P0173 means the fuel trim for cylinder bank 2 is outside the normal range, indicating the PCM is adding too much or too little fuel to keep the air‑fuel ratio at the target. This is usually triggered when the long‑term fuel trim exceeds a threshold (often ±25%).
Leaks in the intake manifold gasket, vacuum hoses, or PCV system allow unmetered air to enter, causing the PCM to increase fuel trim on bank 2.
If the MAF sensor underreports airflow, the PCM adds excess fuel; if it overreports, it may cut fuel, both leading to out‑of‑range trim.
Insufficient fuel pressure or flow makes the PCM raise fuel trim to compensate, especially noticeable on one bank if the fuel rail or injector circuit is asymmetrical.
An injector that drips or fails to open properly changes the actual fuel delivered, causing the PCM to adjust trim beyond limits.
A sluggish or biased oxygen sensor gives incorrect feedback, prompting the PCM to apply excessive fuel correction.
While rare, a corrupted fuel‑trim table or internal PCM fault can cause erroneous trim values; this is considered only after all other causes are ruled out.
No. The PCM is rarely the root cause; most P0173 cases stem from vacuum leaks, MAF sensor issues, fuel delivery problems, or faulty injectors/O₂ sensors. PCM replacement should only be considered after those components have been verified good.
Driving is possible for short distances, but the incorrect fuel mixture can lead to poor performance, increased emissions, and potential catalyst damage. It is advisable to diagnose and repair the issue promptly.
P0173 specifically refers to bank 2. If bank 1 shows similar symptoms, a separate code (P0171/P0172) would likely appear. However, a global issue like a faulty MAF sensor can affect both banks and may set codes on each side.
Clearing the code only turns off the Check Engine Light; if the underlying fault remains, the code will return once the PCM detects the fuel trim out of range again.