Subsystems and Conditions Affecting Fuel Trim

Once you know the driveability symptom and are able to confirm that the air/fuel ratio is excessively lean or rich, it is a fairly easy task to identify all of the sub-systems which can effect the mixture. Check each sub-system to confirm proper operation.

The following chart lists sub-systems and other factors which can cause the oxygen feedback system to make rich or lean corrections and, in some cases, cause fuel trim data to approach its correction limits:

Sub-systems and Factors Affecting Fuel Trim

Fig. 5-46

Negative fuel trim % (low LVF, < 2.5v) Lean Command (Rich AFR Condition)

Positive fuel trim % (high LVF, > 2.5v) Rich Command (Lean AFR Condition)



High altitude operation (except HAC equipped)

Lower than normal fuel pressure

Fuel contamination in crankcase

Air leak into intake system

Loaded or malfunctioning evaporative emissions system

Air leak into exhaust, upstream of 02S

Excessive EGR flow rate

Throttle body/shaft wear

Leaking fuel pressure regulator

High oxygen content in fuel

Higher than normal fuel pressure

Restricted fuel injector or faulty spray pattern

Leaking fuel injector

Water contaminated fuel

Secondary air system improperly routed upstream

NOTE: OBD vehicles without High Altitude Compensation operating at high altitude (> 5000 feet) may operate at the lean fuel trim correction limit. This is a normal condition.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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