Dear Car Talk:

My grandfather has a '97 Ford F-150 with 147,000 miles on it. After the check-engine light came on, he took it to have it scanned, and the scan tool said the upstream O2 sensor was bad. So he replaced it. Five miles or so after he replaced it, the check-engine light came on again. He got it checked again, and the same code came up. So he replaced it again, and he still has the problem. Any idea what else to check that could make this happen over and over again?

-- Craig

Well, two things come to mind. One is that the oxygen sensor itself is not the problem. Maybe there's a wire that broke off near the sensor, and that's why the computer thinks the O2 sensor is bad.

Modern car computers can distinguish between an electrical problem, like an open circuit from a broken wire, and the O2 sensor itself. But a 1997 F-150 might not have that talent. Next time a customer comes in with a '97 F-150, I'll break one of his wires and let you know.

Start by doing what's called a "continuity check" on the wires that go to the O2 sensor. You'll need the wiring diagram. There should be three wires, and you'll test each one.

If they're all good, and you're getting power to the O2 sensor, then perhaps Gramps is buying some bad O2 sensors, or perhaps even the wrong sensors.

In that case, instead of going back to the parts store that scanned the truck, go to the Ford dealer and buy a new O2 sensor from them. If that works, obviously the problem was the replacement sensors he was buying. If that doesn't work, you can return the sensor and consider a fire, Craig. Good luck.