P0174 indicates that the engine control module (ECM) has detected a lean air-fuel mixture on Bank 2 (the side of the engine opposite cylinder #1). This means there is too much air or too little fuel entering the combustion chamber, which can cause performance issues and potential engine damage if left uncorrected.
Unmetered air entering the engine after the mass airflow (MAF) sensor is the most common cause. Leaks can occur at intake manifold gaskets, vacuum hoses, or the PCV system on Bank 2.
A dirty or faulty MAF sensor can under-report incoming air volume, causing the ECM to deliver insufficient fuel. This often affects both banks but may trigger P0174 first on the more sensitive bank.
Clogged fuel injectors, a weak fuel pump, or a restricted fuel filter can reduce fuel pressure or volume, leading to a lean condition on one bank.
An exhaust leak upstream of the Bank 2 oxygen sensor can allow outside air to enter, causing the sensor to read lean and triggering the code.
A degraded or contaminated O2 sensor may send an incorrect lean signal to the ECM, but this is less common than an actual air or fuel issue. The sensor should be tested rather than replaced first.
PCM failure is extremely rare for P0174. Replacing the computer is almost never the correct fix; the underlying mechanical or sensor problem should be thoroughly diagnosed first.
Yes, a contaminated MAF sensor is a common cause. It under-reports airflow, causing the ECM to inject too little fuel. Cleaning the sensor with proper MAF cleaner often resolves the issue.
No. The O2 sensor is usually reporting the lean condition, not causing it. Replacing it without fixing the underlying air or fuel problem will not clear the code permanently.
Not always, but vacuum leaks are the most frequent cause. Other possibilities include MAF sensor issues, fuel delivery problems, or exhaust leaks. A systematic diagnosis is needed.
It is extremely rare. The PCM is often blamed, but in nearly all cases the cause is a mechanical or sensor issue. Only consider PCM replacement after all other possibilities have been ruled out.