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1998 Buick Lesabre P0203 and P0303 Codes: Injector Not Firing, Diagnosis Steps & PCM vs Wiring Troubleshooting

Model: 98 Buick Lesabre Fault Code: P0203 Posted: 2009-06-05 17:28

I recently purchased a 1998 Buick LeSabre Limited from the original owner, which had been driven over 124,000 miles with no prior issues and excellent engine performance. Suddenly, I began experiencing engine misfires and have received two diagnostic trouble codes: P0303 and P0203. After replacing the spark plugs and wires—both of which appeared to be original—I replaced the fuel injector as well, but the problem persists. I used an oscilloscope and a screwdriver to test the injector directly and found that it is not firing at all. When I disconnected the injector and connected it to my CYC 3 plug on the CYC 5 tester, the system still failed to trigger the injector. This indicates that the injector is not receiving the proper signal to fire. The CYC 3 plug shows voltage input, but there is no variation in voltage as the engine runs—this suggests a lack of pulse from the PCM. I’ve seen conflicting advice online about whether the issue lies with the camshaft position sensor (CMP) or the Powertrain Control Module (PCM). Given that P0203 specifically indicates an open or shorted condition on the injector circuit, it points strongly toward a wiring or electronic control module failure. I’m looking for clear, actionable steps to diagnose this issue without unnecessarily replacing parts. I’d prefer to identify the root cause before investing in expensive repairs such as a new PCM. Any insights into how to properly test the injector circuit from the PCM, check for proper pulse using a noid light or scan tool, and determine whether a faulty CMP or PCM is responsible would be greatly appreciated.

Related fault codes

P0203P0303

Comments (5)

Anonymous 2009-06-06 13:13

A failing PCM is unlikely in this case because the issue is isolated to one injector circuit. While possible, it's more probable that there's a wiring fault rather than a single driver failure within the PCM.

Anonymous 2009-06-06 14:10

If the wiring from the injector to the PCM is intact and shows continuity, then the issue likely lies with the PCM itself. A new PCM may require programming before it can function properly—this typically needs a dealer-level scan tool or access to GM’s official software platform.

Anonymous 2009-06-08 15:57

The sudden onset of symptoms is unusual—it was working fine one day and then failed the next. To test the injector circuit, use a noid light to check for pulse from the PCM's injector driver circuit. The CMP sensor is located on the front timing cover and provides input to the PCM for injector firing. If no pulse is detected, either the PCM has a faulty injector driver or the CMP signal is missing.

Anonymous 2009-06-08 18:39

P0203 specifically indicates an open or shorted condition in the injector circuit—this rules out a camshaft position sensor fault. If a noid light does not flash, it doesn’t mean the injector is defective; instead, it means there's no power or ground being delivered to the injector. The wire color should be GRN/BLK and connects to pin 73 on the blue connector at the PCM. A replacement PCM must be programmed by a dealer or someone with access to GM’s official programming tools and website.

Anonymous 2009-06-09 07:38

If continuity tests show no issues but the circuit still fails to deliver current, there may be high resistance in the wiring that prevents proper current flow. This can cause a 'complete' circuit without actual power delivery. A scan tool capable of actuating injectors should be used for accurate diagnosis. Before replacing a PCM, ensure you’ve properly read the wiring diagram and tested with a digital multimeter (DVM) or noid light. If in doubt, take the vehicle to a dealer for professional diagnostics to avoid unnecessary expenses.