P0174
System Too Lean Bank 2.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 1
- AreaFuel and air metering
- 74
- FaultThe specific fault in that area
What it means
P0174 is the generic code for 'System Too Lean, Bank 2'. The engine control unit has seen the long-term and short-term fuel trims on bank 2 add a large amount of fuel to compensate for too much air or too little fuel, and the correction has reached its limit. Bank 2 is the side of the engine that does not contain cylinder 1, and the code only applies to engines with two cylinder banks.
Symptoms
- Engine management warning light on
- Rough or unstable idle
- Hesitation, stumbling or poor acceleration under load
- Engine misfire or occasional stalling
- Poorer fuel economy
- Hissing noise from the engine bay from a vacuum leak
Possible causes
- Vacuum leak on the intake side, such as a split or disconnected vacuum hose, cracked intake pipe or leaking intake manifold gasketlikely
- Faulty or contaminated mass airflow sensor giving a reading that is too low or inaccuratelikely
- Crankcase ventilation (PCV) system fault, such as a leaking hose, a stuck-open valve or a damaged diaphragmlikely
- Low fuel pressure from a weak fuel pump, blocked fuel filter or faulty pressure regulator (where fitted)possible
- Restricted or dirty fuel injectors on bank 2possible
- Exhaust leak ahead of the bank 2 oxygen or air-fuel ratio sensor letting in extra air, or a faulty sensorpossible
- Outdated engine control software on certain models where the manufacturer has issued an update for lean runningrare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting engine load, speed and temperature when the fault set. Check whether P0171 is also stored, as that points to a shared cause.
- Visually inspect the intake pipework, vacuum hoses, PCV hoses, air filter housing and connectors for splits, loose clips and disconnections.
- View live fuel trims for both banks at idle and at about 2,500 rpm. A lean condition that improves at higher revs suggests a vacuum leak, while one that worsens suggests a fuel delivery or airflow measurement problem.
- Check the MAF sensor reading against expected values at idle and inspect the sensor element and wiring for contamination or damage.
- Smoke test the intake and crankcase ventilation systems to locate leaks, including the manifold gaskets.
- Measure fuel pressure and volume against the manufacturer's specification, and check the fuel filter where one is serviceable.
- Inspect the exhaust for leaks upstream of the bank 2 sensor, then check the sensor's live response.
- Check injector operation and spray pattern or balance on bank 2 if the other checks find nothing, and check for any applicable software updates for the model.
How to fix
- Repair or replace leaking vacuum hoses, intake pipes or manifold gaskets, and refit loose clips.
- Clean or replace the MAF sensor if it is proven faulty, or repair its wiring.
- Replace faulty PCV valves, hoses or diaphragms.
- Restore fuel pressure by replacing the fuel pump, filter or regulator as diagnosed.
- Clean or replace faulty injectors on bank 2.
- Repair exhaust leaks, replace a faulty sensor if proven defective, or apply a manufacturer software update where one is known to address the fault.
Can I keep driving?
The car can usually be driven short distances to a workshop, but prolonged lean running can cause misfires, overheating and damage to the catalytic converter. Have it diagnosed soon and avoid heavy loads or hard acceleration until it is fixed.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.