Dear Tom and Ray:

I have a 1985 Mercury Grand Marquis. Recently, the oil-level light came on, so I checked the level on the dipstick. It was in the safe zone. So, which do I trust, the dipstick level or the idiot light? Thanks.

-- Scott

TOM: In dipstick we trust, Scott.

RAY: The dipstick almost never lies. It can give you a false reading if you use it incorrectly -- if you check it while the engine is running, or just after it’s been shut off and there’s still oil in the upper part of the engine that hasn’t drained down yet. But generally speaking, the dipstick tells the truth.

TOM: You’re lucky in that most cars don’t even have an oil-level light, Scott. They have an oil-pressure light, which only comes on once the situation is a “pull-over-right-now-and-shut-off-the-engine” emergency.

RAY: But your car, and some other Mercurys, came with an oil-level sensor that plugged into the side of the oil pan. That gives you an early heads-up, well before it’s an emergency.

TOM: It’s a great thing to have. I don’t know why every car doesn’t have one.

RAY: But after 30 years, for some reason, your oil-level sensor decided to croak.

TOM: I definitely would replace it. It’s inexpensive and easy to install. And it’s great to have, especially on an older car, which is increasingly likely to spring a leak or burn some oil.

RAY: The time to replace the sensor is next time you’re having your oil changed. Once the car is up on the lift, it’s a five-minute job.

TOM: But in the meantime, the dipstick will give you an accurate reading. Check it first thing in the morning, when the engine is cold. In fact, you can pad out to the driveway and wipe the dipstick on your pajamas.