P0174: System Too Lean Bank 2

What does P0174 mean?

With closed-loop control the engine module corrects the injected fuel using the oxygen sensor: short-term fuel trim reacts within fractions of a second, long-term trim stores a learned base correction per load and speed range. Positive values mean fuel is being added because the mixture was lean, negative values mean fuel is being removed; around ±10 % is normal. P0174 is the bank 2 counterpart of P0171 and only exists on V and boxer engines with separate mixture control per bank. The limits match: GM a long-term factor of about 1.30 (engine-specific), Ford above +25 % with short-term trim above +1 %; an older Saab V6 flagged more than 25 % deviation for 20 s. Two trips, no limp mode. P0174 alone points to a leak, injector or exhaust leak on bank 2; together with P0171 to a shared cause such as the air flow sensor, fuel pressure or crankcase ventilation – exactly what several GM bulletins describe. Diagnosis hint: trims very high only at idle and normal under load point to an air leak; trims rising with load point to fuel shortage or an air flow sensor reading low. The oxygen sensor only reports what is in the exhaust and is rarely the cause.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on (after two drive cycles)
• Rough or unsteady idle, possibly stalling
• Hesitation or lack of power when accelerating
• Hissing or whistling from the engine bay (air leak, PCV)

Common causes

• High Air leak after the air flow sensor on bank 2 (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line)
• High Crankcase ventilation leaking or faulty (PCV hose, diaphragm in valve cover, oil cap, dipstick)
• Medium Air flow sensor dirty or drifting (measures too little air)
• Medium Fuel pressure or delivery too low (weak pump, clogged filter, regulator, tank nearly empty)
• Low Injectors clogged, or exhaust leak ahead of the oxygen sensor

Likelihood is a general estimate across vehicles; your make and model may differ.