P0171 and P0174 together -- same fix?

Often yes. Both banks lean at once almost always means a vacuum leak or bad MAF sensor affecting all cylinders. Focus there before looking at bank-specific components.

Can low fuel cause P0174?

A very weak fuel pump can. But a lean code from low fuel would typically accompany P0171 as well.

How serious is running lean compared to running rich?

Lean is often more urgent. Lean combustion runs hotter and is more likely to cause misfires, pre-ignition, and long-term valve or piston damage.

Codes › P0174

P0174: Your engine is not getting enough fuel on the passenger side

System Too Lean (Bank 2)

Moderate severityCan you keep driving?

Yes, for now. But lean misfires can follow if the condition worsens, and lean combustion runs hot, which stresses pistons and valves. Do not ignore it.

What it means

P0174 is the Bank 2 lean counterpart to P0171. It means the engine on the passenger-bank side is getting too much air for the fuel being delivered, and the computer has already added as much fuel correction as it allows. Lean conditions are often harder to diagnose than rich ones because the root cause is almost always too much air coming in somewhere unexpected, not too little fuel. The key insight: if you have both P0171 and P0174 at the same time, the cause is upstream and common to both banks, almost always a vacuum leak or a bad MAF sensor.

Common causes

Vacuum or intake leakUnmetered air after the MAF sensor leans out the mixture. The single most common cause of lean codes.
Dirty or failing MAF sensorA contaminated MAF under-reports airflow, causing a lean trim on one or both banks.
Weak fuel pump or clogged fuel filterInsufficient fuel pressure affects all cylinders. More likely if P0171 is also present.
Clogged fuel injectors on Bank 2 cylindersInjectors that do not fully open reduce fuel delivery to specific cylinders.
Faulty Bank 2 upstream O2 sensorA sensor stuck lean tells the computer the mixture is lean even when it is not, causing unnecessary fuel addition and confusing the diagnosis.

What it costs

Vacuum hose replacement$20 to $150
MAF sensor cleaning or replacement$50 to $300
Fuel filter replacement$50 to $200
Fuel injector service$150 to $600

Symptoms

Check engine light on, steady
Rough idle, especially when cold
Hesitation or stumble on light acceleration
Worse fuel economy
Possible misfires on Bank 2 cylinders

What to do next

If P0171 is also stored, focus on common-bank causes: MAF sensor and vacuum leaks. A smoke test is the most reliable way to find small vacuum leaks. If P0174 is alone, the cause is more likely Bank 2 specific: check the Bank 2 PCV hose and the intake manifold gasket on that side. Clean the MAF sensor first since it is free if you own the spray.

Lewis shows Bank 1 and Bank 2 fuel trim values side by side so you can immediately see whether the lean condition is isolated to one bank or affecting both. Download Lewis.

Related codes

FAQ

P0171 and P0174 together -- same fix?: Often yes. Both banks lean at once almost always means a vacuum leak or bad MAF sensor affecting all cylinders. Focus there before looking at bank-specific components.
Can low fuel cause P0174?: A very weak fuel pump can. But a lean code from low fuel would typically accompany P0171 as well.
How serious is running lean compared to running rich?: Lean is often more urgent. Lean combustion runs hotter and is more likely to cause misfires, pre-ignition, and long-term valve or piston damage.