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1998 Chevy Express 4.3 Vortec P0171 and P0174 OBD2 Code - Check Engine Light Returning After Full Component Replacement

Model: 98 Chevy Express 4.3 Vortec Fault Code: P0171 Posted: 2016-03-15 18:12

My 1998 Chevrolet Express 4.3L Vortec engine is showing persistent P0171 (System Too Lean) and P0174 (System Too Rich) OBD2 codes. The check engine light comes back on repeatedly even after replacing all suspected components: fuel injectors, MAF sensor (cleaned), MAP sensor, air intake hoses, air filter, PCV system, and three oxygen sensors. I've also replaced spark plugs, wires, cap, and rotor. The vehicle runs smoothly with no hesitation or performance issues. I have confirmed there are no visible leaks in the air intake system, and all vacuum lines appear intact. Despite these efforts, the codes persist. I'm at a loss as to what could be causing this issue. Any insights into potential underlying causes would be greatly appreciated.

Related fault codes
P0171P0174
Comments (4)
Anonymous 2016-03-16 06:34

Could it be low fuel pressure? This is a common cause of lean conditions in the 4.3L Vortec engine.

Anonymous 2016-03-16 10:47

A stuck EGR valve or a damaged catalyst inside the EGR valve that keeps it open could also cause these codes. As autojoe suggested, checking fuel pressure is a good next step. I've seen similar issues where there's a vacuum leak near the brake booster connection—sometimes due to an worn O-ring in the fitting.

Anonymous 2016-03-17 18:33

I recently installed a new hose on my PCV line and tested it by blowing into it with a cigar. I discovered a small vacuum leak there—it was causing the lean/rich imbalance despite all other components being replaced.

Anonymous 2016-03-17 18:34

Thanks to everyone who offered helpful suggestions—this has been very informative!