Q: When my sister and I shop, we save time by having lunch in the car. When she drives, if it is winter or summer, she will leave the car running for the half-hour we eat, so she can run the heat or air conditioning. When I drive, I think that practice is not good for the car, so we suffer with the motor off.

Is it acceptable to idle the car so we are comfortable?

A: Idling doesn't harm the car. As long as your cooling system is working (if it wasn't, you'd see a "hot" warning light on the dashboard), cars can idle indefinitely. Or, at least until they run out of gas.

Idling actually is easier on the car than driving. The engine is doing very little work.

But there is a concern — pollution. When you sit there idling, the engine is still putting out carbon monoxide, carbon dioxide, unburned hydrocarbons and nitrous oxide compounds. And because of that pollution, many places have ordinances limiting the amount of time you can let a vehicle idle. So check the local regulations.

That said, if your teeth are chattering, or sweat is dripping off your chin onto your fish tacos, there's no reason to suffer. You don't want to be wasteful, but you also don't want to end up with frostbite or heat stroke.

So the real question is: Is there room for compromise, especially when the weather is more moderate?

If it's, say, 80 degrees out, you can idle the car and run the AC for five minutes and then shut it off until you feel uncomfortably warm again. And if it's 50 degrees you can run the engine and the heater until you're toasty and then shut it off until you need some more heat. And on days when the weather is more extreme, here's an idea: You can live it up by going into a restaurant.

Bad gas a bad bet

Q: I have a 2018 Chevy 2500HD Silverado with a 6.0-liter V8 and the trailering package. I pull a 6,000-pound trailer with it.

After I get gas, hitch up my trailer and hit the open road, within about 40 miles, the "Check engine" light comes on. I have OnStar, so I have them run a diagnostic, and it's always the same code: "P0324 ECM Engine System." This has happened three times. After I unhitch the trailer and get more gas, the "Check engine" light turns off.

I took my truck to the dealer (twice), and they told me it is "bad gas." How can that be? I feel that they just don't want to fix it. Can you tell me what my issue is?

A: It could be bad gas, but if it's happened three times with gas from different gas stations, it sounds like they don't know how to fix this and are just trying to get rid of you.

You can humor them by trying different gas, if you haven't already, but if that doesn't help — and before your warranty runs out — go back to the dealer and tell them they need to look harder.

"P0324" is the code for a knock sensor malfunction. Bad gas could cause knocking, which might turn on the "Check engine" light. But it's also possible the knock sensor is faulty. And it might malfunction only under "load," or high-stress conditions.

For example, if they tested the knock sensor at the dealership, it might have tested fine. But when you're driving uphill at 65 miles per hour, in 80-degree heat, pulling a 6,000-pound trailer, the sensor might misbehave.

It's also possible your engine control module is bad. The knock sensor "reports to" the ECM. Either way, they're going to have to make a more serious effort to fix this.

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