P0170: OBD-II code P0170 flags a general fuel trim malfunction on bank 1 — the PCM's fuel-trim compensation has hit its limit in either the lean or rich direction.

Symptoms

• Check engine light is on; engine may hesitate, hunt at idle, run rough, or feel underpowered under load.
• Rough idle, occasional misfires, and reduced fuel economy from constant PCM compensation.
• Companion codes like P0171 (lean) or P0172 (rich) usually accompany P0170 with more specific diagnostic direction.

Common causes

• On the lean side: vacuum leaks, low fuel pressure, a weak fuel pump, or a contaminated MAF sensor.
• On the rich side: leaking or stuck-open fuel injectors, a stuck EVAP purge valve, or high fuel pressure.
• A failed upstream O2 sensor on bank 1 giving the PCM bad feedback that forces continuous compensation.

What to do?

Read short and long-term fuel trims on bank 1 to determine direction (positive = lean, negative = rich). Smoke-test for vacuum leaks if lean. Check fuel pressure with a gauge. Address companion P0171/P0172 codes as the primary diagnostic path.