OBD-II Code P0203 — What It Means & How to Fix It

What Does This Error Mean?

P0203 means there is an electrical fault in the fuel injector circuit for cylinder 3. The PCM detected that it cannot properly control the injector on cylinder 3. This typically causes a cylinder 3 misfire, rough idle, shaking, and loss of power. The injector, its wiring, or a connector are the most likely causes.

Affected Models

All vehicles 1996+
Common in Toyota Camry and Corolla four-cylinders
Common in Ford Mustang and F-150 V6/V8
Common in Honda Accord and Civic
Common in GM 3.6L V6 engines

Common Causes

Failed fuel injector on cylinder 3 with an open or shorted coil
Corroded or loose wiring connector at the cylinder 3 injector
Chafed or broken wire in the injector harness between connector and PCM
Injector driver circuit failure in the PCM (rare)
Severely clogged injector causing abnormal resistance readings

How to Fix It

Find cylinder 3 using your vehicle's cylinder layout diagram. On a four-cylinder it's the third cylinder from the front. On a V6 or V8, the numbering varies. Inspect the wiring connector for corrosion or damage.

Always confirm cylinder numbering from the owner's manual or a trusted online source before proceeding.
Unplug the cylinder 3 injector connector and test resistance with a multimeter. Normal is typically 11-17 ohms. Zero ohms means a short; infinite ohms means the coil inside is broken.

If you don't have specs, test an adjacent injector as a comparison baseline.
Check the wiring from the connector back toward the engine harness and PCM. Look for damage near hot exhaust components, sharp metal edges, or tight bends that can crack wire insulation over time.

Repair any damaged sections with quality solder connectors or heat-shrink butt connectors — not electrical tape alone.
Swap the cylinder 3 injector with a known-good injector from another cylinder. Clear the code and drive for 20+ miles. If the misfire follows the injector to its new location, the injector is the problem.

If the code and misfire stay on cylinder 3 after the swap, the problem is in the circuit — wiring or PCM.
Replace the faulty injector using a new O-ring lightly coated with clean engine oil. Reinstall carefully, clear all codes, and perform a test drive monitoring cylinder 3 misfire counts on a scan tool.

Misfire counts should drop to zero after repair. If they continue, check compression and spark plug on cylinder 3.

When to Call a Professional

If basic wiring and connector checks don't resolve the problem, seek professional help. A technician can perform a noid light test to confirm the PCM is sending a pulse signal. They can also scope the injector waveform to pinpoint the fault precisely. Diagnosis typically costs $80-$130. Injector replacement runs $150-$400 depending on the vehicle.

Frequently Asked Questions

P0203 and P0303 appeared at the same time — are they related?

Yes, they almost always go together. P0203 is the injector circuit fault. P0303 is the misfire that results from it. Fix the injector circuit and both codes should clear. Always address the circuit code first.

Could a bad spark plug cause P0203?

No, P0203 is specific to the injector circuit — spark plugs have their own codes. However, a bad spark plug can also cause a cylinder 3 misfire. If you fix the injector circuit but still have a misfire, check the spark plug and coil on cylinder 3 next.

How do I know if my PCM is the problem?

PCM failures are rare. First confirm the injector and all wiring are good. If everything checks out but the cylinder 3 injector circuit still has no signal, a technician can test the PCM driver circuit directly. Never replace a PCM without ruling out wiring issues first.