P0204 – Injector Circuit/Open Cylinder 4

P0204 is an OBD-II trouble code that tells you something is wrong with the fuel injector circuit on cylinder 4. When this code sets, your engine computer (ECU) has detected an electrical fault—either too much resistance, a short, or an open circuit—on that injector’s control circuit. You might notice rough running, poor fuel economy, or a flashing check engine light. Left alone, it can damage the catalytic converter or even the engine. This guide walks you through what P0204 means, common causes, diagnosis steps, and how to fix it.

What Does P0204 Mean?

P0204 stands for “Injector Circuit Malfunction – Cylinder 4.” The ECU constantly monitors the electrical signal going to each fuel injector. When the signal for injector #4 is outside the expected range, it stores P0204 and usually turns on the check engine light.

This is an electrical circuit fault, not just a dirty injector. It can be caused by the injector itself, the wiring, the connector, or the driver circuit inside the ECU. On some vehicles, P0204 may show up along with misfire codes or rich/lean mixture codes.

Quick Reference

• Code: P0204
• Definition: Injector Circuit Malfunction – Cylinder 4
• Type: Fuel injector electrical fault
• Severity: Moderate to serious (can cause misfire and converter damage)
• Primary suspects: Injector #4, wiring, connector, ECU driver

Real-World Example / Field Notes

In the shop, I see P0204 fairly often on higher-mileage vehicles or ones that have had engine work done. A typical case: a four-cylinder sedan comes in with a flashing check engine light, rough idle, and low power. Scan shows P0204 and a misfire on cylinder 4. Visual inspection reveals a brittle injector connector with exposed copper. A quick wiggle test makes the engine stumble harder. Replacing the connector pigtail, cleaning corrosion, and clearing the code fixes the issue. Many times it’s that simple—electrical, not mechanical.

Symptoms of P0204

• Check engine light on or flashing – P0204 often appears with misfire or fuel trim codes.
• Rough idle – Engine shakes or feels uneven, especially at stoplights.
• Loss of power – Poor acceleration, hesitation, or stumbling under load.
• Engine misfire – Noticeable skip or jerk, sometimes more obvious when cold.
• Increased fuel consumption – ECU may overcompensate, hurting fuel economy.
• Fuel smell from exhaust – If the injector is stuck open or misfiring, unburned fuel may reach the tailpipe.
• Hard starting – Extended cranking or needing throttle input to start.
• Possible engine knock – Uneven fueling can cause light knocking or pinging in some cases.

Common Causes of P0204

Most Common Causes

• Faulty fuel injector #4 – Internal coil failure, shorted or open windings, or stuck pintle.
• Damaged injector wiring – Broken, chafed, or rubbed-through wires causing open or short circuits.
• Corroded or loose injector connector – Poor contact increases resistance and triggers the code.
• Previous engine work – Harness pinched or not fully reconnected after head gasket, intake, or timing work.
• Poor ground or power supply – Shared power or ground issues affecting injector #4 circuit.

Less Common Causes

• ECU driver failure – The internal transistor that controls injector #4 fails or shorts.
• Aftermarket modifications – Poorly installed remote start, alarm, or performance wiring spliced into injector circuits.
• Rodent damage – Chewed wiring harness near the fuel rail or intake manifold.
• Connector pin fitment issues – Spread or pushed-back terminals inside the connector, especially after repeated unplugging.
• Water intrusion – Moisture in the harness or ECU connector causing intermittent shorts.

Diagnosis: Step-by-Step Guide

For P0204, you’ll want a basic scan tool, a digital multimeter, and ideally a noid light or an oscilloscope. A wiring diagram for your specific vehicle is a big help. Safety first: work on a cool engine, disconnect the battery when probing connectors, and keep fuel away from sparks.

1. Confirm the code and check for companions. Scan the ECU for P0204 and look for related misfire (P0304) or fuel trim codes. Note freeze-frame data (RPM, load, temp) to see when the fault occurred.
2. Perform a visual inspection. Locate injector #4 using a cylinder layout diagram. Inspect the injector, connector, and nearby harness for cracks, burns, tape repairs, or rubbing on metal brackets.
3. Wiggle test. With the engine idling, gently wiggle the injector #4 connector and harness. If the engine stumbles or the MIL flashes, you likely have an intermittent wiring or connector issue.
4. Check injector resistance. Key off, unplug injector #4. Measure resistance across the injector terminals with a multimeter. Compare to spec and to another injector. An open (infinite) or very low resistance indicates a bad injector.
5. Test injector power supply. Key on, back-probe the injector connector. One side should have battery voltage (or close). If not, trace the common power feed, fuse, or relay for the injector circuit.
6. Check ECU control signal. Use a noid light or scope on the control wire while cranking or idling. You should see a pulsed signal. No pulse at injector #4 but present at others points toward a wiring break or ECU driver issue.
7. Continuity test the control wire. With the battery disconnected and ECU unplugged, test resistance from injector #4 control pin at the ECU to the injector connector. High resistance or open circuit means damaged wiring.
8. Inspect connector terminals. Look for green corrosion, bent pins, or loose terminals in both the injector and ECU connectors. Repair or replace as needed.
9. Swap test (if accessible). On some engines, you can swap injector #4 with another cylinder. If the problem moves to that cylinder (code changes to P020X), the injector is faulty.
10. Check Mode $06 data (if available). Some scan tools let you view injector or misfire counters. This can confirm that cylinder 4 is the consistent trouble spot.

Pro tip: Don’t replace the injector just because you see P0204. Verify power, ground, and control signal first. A $10 wiring repair often fixes what looks like a $200 injector problem.

Possible Fixes & Repair Costs

Once you’ve pinpointed the cause, repairs for P0204 are usually straightforward. Common fixes include replacing injector #4, repairing or replacing the injector connector, fixing damaged wiring, or in rare cases, repairing or replacing the ECU. Typical costs vary: an injector replacement can run $150–$400 per injector parts and labor, wiring or connector repairs $80–$250 depending on access, and ECU repair or replacement $400–$1,200 or more. Labor rates, vehicle make, and parts quality all affect the final bill.

• Replace fuel injector #4 – New or quality remanufactured injector, plus sealing O-rings.
• Repair or replace injector connector – Install a new pigtail, solder and heat-shrink, reroute for strain relief.
• Fix damaged wiring – Splice in new wire sections, secure harness away from hot or sharp areas.
• Clean and treat terminals – Remove corrosion, tighten loose pins, use dielectric grease where appropriate.
• ECU repair or replacement – Only after confirming the driver circuit has failed and wiring is good.
• Clear codes and road test – Verify smooth operation, no misfires, and that P0204 does not return.

Can I Still Drive With P0204?

You can often drive a short distance with P0204, but it’s not recommended for long periods. A dead or intermittent injector on cylinder 4 means that cylinder may be running lean, rich, or not firing at all. That can cause rough running, loss of power, and overheating of the catalytic converter. If the check engine light is flashing, treat it as urgent: reduce load, avoid highway speeds, and get the vehicle inspected as soon as possible.

What Happens If You Ignore P0204?

Ignoring P0204 can lead to more expensive damage. Persistent misfires can overheat and melt the catalytic converter, wash down cylinder walls with fuel, dilute engine oil, and in extreme cases cause internal engine damage. What starts as a relatively simple injector or wiring repair can turn into a multi-thousand-dollar exhaust or engine job if you keep driving with a severe misfire.

Last updated: February 13, 2026

Vehicles Commonly Affected by P0204

P0204 can show up on almost any OBD-II vehicle, but it’s especially common on high-mileage four-cylinder and V6 engines. I often see it on Ford (Focus, Fusion, F-150), GM/Chevrolet (Malibu, Silverado, Equinox), Chrysler/Dodge/Jeep (Caravan, Ram, Wrangler), Toyota (Corolla, Camry, RAV4), Honda (Civic, Accord, CR-V), Hyundai/Kia compacts and crossovers, and various European models like VW and BMW. Turbocharged engines and vehicles with tightly packed engine bays tend to have more heat-related wiring and connector issues.