P0174: System Too Lean Bank 2
P0174 means the engine computer found too much air or too little fuel in the mixture on bank 2 (one side of a V or boxer engine) and had to add fuel to compensate. The car is usually drivable for a short time with no limp mode, but the cause should be found soon because it can cause rough running and will fail an emissions test.
You can drive, but get it checked soon
The data lists no limp mode for this code, so the car normally keeps running. A lean mixture can still cause rough idle, stalling and weak acceleration, so keep trips short and get it checked soon, especially if the engine hesitates or stalls.
What it means
The engine computer constantly adjusts how much fuel is injected, using the oxygen sensor in the exhaust to check the result. Short-term fuel trim reacts within fractions of a second. Long-term trim is a learned correction stored for each load and speed range. Positive values mean fuel is being added because the mixture was lean, and around ±10 % is normal. P0174 sets when the correction on bank 2 goes beyond the limit the manufacturer defines, and the light comes on after two trips (drive cycles). The code only exists on V and boxer engines that control the mixture separately for each bank. It is the bank 2 counterpart of P0171. If both codes appear together, the cause is usually shared by both banks, such as the air flow sensor, fuel pressure or crankcase ventilation. If P0174 appears alone, look for a leak, an injector problem or an exhaust leak on bank 2.
Likely causes
- Air leak after the air flow sensor on bank 2 (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line)Very likely
Air that enters after the sensor is not measured, so the computer injects too little fuel for the actual airflow.
- Crankcase ventilation leaking or faulty (PCV hose, diaphragm in valve cover, oil cap, dipstick)Very likely
A leak in this system lets unmetered air into the engine, which leans out the mixture.
- Air flow sensor dirty or driftingPossible
A sensor that measures too little air makes the computer deliver too little fuel.
- Fuel pressure or delivery too low (weak pump, clogged filter, regulator, tank nearly empty)Possible
If not enough fuel reaches the injectors, the mixture runs lean even when the air measurement is correct.
- Clogged injectors, or an exhaust leak ahead of the oxygen sensorLess likely
Restricted injectors deliver less fuel, and an exhaust leak can pull in outside air that makes the sensor read lean.
What you may notice
- 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)
What to check, cheapest first
- 1Check that the oil cap and dipstick are seated properly, and look over the PCV hose and other vacuum hoses for cracks, splits or loose ends. · you can do it
- 2Listen at idle for hissing or whistling around the intake manifold, throttle body and brake booster line. · you can do it
- 3Make sure the tank is not nearly empty, then note whether P0171 is also stored, since both codes together point to a shared cause. · you can do it
- 4Read the fuel trim values with a scan tool. Very high trims only at idle suggest an air leak, while trims that rise with load suggest low fuel supply or an air flow sensor reading low. · mechanic
- 5Inspect or clean the air flow sensor and check it for a low reading, and look for an exhaust leak ahead of the oxygen sensor. · mechanic
- 6Test fuel pressure and delivery, and check the injectors if the other checks find nothing. · mechanic
Related codes
Frequently asked questions
Can I drive with P0174?+
Usually yes for a short time, since there is no limp mode. A lean engine can run rough or stall, so have it checked soon.
Can a loose oil cap or gas cap cause P0174?+
A loose or missing oil cap or dipstick can, because the crankcase ventilation then lets in extra air. The data does not list the gas cap as a cause.
Will P0174 pass an emissions test?+
Not with the light on. The code is emissions-relevant and turns the check engine light on, so it needs repair first.
Is the oxygen sensor the problem?+
Rarely. The sensor only reports what is in the exhaust, so look for a leak, fuel supply problem or air flow sensor first.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10