Engine codes
P0171: system too lean (bank 1)
The engine computer corrects the fuel mixture all the time, adding or removing fuel to stay at the right ratio. P0171 means it has added as much as it is allowed on bank 1 and the oxygen sensor still reads lean.
P0171: can I drive, and what will the labor cost?
Estimated labor, find and replace a leaking vacuum or pcv hose
$33 to $110
| Labor time (estimate) | 0.3 to 1 h |
| Rate used | $110/h |
| Technician wage, United States (average) | $27.13/h |
| Moderate: fix within weeks Drivable for now; a lean engine can hesitate and misfire, so fix within a few weeks, sooner if misfire codes appear. | |
Labor only: parts and the diagnostic fee are not included. Hours are our estimate for a typical car; book times vary by model. Wage: BLS, May 2025.
P0171 is the generic OBD-II code for "system too lean, bank 1". It is set by the fuel system monitor, which watches the fuel trims: the percentage the computer adds to or removes from its base fuel calculation. When the long-term trim on bank 1 reaches its upper limit, often around 25 percent, and the mixture is still lean, the code is stored. The engine is getting too much air for the fuel it measures, or too little fuel for the air. The most frequent causes are air that bypasses the air flow sensor, through a cracked vacuum hose, a failed PCV valve or hose, or a leaking intake gasket, and an air flow sensor that under-reads. Low fuel pressure from a weak pump or clogged filter, and restricted injectors, follow. It is drivable for now, but a lean engine hesitates and can misfire. Look at whether P0174 is stored too: both banks lean changes the likely cause.
Read your fuel trims (from a scanner's live data)
Total trim at idle / 2,500 rpm
+20% / +6%
| Reading: Lean at idle that improves with engine speed: the pattern of a vacuum leak. |
Positive trims mean the computer is adding fuel. This idle-versus-speed comparison is a common shop test, not a rule: confirm with a smoke test or a fuel pressure gauge. Negative values can be typed with a minus sign.
Codes, rules and labor data checked by Radif Partners · Editorial policy · Method and sources
P0171 at a glance
| Code | P0171 |
| Standard description | System Too Lean (Bank 1) |
| System | Fuel mixture and delivery |
| Severity | Moderate: fix within weeks |
| Common symptoms | hesitation, rough idle, misfire under load, hissing from the engine bay |
Reading the fuel trims
A basic scanner shows short-term and long-term fuel trim for each bank. Their sum tells you how hard the computer is correcting. The pattern matters more than the number: a high correction at idle that falls toward normal at 2,500 rpm points to a vacuum leak, because a fixed leak is a large share of the small airflow at idle and a small share at speed. A high correction everywhere points to fuel delivery or to the air flow sensor. The reader on this page applies that logic to your four readings.
Finding a vacuum leak
Listen for a hiss at idle around the intake, and look at every rubber hose connected to it, especially the PCV hose, which hardens with heat. Pinching the PCV hose briefly and watching the idle change is a quick test on many engines. A shop uses a smoke machine, which fills the intake with visible vapor and shows the leak in minutes; it is usually the best value hour of diagnosis for this code.
One bank or two
On a V engine, a leak or fault that affects only one side sets only that side's code. A cause shared by both banks, such as a weak fuel pump, a bad air flow sensor or a leak at the throttle body, usually sets P0171 and P0174 together. On an inline four or six cylinder there is only bank 1, so P0171 alone tells you less.
Likely causes of P0171, in order
| Cause | How often |
|---|---|
| Vacuum leak: cracked hose, PCV valve or hose, intake gasket | most frequent |
| Dirty or failing MAF sensor reading low | most frequent |
| Low fuel pressure: weak pump, clogged filter, regulator | frequent |
| Restricted fuel injectors | frequent |
| Exhaust leak ahead of the upstream oxygen sensor | less frequent |
P0171 repairs and labor time
| Repair | Labor (estimate) | At $110/h |
|---|---|---|
| Find and replace a leaking vacuum or PCV hose | 0.3 to 1 h | $33 to $110 |
| Replace intake manifold gasket | 1.5 to 4 h | $165 to $440 |
| Replace the MAF sensor | 0.3 to 0.6 h | $33 to $66 |
| Replace fuel pump module | 1.5 to 3.5 h | $165 to $385 |