Reading Scan Data Patterns

Fuel trim is the PCM's continuous correction to the air-fuel mixture. Positive trims mean lean — the PCM is adding fuel. Negative means rich — the PCM is removing fuel. The pattern of where the trim is high tells you where the fault is. High positive trims only at idle that normalize at cruise — vacuum leak. High positive trims at all speeds — fuel delivery. Negative trims at all speeds — rich condition: leaking injector, contaminated MAF, fuel pressure too high.

A coolant temperature sensor that reads abnormally low when the engine is clearly warm means the sensor or its circuit has failed high resistance — replace the sensor. An oxygen sensor that switches between rich and lean slowly instead of rapidly is lazy — low switching frequency on a warm engine is a failed O2 sensor. An O2 sensor stuck at a fixed voltage with no switching is dead.

Short term trims react in real time. Long term trims are the averaged learned correction. If STFT is jumping wildly but LTFT is near zero, the fault is intermittent — the PCM has not had time to learn it. If both STFT and LTFT are high positive, the fault has been present long enough for the PCM to learn a large correction — it has been a problem for a while.