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2008 Silverado 1500 with P0455, P0172, and P0175 Codes: Causes and Diagnosis

Model: 08 Silverado 1500 Fault Code: P0172 Posted: 2018-09-27 16:23

My 2008 Chevrolet Silverado 1500 (4.8L V8, 4x4, not flexfuel) has recently started showing three diagnostic trouble codes simultaneously: P0455, P0172, and P0175. The P0455 code has been present for some time, while the P0172 and P0175 codes appeared today. The truck drives normally under regular conditions, but it hesitates when starting after refueling—this occurs only during the first crank following fueling, with no issues otherwise. I've already replaced the evaporative emissions canister vent valve and the kinked fuel filler neck hose to the tank, yet the issue persists. The vehicle still experiences difficulty getting gas in smoothly at times, sometimes shutting off mid-fill. I'm now concerned that these codes are related and need a clear diagnosis. I'm seeking input on whether these codes are interconnected—especially P0172 and P0175 (indicating a rich fuel condition) and P0455 (EVAP system leak). Could a vacuum leak, faulty MAF sensor, or purge line issue be responsible? I'd also appreciate guidance on how to test the purge valve by pinching off its vacuum hose at idle and under load to determine if excess fuel is being introduced into the intake.

Related fault codes
P0172P0175P0455
Comments (11)
Anonymous 2018-09-27 20:26

To better diagnose this, please share your MAF sensor readings and fuel trims at idle and under load. Also, provide freeze frame data for the P0172 and P0175 codes. P0172 and P0175 are commonly caused by vacuum leaks, issues with the Mass Airflow (MAF) sensor, or incorrect fuel delivery. P0455 typically indicates one of these conditions: - A loose or improperly sealed gas cap - A non-factory or incompatible gas cap - A leak or damage in the EVAP system The hesitation after refueling may be due to excess fuel being drawn from the purge line into the intake manifold. More details: P0455 explanation

Anonymous 2018-09-27 22:26

What is the engine size and fuel type? (e.g., 4.8L V8, not flexfuel)

Anonymous 2018-09-27 23:10

@cj1 Thank you for your input—I’ll upload photos and data as requested tomorrow.

Anonymous 2018-09-27 23:13

@autojo It's a 4.8L V8, 4x4, not flexfuel.

Anonymous 2018-09-27 23:38

I’ve always had trouble refueling my truck—this issue alone previously only triggered occasional P0455 codes and didn’t stay active. Sometimes when I’m adding fuel, the engine shuts off as if full, requiring me to keep pouring gas in continuously. After replacing both the EVAP canister vent valve and the kinked fuel filler neck hose (which connects to the tank), the problem still occurs frequently. Now, today, new P0172 and P0175 codes have appeared alongside the emissions code—this suggests a deeper system issue.

Anonymous 2018-09-28 17:47

I’ve attached what I believe are the requested photos—if they’re not sufficient, please let me know. Thanks for your help!

Anonymous 2018-09-28 17:52

I apologize—I may have misphrased earlier. Let me clarify this point again.

Anonymous 2018-09-28 19:18

Regarding the negative fuel trims (P0172 and P0175): Try pinching off the vacuum hose from the purge valve to the intake manifold. Locate it based on your vehicle’s layout. Observe the fuel trims for about a minute—watch if they shift into positive territory and stabilize around ±10. If the fuel trim total (LT + ST) exceeds -25, then P0172 or P0175 will be triggered. This test helps determine whether excess fuel is coming from the purge line.

Anonymous 2018-09-28 20:09

Got it—thank you! What does it mean if the fuel trims go positive? Or if they don’t improve?

Anonymous 2018-09-28 21:39

If the fuel trims become positive, it indicates that excess fuel is being introduced from the purge line into the intake. If there's no improvement in trim values after pinching off the hose, then the purge line is likely not the source of the problem—further investigation into other components (like vacuum leaks or MAF sensor) may be needed. P0172: This code means that an oxygen sensor in Bank 1 detected a rich condition (too little oxygen in exhaust). Note: P0175 is very similar and often appears alongside it. Your vehicle may show both codes simultaneously. More details: P0172 explanation

Anonymous 2018-09-29 03:09

Thank you for the clear explanation—this helps a lot!