P0171: System too lean (bank 1)
The engine computer sees too much air or too little fuel and is adding fuel to compensate.
Updated
What P0171 means
The engine computer aims for a precise mix of air and fuel. An oxygen sensor in the exhaust tells it how the mix came out, and the computer corrects the amount of fuel it injects. That correction is called fuel trim.
When it has to add more fuel than its limit allows, it stores P0171: the engine gets too much air or too little fuel, and the computer can’t correct it any further.
Read My Car shows short and long term fuel trim under Live data, if your car reports them. Positive numbers mean the computer is adding fuel.
P0174: the same fault on bank 2
P0174 is the same fault on bank 2, the side without cylinder 1, which only V and flat engines have. When P0171 and P0174 are stored together, look for a cause that affects the whole engine, such as a leak, the air flow sensor or the fuel supply, rather than one bank.
P0174: System too lean (bank 2)
Can I keep driving?
Check soon
Probably OK to drive short-term, but have it checked soon. Fuel use or emissions may be worse.
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
A lean engine can run rough or misfire, so look out for misfire codes (P0300 to P0312) as well: those need attention sooner.
Symptoms
- Often none besides the check engine light.
- A rough or uneven idle.
- Hesitation when accelerating.
- Misfire codes stored at the same time.
Common causes
- Vacuum or intake air leak. Air that enters after the air flow sensor isn’t measured, so the computer injects too little fuel for it. A cracked hose is a typical example.
- Dirty or faulty air flow (MAF) sensor. If it under-reads the air, the computer injects too little fuel.
- Weak fuel pump or clogged fuel filter. The injectors get too little pressure, which tends to show most under load.
- Leaking intake gasket or PCV hose. Another way in for unmeasured air. The PCV system ventilates the crankcase into the intake.
Typical fixes
- Look and listen for vacuum leaks around the intake, its hoses and the PCV (crankcase ventilation) system, with the engine idling.
- Check the air flow sensor and the intake ducting between it and the engine.
- Have the fuel pressure tested if the leak and sensor checks find nothing.
- Clear the codes after the repair and check in live data that fuel trim moves back towards zero.
Questions
Can I drive with P0171?
Probably for a short time: the app’s advice is to have it checked soon, as fuel use or emissions may be worse. If misfire codes appear too, avoid hard driving, and if the check engine light flashes, stop when it’s safe to do so.
What is the difference between P0171 and P0174?
The same fault on different banks. P0171 is bank 1, the side with cylinder 1; P0174 is bank 2 of a V or flat engine. Both together point to a cause that affects the whole engine.