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Mazda 626 2.0L - P0303 and P0172 Codes: Intermittent Rough Idle & Acceleration Hesitation

Model: mazda 626 Fault Code: P0172 Posted: 2006-12-10 15:39

I'm experiencing both P0303 (cylinder 3 misfire) and P0172 (rich air-fuel mixture) codes on my Mazda 626 2.0L engine. Symptoms include intermittent rough idle and a growing hesitation during acceleration. I've already checked fuel pressure, fuel pressure regulator, vacuum lines, and the intake system — all appear to be functioning normally. Fuel pressure remains stable at 20 psi for over an hour, which suggests no significant injector leaks. However, I noticed something unusual: the fuel pump does not activate when the engine is turned off (KOEO) — there's instant fuel pressure upon starting, but no activation during idle. I've also tested the upper oxygen sensor; readings initially drop to very low levels for 10 minutes before stabilizing between .1 and .9. I cleaned both the MAF sensor and throttle body using Sea Foam, added a can of injector cleaner to the fuel tank, and replaced the spark plugs — all with no improvement. The wiring and spark plug resistance are within normal ranges. My question is: Can a faulty oxygen sensor cause these symptoms? I'm looking for troubleshooting advice before replacing parts. Any insights would be greatly appreciated.

Related fault codes
P0172P0303
Comments (4)
Anonymous 2006-12-11 09:38

Could the P0303 misfire code be related to fuel delivery issues? What's your current fuel pressure reading at idle and under load?

Anonymous 2006-12-11 23:23

The issue might stem from a faulty fuel pressure regulator. This part costs around $170. If you have Bosch spark plugs, they are known to be problematic in Mazda engines. The root cause of the misfire is likely low fuel pressure — perform a simple test: disconnect the vacuum line from the fuel pressure regulator and check if fuel flows into it. If fuel is present, the regulator is defective and needs replacement.

Anonymous 2006-12-12 12:10

What does 'behaving normally' mean in terms of actual fuel pressure? Can you provide a specific reading at idle and under load?

Anonymous 2007-01-02 18:19

Jeff Compton wrote: What does 'behaving normally' mean anyway? What's its pressure? I've encountered several defective new spark plugs over time — check the plug condition. Are they all black, or is only one showing signs of fouling?