Q: My check-engine light came on this week. My neighbor used a scanner to read a code P0171. What could cause this?

— Paul Guthrie

A: A P0171 indicates the engine’s cylinder bank 1 is running too lean (too much air or not enough fuel). If you have a V-6, V-8 or V-10 engine and a different code, P0174, also appears, this means bank 2 is also lean. Common causes can be insufficient fuel delivery (dirty fuel filter, weak fuel pump, dirty pump strainer, faulty pressure regulator), air leaks (cracked/loose vacuum hoses, leaking gaskets), or a leaking intake air duct on a mass airflow sensor equipped vehicle, a dirty MAF sensor element, faulty oxygen sensor — or air sneaking into the exhaust, fooling the sensor. Other causes can be tight valves, a stuck open PCV valve and dirty fuel injectors.

As you can see, careful testing is needed in order to track down the actual cause, avoiding unnecessary parts replacement. The first step, using the scan tool, is to observe the freeze frame that’s recorded when the trouble code is set. This indicates a specific driving condition, allowing data to be observed when the problem is at its worst during a diagnostic road test. The freeze frame is erased when the code is cleared — write down the info first! If only the P0171 occurs on a V-something engine, this rules out a lot of possibilities that would have affected both banks of cylinders. If it’s a four cylinder, single bank engine, all are possible.

This problem is tricky and would best be fixed by a pro. It would take about 10 times the length of this column to talk you through all the possibly needed test steps.

Q: I was wondering if you believe in magnetic drain plugs? I know that manufacturers sometimes place magnets in the transmission drain pan. It seems like they might help filter the engine oil and extend its lifetime, but perhaps the oil filter already accomplishes this. On a separate note, the car dealers like to flush the power steering fluid every 75,000 miles. Is this worth the $100, or should I take my wife out to dinner instead? Maybe I could flush it myself?

— Bruce

A: A magnetic drain plug certainly can’t hurt, if you can find one that correctly fits your vehicle. This strategy is of a bit less value on a modern engine due to the greater use of aluminum parts — their metal particles are non-ferrous. You’re correct the oil filter should grab any shavings, along with other debri.

I’m on the fence regarding power-steering system flushing. It makes sense, but the cost versus benefit is high unless you’ll be keeping the vehicle for a long time. Yes, you could achieve similar results by suctioning out the power steering reservoir (use an old turkey baster) and renewing that quantity of fluid. Next, drive around the block, being sure to work the steering wheel in both directions. Suction again, and renew fluid, repeat perhaps three or four times. I’ll bet you can achieve perhaps a 90 percent fluid exchange, for the cost of two or three quarts of power steering fluid. Be sure to use the manufacturer’s specified fluid or a generic fluid meeting their specifications. The old fluid can be recycled, along with your motor oil.