Injector Circuit/Open Cylinder 3
The P0203 code indicates that the Powertrain Control Module (PCM) has detected a malfunction, specifically an open circuit or electrical fault, in the fuel injector circuit for cylinder number 3. The PCM sets this code when it monitors the voltage or resistance of the injector circuit and finds it falls outside the manufacturer's expected operating range.
This code requires immediate attention because driving with a disabled cylinder can cause severe drivability issues and potentially damage the catalytic converter if unburned fuel enters the exhaust.
Common symptoms
- Illuminated or flashing Check Engine Light
- Engine misfire resulting in a rough idle
- Noticeable lack of engine power during acceleration
- Decreased fuel economy
- Engine stalling or severe hesitation
Likely causes
- Faulty or failed fuel injector on cylinder 3
- Damaged, broken, or shorted wiring in the cylinder 3 injector circuit
- Loose, disconnected, or corroded electrical connector at the cylinder 3 fuel injector
- Open circuit in the main fuel injector wiring harness
- Faulty Powertrain Control Module (PCM) or internal injector driver (rare)
What to check before replacing parts
Visually inspect the wiring and electrical connector at the cylinder 3 fuel injector.
Look for: Look for broken wires, melted insulation, pushed-out pins, or a loose/corroded connector that could cause an open circuit.
Listen to the cylinder 3 injector with an automotive stethoscope while the engine is running.
Look for: Listen for a steady, rhythmic clicking sound. Silence indicates the injector is not physically firing, pointing to an electrical or internal mechanical failure.
Test the cylinder 3 fuel injector resistance using a digital multimeter.
Look for: Disconnect the injector and measure across its terminals. A reading outside the manufacturer's specified range (often 10-18 ohms) or an 'OL' (open loop) reading indicates a failed internal injector coil.
Check for proper supply voltage at the injector connector.
Look for: With the key in the ON position, probe the power feed wire at the disconnected harness. You should see battery voltage (around 12V); a lack of voltage indicates a blown fuse, bad relay, or broken supply wire.
Test the ground pulse signal from the PCM using a noid light.
Look for: Plug a noid light into the injector harness connector and crank the engine. If the light does not flash rapidly, the PCM is not sending the ground pulse, indicating a wiring issue between the injector and PCM, or a faulty PCM driver.
This page explains the generic P0203 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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