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Mazda Protege 2001 PO303 Code: Open Loop Running at 186°F - How to Diagnose and Fix

Model: Mazda Protege Fault Code: P0303 Posted: 2007-05-30 21:31

I'm experiencing a persistent issue with my 2001 Mazda Protege MP3. The symptoms began with a rough idle and noticeable loss of engine power. The check engine light (MIL) activated once, then turned off before I could retrieve the code. On subsequent drives, the fault code displayed was PO303, indicating a misfire in cylinder 3. The vehicle is currently running in open loop mode with a calculated load of 87.4%, engine coolant temperature at 186°F, STFT Bank 1 at 0.0%, and engine RPM at 5701. Vehicle speed data is not available. I've researched common issues for this model year and found that problems often stem from spark plugs, coil packs, or plug wires. However, the fact that the engine is running in open loop at such a high coolant temperature (186°F) raises concern about a failing oxygen sensor—particularly the upstream sensor on Bank 1. Given the vehicle's history of over 12,000 miles driven under hard use (race car conditions), I'm looking for guidance on where to begin troubleshooting. What are the most effective steps to diagnose and resolve this issue? Any specific tools or procedures recommended?

Related fault codes
P0303
Comments (2)
Anonymous 2007-05-31 07:03

PO303 is a misfire code indicating cylinder 3 is not firing properly. If you're using your car for high-performance driving, it's critical to diagnose and fix this issue promptly to avoid further damage. From OBD-II knowledge (though based on memory), the engine typically enters open loop mode at full throttle—especially during sustained wide-open-throttle operation. This is normal behavior under such conditions. However, if the vehicle remains in open loop after releasing the throttle or returning to normal driving, it may indicate a sensor issue. I recommend testing whether the system returns to closed-loop mode when you gently release the accelerator. If it doesn't transition back, this could point to an oxygen sensor failure—particularly the upstream O2 sensor on Bank 1. This would be a strong indicator for further inspection or replacement.

Anonymous 2007-05-31 07:07

Yes, many vehicles enter open loop mode at wide-open throttle (WOT), especially during prolonged high-load driving. The 2001 Mazda Protege is known to exhibit this behavior under extreme conditions, so it's not unusual—but sustained open-loop operation should be investigated further.