P1055

What the Code Means

Code P1055 indicates that long-term fuel trim has reached its maximum adjustment limit. The engine control unit (DME) is constantly correcting the mixture to compensate for a deviation from normal, and the available adaptation range is exhausted. Typical causes are unmetered air leaks (vacuum leaks), a faulty mass air flow sensor (MAF), or fuel pressure problems.

Symptoms

• Check engine light is on
• Rough, unstable idle
• Increased fuel consumption

Diagnosis and Repair

1. Check the system for vacuum leaks with a smoke test — this is the most likely reason the adaptations have maxed out.
2. Inspect the MAF sensor; clean it if dirty, replace it if faulty.
3. Check fuel pressure and the operation of the pressure regulator.
4. After fixing the cause, reset the fuel adaptations through ISTA.

A common diagnostic mistake is resetting the adaptations without finding and fixing the root cause: the trim will return to its limit and the code will come back.

Secondary Problems

• If you ignore the code, the engine runs either too lean or too rich.
• A lean mixture leads to engine knocking — destructive shock waves in the combustion chamber.
• Both conditions (lean and rich) damage the catalytic converter, and replacing it is an expensive repair.

Don't put off diagnosis: the longer the DME runs at the adaptation limit, the higher the risk of the damage listed above.