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P0300 P0175 P0172 Troubleshooting for 1996 Chevrolet Sierra 5.7L Engine

Model: 1996 Sierra 5.7 Fault Code: P0172 Posted: 2012-01-08 15:11

I've been reading the forum for some time and now need help with a persistent issue on my 1996 Chevrolet Sierra 1500 with a 5.7L V8 engine. After initially thinking it just needed a tune-up, I'm now seeing trouble codes P0300 (random misfire), P0175 (rich condition bank 2), and P0172 (rich condition bank 1). I've already replaced the distributor cap, rotor, spark plugs, cleaned the throttle body, and used injector cleaner in the fuel tank. I have a OBD-II scanner with freeze frame data, which shows the following: Fuel System Control Using HO2S - Bank 1, Fuel System Control Using HO2S - Bank 2, Load 3.5%, Coolant Temperature 89°F, Fan Trigger 1: -12.5°C (LT), Fan Trigger 2: -12.5°C (LT), MAP pressure at 33 kPa, RPM at 717, Vehicle Speed at 19 mph (not moving), MAF reading at 7.930 g/s. I'm wondering if this data is useful for diagnosing the problem. Specifically, is a MAP pressure of 33 kPa considered high? Any insights would be greatly appreciated. Thanks, Mike.

Related fault codes
P0172P0175P0300
Comments (3)
Anonymous 2012-01-09 06:03

Most likely cause: a faulty fuel pressure regulator. The consistent misfire combined with rich conditions on both banks strongly suggests the engine is receiving excessive fuel pressure.

Anonymous 2012-02-11 14:58

Sorry for the delayed reply. I recently replaced the fuel pressure regulator with a Bosch unit from Injector Warehouse — cost was $200 minus a $50 core charge. Since the replacement, the truck runs much smoother and no misfire or rich code issues have returned.

Anonymous 2012-02-11 18:21

Great to hear! Thanks for sharing your solution — it's helpful for others facing similar issues with older Sierra models.