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2001 Pontiac Grand Am P0303 P0137 P0420 Diagnosis and Repair Guide

Model: 2001 Pontiac Grand Am Fault Code: P0137 Posted: 2016-04-08 21:16

Hi everyone, I'm a new user here and am looking for help diagnosing issues with my 2001 Pontiac Grand Am. My engine runs roughly and feels unstable during operation. A diagnostic scan revealed three trouble codes: P0303 (cylinder 3 misfire), P0137 (downstream oxygen sensor failure), and P0420 (catalytic converter efficiency below threshold). I've attached a screenshot of the diagnostic results for reference. image.jpeg (641.17 KiB) Viewed 1302 times I'm not very experienced with car diagnostics, so I'd appreciate any advice on how to properly troubleshoot and fix these issues step by step.

Related fault codes
P0137P0303P0420
Comments (5)
Anonymous 2016-04-09 08:28

Likely a 2.4L engine? Start with addressing the P0303 misfire, as it's likely the root cause affecting the other codes. Check for spark at ignition components, inspect spark plugs (look for wet or black deposits), and then evaluate fuel injectors — verify injector pulse and check fuel pressure under KOEO conditions.

Anonymous 2016-04-09 10:57

Yes, it's a 2.4L engine. I was told my downstream O2 sensor is faulty — could replacing it resolve any of these issues on its own?

Anonymous 2016-04-09 11:09

Begin with resolving the P0303 code. Why do you believe the oxygen sensor is faulty?

Anonymous 2016-04-09 11:35

A few weeks ago, my vehicle was tested and it was reported that the O2 sensor was failing — however, I didn't replace it at the time. Previously, this was the only code appearing on the scan.

Anonymous 2016-04-09 12:05

That sounds concerning — how exactly was the oxygen sensor tested? Regardless, replacing the sensor may help with P0420, but it won't fix the P0303. And since P0303 is likely the root issue, fixing that first is essential to resolve all problems. Avoid using cheap aftermarket parts from discount stores — they often don’t meet quality standards and can lead to recurring issues. The most significant concern here remains the P0420 code; however, it should only be addressed after resolving the underlying misfire (P0303).