/ground codes that might point to a bigger issue like a shared power feed problem.
3. Perform a visual inspection. With the engine off, inspect cylinder 4 injector and harness. Look for broken clips, stretched or melted wiring, oil-soaked connectors, or obvious damage near brackets and valve covers.
4. Wiggle test the harness. With the engine idling, gently move the injector harness and connectors. If the idle changes or the engine stumbles, you likely have an intermittent wiring or connector fault.
5. Check injector resistance. Disconnect the cylinder 4 injector and measure resistance across the terminals with a multimeter. Compare to factory specs and to another known-good injector. A reading out of range points to a bad injector.
6. Test power and ground/control. With key on, engine off, verify proper voltage at the injector connector on the power side. Then check continuity and resistance from the control wire back to the ECU connector using a wiring diagram.
7. Use a noid light or scope if available. A noid light or oscilloscope can confirm that the ECU is pulsing the injector correctly. If the signal is missing or abnormal only on cylinder 4 while wiring checks out, suspect the ECU driver.
8. Swap components for confirmation (when practical). On some engines, you can swap injector 4 with another cylinder. If the problem and code move to the new cylinder, the injector is at fault. If it stays on cylinder 4, focus on wiring or ECU.
9. Check Mode $06 data. Some scan tools will show cylinder-specific misfire and injector test data. This can confirm that cylinder 4 is consistently underperforming.
10. Recheck after each repair. After repairing wiring or replacing parts, clear codes and road test under similar conditions to the freeze frame to confirm the fix.

Pro tip: Don’t overlook ground points. A corroded or loose engine ground can cause strange injector circuit readings and intermittent P0271 even when the injector and harness look perfect.

Possible Fixes & Repair Costs

Fixing P0271 can be straightforward once you pinpoint the root cause. Common repairs include replacing the cylinder 4 injector, repairing or replacing sections of the injector harness, cleaning and tightening connectors, or in rare cases replacing the ECU/PCM and programming it. Typical repair costs vary widely: a simple wiring repair might run $100–$250, a new injector installed $200–$500 depending on engine design, and an ECU with programming can reach $600–$1,200 or more. Labor rates, parts quality (OEM vs aftermarket), and how buried the injector is all affect the final bill.

Can I Still Drive With P0271?

You can often still drive with P0271, but it’s not a good idea to ignore it or take long trips. A misfiring or under-fueled cylinder can cause rough running, loss of power, and poor fuel economy. More importantly, unburned fuel or uneven combustion can overheat and damage the catalytic converter, and long-term misfires can stress internal engine components. If the engine runs very rough, stalls, or the check engine light is flashing, you should avoid driving and have it towed to a shop.

What Happens If You Ignore P0271?

Ignoring P0271 can turn a moderate electrical or injector issue into expensive engine or catalytic converter damage. Continuous misfires can melt the catalyst, foul spark plugs, contaminate engine oil, and lead to hard starting and poor drivability. Over time, you may also fail emissions tests and risk being stranded when the injector or wiring finally fails completely.

Last updated: January 22, 2026

Vehicles Commonly Affected by P0271

P0271 can show up on almost any OBD-II vehicle, but it’s more common on multi-cylinder engines where injectors and harnesses see a lot of heat and vibration. You’ll often see this code on Ford and GM trucks and SUVs (F-150, Silverado, Suburban), Dodge/Ram pickups, and various Jeep models. It also appears on some Volkswagen, Audi, and BMW gasoline engines, as well as common Japanese brands like Toyota, Nissan, and Honda when mileage is high or wiring has been disturbed during other repairs. Turbocharged and workhorse vehicles that tow or haul regularly are especially prone due to higher under-hood stress.