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2000 GMC Silverado 5.3L P0171 P0174 P0420 Fault: Diagnosis & Solutions After Intake Gasket Replacement

Model: 2000 Silverado Fault Code: P0171 Posted: 2013-02-05 16:01

My 2000 GMC Silverado with a 5.3L V8 engine is currently showing persistent OBD-II fault codes: P0171 (System Too Lean Bank 1), P0174 (System Too Lean Bank 2), and P0420 (Catalyst Efficiency Below Threshold). The vehicle has no aftermarket modifications except for a Flowmaster exhaust system. A service bulletin previously prompted the replacement of the intake manifold gasket, which was completed — however, the codes persist. I've already performed several repairs including: - Replacement of the ACDelco EGR valve - Installation of four Bosch oxygen sensors (purchased at AutoZone) - Two replacements of the Mass Air Flow (MAF) sensor - Intake manifold gasket replacement - New fuel filter installation - Fuel system cleaning - Cleaning of the Idle Air Control (IAC) valve The engine produces a consistent hissing noise on the passenger side, which has been present since around 193,000 miles. Spark plugs were replaced previously, though I'm uncertain about the exact type used. I do not currently own a fuel pressure tester due to concerns about handling it incorrectly — but I have a reliable OBD-II scanner and collected diagnostic data during a drive at a red light: - RPM: 527 - Load Calculation: 2.0% - MAF (lbs/m³): 0.67 - MAP (inHg): 11.5 - IAT: 88°F - Ignition Advance: 21.5° - Second Air System: Atmospheric - ST FTRM1/ST FTRM2: -0.8 - LT FTRM1/LT FTRM2: 12.5 / 11.7 - Fuel System 1 & 2: Closed (CLSD) - O2S11: 0.110 (fluctuates between ~0.110 and 0.900) - O2S12: 0.680 - O2S21: 0.625 - O2S22: 0.645 Symptoms include: - Difficult engine starts - Idle fluctuation (ranging from 200 to 1300 RPM) - Engine stalling - Once, exhaust smelled like gasoline - Surging during acceleration I'm seeking advice on potential root causes. Given the persistent lean codes and catalytic converter inefficiency, could this be related to a cracked or warped plastic intake manifold? I've heard that vacuum leaks can cause similar symptoms — especially around the intake manifold area. The hissing noise may indicate such a leak. I plan to replace both catalytic converters on Thursday, but I'd like to understand if there's a more fundamental issue before proceeding with costly repairs.

Related fault codes
P0171P0174P0420
Comments (5)
Anonymous 2013-02-06 09:17

Fuel pressure testing is essential — it’s not difficult and can be done with a simple gauge (like those sold at Harbor Freight for around $30). The O2 sensor reading on Bank 1 Sensor 1 appears to be stable at 0.680 V, which suggests the upstream sensor may be functioning correctly. However, I don’t see Engine Coolant Temperature (ECT) in your data — could you please share that? Also, your MAP reading is low; typical idle MAP values are around 14–18 inches of mercury. Have you tried using carb cleaner to test for vacuum leaks? This is a standard and inexpensive diagnostic method.

Anonymous 2013-02-06 11:16

I plan to get a fuel pressure gauge soon — I saw one on YouTube at Harbor Freight priced around $30. What exactly is ECT? I’m not very experienced with vehicle repairs. Regarding MAP, what should the normal idle range be? I’ve heard stories about spraying carb cleaner and causing fires — would water work instead? The O2 sensors do fluctuate between 0.110V and 0.890V. Could it be that the plastic intake manifold has cracked or warped, especially since it was replaced only recently?

Anonymous 2013-02-06 12:38

ECT stands for Engine Coolant Temperature — this is a critical value to monitor. For fuel pressure testing, look into Sears or similar retailers — prices may surprise you. At idle, vacuum levels are typically highest. A good baseline is around 18 inHg — I’m not quoting numbers without double-checking, but that’s where most diagnostics start. The carb cleaner 'trick' is a classic and reliable method — it's not about using starter fluid or letting liquid pool; instead, spray directly onto suspected leak points (like the intake manifold, MAF sensor, vacuum lines, PCV valve, and purge valve). A change in engine sound or RPM will indicate a leak. For intake manifold checks, especially on plastic manifolds, warping is part of known service bulletins. I recommend spraying air tubing after the MAF to test for leaks at key junctions.

Anonymous 2013-02-06 12:52

ECT is currently reading 198°F. Both upstream oxygen sensors do fluctuate between 0.110V and 0.890V. I’ve also noticed a hissing noise near the intake manifold — I’m wondering if the mechanic who replaced the gasket performed a warp test. The repair was not done at a dealership, so it may have been less thorough. Additionally, when I quickly rev the engine, I hear a noticeable 'sucking' sound — this could indicate air ingestion due to a vacuum leak.

Anonymous 2013-02-06 13:50

Try spraying suspected vacuum leak areas with carb cleaner — it's an easy, cost-effective test. Just apply directly to potential leak points and listen for changes in engine behavior or RPM.