Pontiac Grand Prix GTP Cylinder 4 Misfire and Injector Malfunction - P0304 & P0204 Codes

What year and engine type? (3.1L or 3.8L?) The 8th character of the VIN will confirm this. Also, please share the exact fault codes you're seeing—P0304 and P0204 are key here. Misfires and injector circuit faults often stem from wiring or connector issues. Injector connectors are known to fail due to poor connections or wire breaks within inches of the plug—available at most auto parts stores. Use a digital multimeter (DVOM) to test the signal wire from the injector to the PCM by gently shaking the harness to simulate engine vibrations.

With the key on and engine off, should there be 12 volts present on one of the injector wires? I'll need to wait for the issue to occur again to capture the actual codes, but this voltage check is essential for diagnosing the circuit.

When the key is on and engine off (KOEO), both the signal wire and power wire should have 12 volts when plugged in. If unplugged, only the power wire should show 12 volts—this helps verify proper wiring and grounding.

The fault codes I'm seeing are P0304 (cylinder 4 misfire) and P0204 (injector circuit malfunction for cylinder 4). This strongly suggests a problem with the injector or its wiring in cylinder 4.

P0304 indicates cylinder 4 is misfiring, while P0204 points to an issue in the injector circuit for that same cylinder. First, check the wiring and connector at cylinder 4. Is there a consistent 12-volt supply? Use a 'noid light' to test the injector signal—does it show steady on (indicating proper operation), or does it flash rapidly like other injectors? A fast flash may indicate poor grounding or ECU-related issues.

I haven't tested voltage yet, but when I used a tester during rough running, the signal appeared to be working—though not steady. It flashed quickly, similar to other injectors. The brightness of the noid light on cylinder 4 is consistent with others. This makes me suspect either a failing ECU or a bad ground connection. It's odd that this only happens when warm—bad wires or injectors usually show symptoms at cold start too.

Injector connectors are known to fail—both in the plug contacts and wiring near the connector. New connectors are available at auto parts stores. Try gently shaking or wiggling the harness while the engine is off to simulate vibration. Ensure the connector is fully seated and tight.

I’ve already tested the harness by shaking it and confirmed no improvement. I also used a noid light between cylinder 4’s injector and spark plug—its behavior matches that of other cylinders. I tried swapping in a known-good injector from another cylinder, but the issue persists. This suggests either an ECU problem (such as a failing internal switch or ground) or a deeper wiring/ground fault. Some users suggest a failing ECU could cause erratic firing patterns due to poor grounding.

While a failing ECU is possible, replacing the PCM without confirming the root cause is not recommended. Let’s start with basics: confirm engine type (3.1L vs 3.8L) using the 8th character of the VIN. Also, identify cylinder 4's position—front or rear bank? The user has confirmed it's a stock 3.8L supercharged engine with cylinder 4 in the front bank, center position. This helps narrow down potential issues.

All components are original and stock: 3.8L supercharged engine, AC Delco spark plugs, paper air filter—no modifications or aftermarket parts installed.

I found a likely reference issue here: [https://arrc.ebscohost.com/statics/gm/im/194244.png](https://arrc.ebscohost.com/statics/gm/im/194244.png) and [http://www.wellsve.com/custom_searches/car_23.pdf](http://www.wellsve.com/custom_searches/car_23.pdf). These resources highlight common wiring and connector failure points in the 3.8L GTP, especially around cylinder 4's injector circuit.

That appears to be a plausible issue—especially given the location of cylinder 4 and known connector vulnerabilities. I’ll go back through all connections and perform a full inspection before considering any ECU or PCM replacement.