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2000 Chevrolet S10 P0171 & P0172: Rich and Lean Codes Explained - Fuel, MAF, or Sensor Issues?

Model: 2000 chevrolet s10 Fault Code: P0171 Posted: 2014-06-10 01:59

2000 Chevrolet S10 LS Extended Cab Pickup, 3-Door, 2.2L 2190CC Naturally Aspirated Engine (134 cu.in., Flex OHV). Currently at 121,000 miles. The vehicle has started exhibiting intermittent stalling during idle and difficulty cranking, occurring twice in the past week. This issue began shortly after a Seafoam treatment was performed—half the bottle added to the fuel tank, the other half sucked through the brake vacuum hose—and a spray of Seafoam (similar to Deep Creep) applied to the engine top. Prior to the treatment, the truck felt slightly sluggish, which led me to perform the cleaning. After checking all vacuum hoses and ensuring they were properly connected, I sprayed WD-40 on the connection point at the brake vacuum line and reinstalled it securely. Upon clearing the check engine light, I drove about one mile and then checked for codes again—both P0171 (too lean) and P0172 (system too rich) returned. This raises a key question: Was the vacuum connection faulty? Or is the issue more complex? If the codes persist after addressing the vacuum system, what should I check next? Possible causes include: - A dirty or clogged MAF sensor — where exactly is it located? - Fuel filter blockage - Faulty oxygen sensors (especially upstream and downstream) - Leaking intake manifold or EGR valve - Contaminated fuel injectors - Poor engine temperature input from ECT/ECT sensor I’ve heard that Seafoam can sometimes cause issues by clogging the catalytic converter or affecting fuel system components. Does this apply? Additionally, I notice a significant drop in power under load and hard starting—could these symptoms point to deeper fuel delivery or sensor problems? Any guidance on diagnosing P0171/P0172 in a 2000 S10 would be greatly appreciated. Thanks for reading, Chris

Related fault codes
P0171P0172
Comments (2)
Anonymous 2014-06-10 08:47

A dirty or restricted fuel filter is a common cause of P0171. Note that one code may appear in history and another as current—using a scan tool with live data stream is essential to evaluate sensor readings. Check the short-term and long-term fuel trims across different RPM ranges to determine if the PCM is adding excessive fuel or reducing it. I've seen cases where Seafoam causes clogs, especially in catalytic converters. If you're experiencing low power under load or hard starts, consider these factors: ECT/Map sensor input errors, dirty injectors, contaminated O2 sensors. Try a Techron Total Fuel System Cleaner (available at AutoZone) and add it to the fuel tank. Specifically, check if your pre-cat O2 sensor signal is fluctuating between 0.02 and 0.08 volts at 2000 RPM—this could indicate a faulty or misbehaving sensor.

Anonymous 2017-06-17 12:52

I experienced the same P0171 issue on my 1998 S10 with a 2.2L engine. I replaced all sensors except the O2 sensors, swapped out the EGR valve and EGR tube. The truck ran fine when cold but would trigger a P0171 code once warmed up. After checking intake and vacuum lines (all intact), I measured fuel pressure at 41 psi—normal—and replaced the fuel filter; the check engine light still came back on with the same code. Eventually, replacing both upstream and downstream O2 sensors resolved the issue. The truck has run smoothly ever since. This experience may help others facing P0171 in their S10s.