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Mazda Protege 5 2002 - Troubleshooting PO300, P1125, and PO420 Codes: Random Misfire, Catalyst Efficiency, and Fuel Pressure Issues

Model: Mazda Protege 5 Fault Code: P1125 Posted: 2007-09-14 18:14

Hi there, I own a 2002 Mazda Protege 5 with a 2.0L engine. Recently, I've been experiencing intermittent running issues — the car runs smoothly for a while, then suddenly stutters and dies. After extensive checks, my OBD2 scanner consistently shows trouble codes PO300, P1125, and PO421. I’ve already replaced the EGR valve with no improvement. The current codes are PO300 (random misfire), P1125 (catalyst system efficiency below threshold), and PO421 (diesel-like exhaust emissions — catalyst system efficiency). I'm seeking advice on how to properly diagnose and resolve these issues. Specifically, I'd like to understand whether the misfires are related to fuel pressure regulation or if they stem from other engine components. Additionally, I want to know if the catalytic converter is still within warranty (8 years / 80,000 miles for original owners) and what steps I should take to verify exhaust system integrity. Any insights or diagnostic suggestions would be greatly appreciated.

Related fault codes
P1125
Comments (2)
Anonymous 2007-09-14 23:22

PO300 indicates a random misfire, which can stem from multiple sources. PO421 points to poor catalyst efficiency — this issue is typically covered under the manufacturer's 8-year or 80,000-mile warranty for original owners. P1125 (note: corrected from P1250 in original) relates to a problem with the Pressure Regulator Control (PRC) solenoid valve. The PRC manages vacuum supply to the fuel pressure regulator; failure here can lead to improper fuel mixture, causing misfires and poor combustion. Possible causes include faulty wiring, a malfunctioning ECU, a defective PRC unit, or a vacuum leak in the hose supplying vacuum to the PRC. A vacuum leak is also known to trigger PO300 codes due to incorrect air-fuel ratios. It's important to check for any leaks in the vacuum lines connected to the fuel pressure regulator and ensure all components are functioning correctly. Good luck with your diagnosis.

Anonymous 2007-09-15 12:58

I believe you may be dealing with two distinct issues, and a systematic approach is needed. First, verify whether the misfire is directly caused by an improperly regulating fuel pressure system — if the PRC is failing, it could lead to inconsistent air-fuel mixtures, resulting in pre-ignition or detonation within the combustion chamber. This abnormal mixture might also negatively affect catalytic converter performance, which would explain the PO421 and P1125 codes. However, it's possible that the second oxygen sensor is malfunctioning independently, or the catalyst itself has degraded due to age or exposure. I recommend inspecting both the catalytic converter and the exhaust system components near the converter — look for leaks, damaged pipes, broken catalytic converters, or cracked exhaust tubes. These physical issues can cause elevated readings from the second oxygen sensor, leading to false emissions codes. A thorough visual inspection combined with a proper diagnostic scan will help determine whether these are separate problems or part of one underlying issue.