P0174: OBD-II code P0174 means the PCM is adding significant extra fuel on bank 2 to keep the mixture stoichiometric — the engine is running lean with fuel trims maxed out.

Symptoms

• Check engine light is on; engine may hesitate, hunt at idle, or feel underpowered under load.
• Rough idle, occasional misfires, and reduced fuel economy from constant PCM lean compensation.
• The lean condition typically worsens at idle and light load, where small vacuum leaks have the biggest effect.

Common causes

• A vacuum leak on bank 2 — a cracked intake boot, torn PCV hose, or leaking intake manifold gasket.
• Low fuel pressure, a clogged fuel filter, or a weak fuel pump reducing supply to that bank.
• A contaminated bank 2 MAF sensor (on V-engines with dual MAFs) or a fouled O2 sensor reading falsely lean.

What to do?

Smoke-test the intake for vacuum leaks — the single most common P0174 cause. Compare long-term fuel trims on bank 1 vs bank 2; a large split confirms the fault is isolated to bank 2. Check fuel pressure at idle and under load. Replace the bank 2 O2 sensor last, only after ruling out air and fuel side causes.