#fuel trim
8 posts in Car Diagnostics blog about "fuel trim" — OBD-II diagnostics, DTC codes, repair. ← All posts
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P2098 and P2099: post-catalyst fuel trim bank 2 lean and rich
Bank 2 versions of a code that catches people out, because it is about a correction the ECU makes after the catalyst — not about the catalyst itself.
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P2178 and P2180: system too rich off idle
A rich condition that appears once you are off idle points somewhere different from one that is present all the time. The load dependency is the diagnosis.
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P2195 and P2197: oxygen sensor signal biased or stuck lean
The sensor is not out of range and not dead. It is simply not moving the way a working sensor moves, and the ECU has noticed the difference.
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Surging at a constant speed: fuel trim, torque converter or EGR
A gentle rhythmic pulsing at a steady cruise, with your foot still. The frequency of the pulse points at which system is hunting.
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P0170 and P0173: fuel trim malfunction, and why it is not the same as lean
P0171 says the mixture is lean. P0170 says the correction system itself has run out of authority. The difference decides where you look.
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P2096 and P2097: post-catalyst fuel trim too lean or too rich
These codes come from the downstream sensor asking the ECU to correct the mixture. They are frequently mistaken for catalyst failure when the real cause is an exhaust leak.
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GDI and FSI Carbon Buildup on Intake Valves: OBD-II Diagnosis
How carbon deposits on intake valves from GDI/FSI/TFSI/TSI engines appear in OBD-II: P2187, P0300, fuel trim Live Data, and solutions — Walnut Blasting vs chemical cleaning.
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Fuel Trim (STFT and LTFT): how the fuel correction tells you about engine health
What Short-Term (STFT) and Long-Term (LTFT) Fuel Trim mean, which values are normal, and how to diagnose a vacuum leak, dirty MAF or clogged injector from Fuel Trim numbers alone.