2002 Chevy Cavalier L61 2.2 Ecotec PO171 Code: Diagnosis and Fixes After Exhaust & Intake Mods

mattmn07 wrote: I'm currently troubleshooting a persistent PO171 code on my 2002 Chevrolet Cavalier equipped with the L61 2.0L Ecotec engine. I've installed several performance modifications, including a custom header, catalytic converter, exhaust system, intake manifold, and pulley setup. After previously replacing the oxygen sensor (which was faulty), I also replaced the fuel filter—yet the PO171 code persists. I've already cleaned the Mass Air Flow (MAF) sensor, but no improvement has been observed. The issue appears to be a lean air-fuel mixture, which is commonly linked to vacuum leaks or incorrect airflow readings. Given my modifications, I suspect that one of the installed components may be introducing an unintended leak downstream from the MAF sensor. I'm seeking advice on how to properly diagnose and resolve this issue. Check for a vacuum leak downstream from the MAF sensor, and also verify fuel pressure—either of these could cause the lean condition reported by PO171.

I performed a fuel pressure test and found it reading normally at 58 psi with or without vacuum. However, when using a vacuum pump to simulate engine load, the pressure dropped from 58 psi to 51 psi. This indicates that the intake system is not generating sufficient vacuum, which may point to an airflow restriction or a leak in the intake path.

That's likely your issue—there's a vacuum leak somewhere in one of the modified components (such as the header, intake, or ducting). Unless the vehicle runs noticeably rough under load, fuel pressure readings should not result in a lean condition. The duct housing the MAF sensor must be completely sealed from the sensor to the intake manifold to ensure accurate airflow measurement. A vacuum leak here will directly cause a PO171 code due to incorrect air-fuel ratio detection.