P0171

What P0171 means

This code means the computer is getting more oxygen in the exhaust than it expects for the amount of fuel it's putting in. It's telling you the mixture is running lean on Bank 1 (the side of the engine with cylinder 1, on V6 and V8 engines - on a four cylinder there's only one bank). The computer tries to correct this by adding more fuel, and P0171 shows up when it's had to add more than it's allowed to and still can't get the mixture right.

Important thing to understand: this code does not tell you what part failed. It tells you the result. The fix could be a fifteen dollar part or something bigger, and you won't know which until it's actually looked at.

Is it safe to drive

Usually yes, for a while. A lean condition can cause rough idle, hesitation, or a slight loss of power, and it's not great for the engine over the long run because lean mixtures burn hotter. Watch for the check engine light flashing (as opposed to staying on steady) - flashing means the engine is misfiring and you should stop driving it and get it looked at right away. Steady light with no other symptoms can wait a bit, but don't ignore it for months.

Common causes, most likely first

A vacuum leak is the most common cause - a cracked hose, a loose or aging intake boot, a bad gasket somewhere air can get sucked in past the sensors. Next most common is a dirty or failing mass airflow sensor, which mismeasures the incoming air. After that: a weak fuel pump or clogged fuel filter not supplying enough fuel, a failing oxygen sensor giving a bad reading, or on some engines a leaking PCV valve or hose. On older vehicles, intake manifold gaskets are a known cause on certain engine families. Which of these is likely depends a lot on the specific engine - some engines are known for plastic intake parts that crack with age, others have PCV systems that fail in a particular way. That's part of what a proper diagnosis sorts out.

What diagnosis involves and rough cost

Diagnosing this right usually means pulling live data (fuel trims, oxygen sensor readings) and then physically checking for vacuum leaks, often with a smoke machine that pushes smoke into the intake to show exactly where air is getting in where it shouldn't. That's the difference between guessing and knowing. Diagnostic time is usually charged separately from any repair.

Cost to fix varies a lot depending on what's actually wrong - a vacuum hose or small intake gasket is a relatively minor repair, while a mass airflow sensor or fuel pump costs more, and an intake manifold job on some engines takes real labor time. We'd rather tell you that range after we've actually found the cause than guess at a number now. You can call Auto Star Service Center at (253) 375-7057, Monday through Saturday 8am to 6pm, and we can get it on the scanner and take a look.