What’s the worst you can do?

Dear Car Talk:

My brother-in-law, who thinks that I am mistaken if my lips are moving, tells me that the worst thing I can do to my automatic transmission is start the car and immediately back out of my garage. His reasoning is that when the engine is warming up, it's running faster than usual. So when my foot's on the brake and I put the car in reverse, wham, the tranny slams up against wheels that won't move. What do you say? – Rolf

RAY: That's far from the worst thing you can do to your automatic transmission, Rolf. Putting bananas in it instead of automatic transmission fluid jumps immediately to mind.

Your annoying brother-in-law may have had a point 30 or 40 years ago. In the old days, when we had carburetors and mechanical chokes, you’d start up a cold car, and the engine would run at 2500 rpm for the first five minutes. And sure, slamming the transmission into gear when the engine was running that fast wasn’t exactly TLC for a transmission.

But now that engines are computer-controlled, an engine that’s warming up will run at 1000 rpm for 30 seconds, and then return to 750 rpm. So whatever strain you put on the transmission is minimal to nonexistent.

Plus, you’re not going to sit there for that 30 seconds “straining” the transmission. Unless you have a garage fetish, you’re going to release the brake and start backing up.

So tell your brother-in-law that he’s wrong. That’s far from the worst thing you can do to your transmission. Driving the car hard, especially accelerating sharply and suddenly, is worse for the transmission. And not only the transmission, but all of the other drivetrain components. Everything except the radio knobs – they can handle hard acceleration.

So if you decide to pass a garbage truck and you pull out and put your foot to the floor, and the transmission downshifts three gears and your back gets pinned to the seat, you’re shortening the life of your transmission, and the rest of your car, far more than you ever could by putting it in reverse in your garage.

And here’s another thing that’s worse than what your brother-in-law suggests: Backing out of your driveway and using the transmission to shift directions instead of stopping first. And don’t forget about neglecting to change the transmission fluid when the manufacturer recommends. That often leads to bananas (see above).

Car keeps giving same error code even after replacing part

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

RAY: 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.

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