Erroneous P0336 Code on 2003 Chevrolet Bonneville: Diagnosis and ECU Flash Concerns

Have you performed a CKP relearn? Is there a P1374 code present? Have you examined the CKP waveform using an oscilloscope for both 3x and 18x signals—only one may be faulty. There's also a known note about EMI/RFI interference from secondary ignition wires or alternator. *Oscilloscope required for accurate diagnosis.*

Thank you for the response. I believe a relearn was performed—likely involving full-throttle operation—as that's what they described. There is no P1374 code. I don't think their diagnostic process includes an oscilloscope. My code reader shows two instances of P0336 most times, but once I saw only one instance with the MIL off; it lit up immediately after the second occurrence. The light typically comes on during the second engine cycle, and there's a slight stumble sometimes—almost like a transmission shift. My main concern is that paying for an ECU flash may not resolve the root cause. Thanks again for your insight.

The presence of multiple P0336 entries (either active or pending) strongly suggests the issue lies in a recurring sensor test failure—this is consistent with a persistent fault condition. A basic code reader lacks the depth to detect such nuances; a professional scanner with freeze frame data analysis would be far more informative. Freeze frame data might reveal critical clues about when and under what conditions the fault occurs.

The last time P0336 was set, the vehicle was idling. Freeze frame data: fuel system—open loop; load percentage—3.9%; engine temp—149°F; short term fuel trim—0.0%; long term fuel trim—-3.9%; MAP—13.6 psi; RPM—743; VSS—6; MAF—0.948 g/s; throttle position—0%. The code may have been triggered by the intermittent stumble, which sometimes feels like a transmission shift. Also, P0336 typically doesn't set during the first engine cycle—it usually appears on the second start. The readiness monitors show all systems as complete except catalytic and evaporative systems, likely due to insufficient driving cycles.

After reviewing additional resources, I should note that my tachometer has been very erratic—spiking or dropping to zero intermittently. Initially, I thought it was a mechanical sticking issue, but now I'm wondering if this could be related to sensor signal disruption.

The PCM cannot be reprogrammed (flashed) without an official GM technical service bulletin (TSB) or product instruction. Reprogramming must occur via the manufacturer’s website and only if the PCM is confirmed faulty—verified by checking input signals, power, and ground connections. If the issue lies in signal transmission from the ignition control module (ICM) to the PCM, a new PCM may be required. It's essential to verify the 18x signal at the ignition module using an oscilloscope and confirm crank sensor wire integrity. A properly ASE-certified shop should perform this diagnostic—especially if the issue is intermittent. Testing must occur while symptoms are present.

I'm not concerned about the code recurring—it always sets on the second engine start. Based on your input, it seems my only viable option is to visit a GM dealership, which I’m hesitant to do. While many repair shops advertise ASE-certified technicians, finding one with expertise in advanced computer diagnostics—especially for older vehicles—is extremely difficult.

Check the harmonic balancer. On several 2003 Bonneville models with 3.8L engines, the rubber mounting has degraded and can separate over time. This could interfere with crankshaft sensor operation or damage the reluctor ring.

To everyone who responded—thank you so much! I have an appointment Monday with a repair shop that assured me their ASE-certified technician can identify the root cause and has all necessary diagnostic tools. I’ll update this thread after the diagnosis and final fix. Thanks again for your valuable input—I truly appreciate forums like this providing accessible, expert-level advice.

After paying a diagnostic fee, I was told that the issue is with the ignition control module (ICM). The crankshaft sensor sends signals to the ICM, but those signals aren't being relayed to the PCM—this triggers P0336. Is it possible for the vehicle to run normally if the PCM isn’t receiving signals from the ICM?

I wasn’t sure if this model had an ICM, but on older vehicles, the camshaft sensor magnet was located on the cam sprocket. The ICM was notoriously difficult to diagnose—common issues included poor grounding or a broken mounting bracket not properly connected to the engine. In some cases, the vehicle could run without full signal input.

The exact diagnostic note recommending ICM replacement states: 'Medium resolution speed signal not being relayed from ICM to PCM—RECOMMEND.'