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P0171 OBD2 Code on 2002 Toyota Bb/XB: Idle Vibration and Engine Light On Only at Stop

Model: Toyota Bb Fault Code: P0171 Posted: 2011-12-26 12:45

Hi everyone, I own a 2002 Toyota Bb (also known as XB in the US). About six months ago, I started noticing the engine light coming on, accompanied by severe vibration when the car is stopped — such as at traffic lights, during traffic jams, or while parking. Initially, I cleaned the throttle body, but the issue persisted. A mechanic later inspected the wiring and temporarily resolved the problem for about four months. However, recently, the engine light has started appearing frequently at idle speed, with significant vibrations. After checking the OBD2 code, it reads P0171 — indicating a lean air-fuel mixture. I've already replaced several components including the Mass Airflow Sensor (MAF), Oil Filter, Oxygen Sensor, throttle body (used second-hand), and fuel injectors. I’ve also thoroughly inspected the intake manifold for leaks or blockages. Despite these efforts, the problem remains unresolved. Notably, the engine light only comes on when idle — during stops or slow driving without accelerating. There are no symptoms during normal highway driving. This makes me wonder if there’s an issue with the Idle Speed Control Valve (ISCV) or another component regulating idle speed. However, my mechanics have told me that if the ISCV were faulty, the engine light would not illuminate — is this accurate? I’ve consulted two local experts familiar with 2002 Toyota Bb models in Hong Kong, and they suggest that a vacuum leak in the intake system or catalyst blockage could also trigger P0171. Given that the issue only manifests at idle, could it be related to an idle control mechanism? I’m frustrated because this has been going on for half a year, and I’ve spent considerable time and money troubleshooting without success. Any insights or suggestions would be greatly appreciated. Thank you in advance!

Related fault codes
P0171
Comments (6)
Anonymous 2011-12-26 22:15

The Idle Speed Control Valve (ISCV) is unlikely to be the root cause — if it were faulty, the engine light typically wouldn't illuminate. Start by visiting a more experienced mechanic or dealership. P0171 is not an obscure code and can usually be resolved with proper diagnostics, especially when common parts have already been replaced.

Anonymous 2011-12-27 11:02

Thank you for your response! However, I’ve already replaced nearly all components commonly associated with this issue, and both local mechanics I consulted have advised me to try further solutions. I’m hesitant to spend more money since there are no drivability issues during normal driving. One of the experts mentioned that a clogged catalytic converter might also trigger P0171 — is that plausible? Given that symptoms only appear at idle, could an idle control system component be malfunctioning?

Anonymous 2011-12-27 14:46

Could you clarify which two mechanics you're referring to? We’re not here to debate — just provide helpful input.

Anonymous 2011-12-28 11:17

I apologize for any misunderstanding. I’m from Hong Kong, and the 'experts' are local mechanics with extensive experience on 2002 Toyota Bb models in that region. I’m not arguing — I simply hope to understand why P0171 persists after replacing so many parts and whether others face similar issues.

Anonymous 2011-12-28 19:24

I’d recommend asking your local mechanics to recheck the intake gasket for vacuum leaks. Even a small leak can cause a lean condition and trigger P0171, especially at idle.

Anonymous 2011-12-29 09:49

Thank you for the suggestion! I’ll have my mechanic inspect the intake gasket again — that’s a good point to consider.