1996 Chevrolet 5.7L Engine Random Misfire and Running Lean: P0300, P0132, P0134, P0171 Diagnosis and Fixes

DTC P0132: HO2S Circuit High Voltage - Bank 1 Sensor 1 DTC P0134: HO2S Circuit Insufficient Activity - Bank 1 Sensor 1 DTC P0171: Fuel Trim System Running Lean - Bank 1 The B1S1 sensor appears to be malfunctioning. Check wiring for pinches, opens, or shorts—especially in the heater circuit. For P0171 (running lean), inspect for vacuum leaks in Bank 1. This issue often returns after other fixes are attempted. Note: Did you use AC Delco components? Specifically, spark plugs, as they can affect performance on older GM engines.

No, I didn’t use AC Delco parts—my spark plugs were OEM before. The issue still persists. I’ve already inspected the wiring harness and found no visible damage. I’m now trying SeaFoam in the fuel system to clean injectors and will check the distributor gear next. Any additional tips would be helpful.

I haven’t found a definitive solution yet. After checking all wiring, I still have the same symptoms. The engine runs fine after warming up but feels sluggish during cold starts—like it lacks power and backfires at startup. I’m not sure how to explain this behavior clearly. It only happens when starting from cold; once warmed, it runs smoothly. I suspect a fuel delivery issue.

On any GM truck with lean DTCs like P0171, fuel pressure and fuel flow must be checked first—this is a fundamental diagnostic step.

Agreed—fuel pressure should be the starting point. What PSI are you reading? Without a scan tool, we’re relying on workarounds. Your B1S1 sensor readings are inconsistent: P0132 shows high voltage, P0134 indicates fixed mid-range voltage. This discrepancy suggests the sensor may not be OEM—perhaps a non-OEM or after-market unit installed incorrectly (e.g., self-wired). Is this possible? If so, it could explain many of these symptoms.

I just learned that I had a Bosch O2 sensor. When unplugged, the code doesn’t change—so it’s not the sensor itself. Wiring is intact with no pinches or breaks. The issue is intermittent: one day it runs fine, then after a few seconds of startup, it feels boggy and backfires. After warming up, it runs smoothly. I can't explain this behavior clearly. Only during cold starts does it feel weak and lack power—once warm, everything improves. I suspect internal fuel system issues or intake manifold leaks.

Fuel pressure bounces between 55 and 60 psi when the key is off—it drops quickly, suggesting a significant leak in the system.

Correction: I meant 50 psi.

I keep forgetting to add SeaFoam to the fuel system—will do that again next time.

Fuel Pump Pressure: 60–66 psi when key is ON and engine OFF. This reading is too low. Rapid pressure drop (leak-down) indicates a problem with the intake manifold or internal components—likely in the upper intake manifold area. Recommendation: - Replace the fuel pump with an OEM unit. - Inspect for leaks inside the upper intake manifold—possible issues include a stuck injector, faulty regulator, or cracked tube. After fixing these, re-evaluate O2 sensor readings. Fuel pressure must be corrected first.

One key question: Why does this only happen during cold starts and resolve once the engine warms up? This behavior suggests a fuel delivery issue at startup, possibly due to low pressure or poor atomization in cold conditions.

To clarify: - Fuel pressure should be between 50–60 psi. Your current reading is too low. - Significant leak-down indicates internal intake manifold issues—components such as injectors, regulator, or tubes are likely at fault. Test procedure: 1. Use a 10-amp jumper to apply B+ to the fuel pump test terminal when key is ON. 2. Once pressure reaches max, pinch off the flexible fuel feed hose with vise grips and remove the jumper—observe if pressure drops. 3. Repeat by pinching only the return line. 4. Pinch both lines simultaneously—check if pressure holds. If pressure drops when both lines are pinched, upper intake manifold must be removed for inspection. Do you smell fuel? If pressure stops dropping after pinching feed line only, suspect a faulty fuel pump check valve or torn fuel pulsator/damper in the tank. Ensure steady B+ to the fuel pump and perform a ground-to-ground voltage drop test on the ground wire (less than 0.100 mV).

Summary: - Fuel pressure is too low—between 50–60 psi, which is below specification. - Significant fuel leak-down points to internal components in the upper intake manifold. - O2 sensor readings must be corrected after fixing fuel pressure. - Replace the fuel pump with an OEM unit for reliable performance and consistent cold-start behavior.