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1990s Chevrolet Trans Am P0171 and P0174 OBD2 Codes: Causes and Fixes After Fuel Fill-Up

Model: 01 Trans Am Fault Code: P0171 Posted: 2010-09-06 09:33

My check engine light came on with OBD2 diagnostic codes P0171 and P0174. This occurred immediately after I refilled the fuel tank. I cleared the codes, and they remained off for about two weeks before returning when my fuel level dropped below half. The check engine light now turns off on its own but reappears after a refill. I've noticed a pinging sound during acceleration. I'm wondering if this is due to poor-quality gasoline or dirty oxygen sensors. Is it safe to continue driving the vehicle under these conditions? What steps should I take to resolve this issue? The P0171 and P0174 codes indicate a lean air-fuel mixture in the exhaust, meaning too much oxygen is present. On V6/V8 engines, Bank 1 typically refers to the side with cylinder #1, while Bank 2 is on the opposite side. These codes often appear together. I’ve read that using higher octane fuel (92 or 93) and adding a gas treatment like Heet (yellow bottle, 12 oz) or STP Gas Treatment can help. However, I’m unsure whether this resolves the root cause or just masks symptoms. I've also had both oxygen sensors replaced recently, and my MAF sensor was cleaned — but the problem persists. I'm concerned that bad gas might be contributing to the issue, especially since it reappears after refueling. Is it safe to drive the car with these codes active? What are the long-term risks of ignoring this fault?

Related fault codes
P0171P0174
Comments (6)
Anonymous 2010-09-19 18:41

Try using 92 or 93 octane fuel and add one bottle of rubbing alcohol or SeaForm to your tank. This can help reduce moisture and improve combustion efficiency, especially if the issue is related to water content in the fuel.

Anonymous 2011-08-10 21:00

P0171 and P0174 indicate a lean condition detected by oxygen sensors in Bank 1 (cylinder #1 side) and Bank 2 (opposite side), respectively. On V6/V8 engines, these codes often appear together. Do not have oxygen sensors replaced by auto mechanics — they are generally reliable and only need replacement if physically damaged or failing. Bad gas can contribute to this issue. Use a dry fuel additive such as Heet (yellow 12 oz bottle) or STP Gas Treatment, which helps remove moisture and improve combustion. Add it directly into the tank after refueling and monitor whether symptoms improve.

Anonymous 2012-11-06 05:00

I recently purchased a 1999 Chevrolet Trans Am with 86,000 miles and am experiencing the exact same issue: after filling up with high-octane fuel, the car begins to almost stall during acceleration. The check engine light has been on continuously. Previously, it would bog down slightly when accelerating gently — but now it's much worse. I had both oxygen sensors replaced a few weeks ago and my MAF sensor was cleaned at purchase. I suspect bad gas isn't the root cause since symptoms started before refueling. I'm still looking for solutions. Does anyone have experience with this issue on older Trans Am models? Any advice on next steps would be appreciated.

Anonymous 2012-11-06 09:04

Bjhorner — please start a new topic and provide full details: all current OBD2 codes, recent maintenance work, modifications, or aftermarket parts installed. This helps ensure accurate diagnosis and troubleshooting.

Anonymous 2012-11-13 21:10

I have a 1998 Ford F-150 with a P0171 code. Many sources suggest the PCV valve is the culprit, especially when accompanied by pinging and loss of power during acceleration. I'm wondering if this applies to my vehicle or whether another component might be at fault.

Anonymous 2012-11-14 08:32

andy_garcia23 — please start a new topic and include all relevant details: current OBD codes, recent repairs, modifications, or aftermarket parts. This will help others provide more accurate advice.