1997 Mazda 626 ES (6-cylinder) with P0400 Code: Troubleshooting Before Replacing EGR

Model: 1997 Mazda 626 ES (6 cyl) Fault Code: P0400 Posted: 2005-10-31 11:49

My check engine light is on, and an OBD2 scan has revealed code P0400. I'm wondering if there's anything I can try before replacing the EGR valve. The car runs fine under normal conditions, though I do notice occasional engine knocking when climbing hills—this makes me question whether the issue is directly related to the EGR system. Idle performance is generally acceptable but sometimes feels rough (not consistently). Mileage has declined noticeably. Additionally, I've found oil in the spark plug well, which may indicate a failing valve cover gasket. Could this oil leak be contributing to or linked with the P0400 code? I'm looking for advice on diagnosing the root cause of the P0400 fault without immediately replacing the EGR system.