P0174

What P0174 means

Your car's computer watches the oxygen sensors to see how much fuel is burning compared to how much air is going into the engine. P0174 means that on Bank 2 (the side of the engine that doesn't hold cylinder 1), the computer is seeing more air than expected for the amount of fuel it's delivering. That's called running lean. On a four cylinder engine there's no Bank 2, so this code shows up on V6 and V8 engines with two banks of cylinders. If your car also has P0171 at the same time, that's the same problem showing up on both sides, which usually points to something affecting the whole engine rather than just one bank.

Is it safe to drive

Usually yes, for a while. A mild lean condition often just costs you a little fuel economy and a rougher idle. But a lean condition can also cause the engine to run hotter than it should and can lead to misfires over time, which does damage. If the check engine light is flashing, or you feel the engine stumbling or shaking, stop driving it and get it looked at. If the light is steady and the car drives normally, it's fine to get it in within a week or two rather than tonight.

What commonly causes it

Most often it's a vacuum leak: a cracked hose, a loose or split intake boot, or a gasket that's stopped sealing. That's true across most engines. Beyond that, common causes include a dirty or failing mass airflow sensor, a weak fuel pump or clogged fuel injector on that bank, a failing oxygen sensor giving a false reading, or on some engines a PCV hose or valve that's leaking air. Which of these is likely depends on the engine. Some V6 engines are known for intake manifold gaskets that leak vacuum as they age. Others are more prone to carbon buildup around the throttle body creating a small leak. We wouldn't guess which one applies to your car without looking at it and the mileage and history.

What diagnosis involves and what it tends to cost

Diagnosis starts with a smoke test or a scan for fuel trim data to find out whether it's a vacuum leak, a sensor problem, or a fuel delivery issue on that bank. That's typically an hour or so of shop time. If it turns out to be a hose or a gasket, the repair itself can be inexpensive, in the range of a hundred to a few hundred dollars depending on what needs to come off to get at it. If it's an intake manifold gasket or a fuel injector, cost goes up because there's more labor involved, and parts vary a lot by engine. We can't give you a real number until we know which one it is, and anyone quoting a firm price before diagnosis is guessing.

Auto Star Service Center in Graham runs this kind of diagnostic during the week and on Saturdays, 8 to 6. Nick Porter handles the driveability and engine diagnostic work there.