Ray Magliozzi
Dear Car Talk:
What good are the idiot lights on a dashboard anymore? I’ve had batteries die with no pre-warning, and now I discovered that my car was practically out of oil (it seems that at my last oil change, the filter was not fully tightened!), but the oil light never came on!
I thought the purpose of these lights was to warn you before you had a problem. Now it seems they only come on after the fact.
What’s the point of that?
— Mike
I guess we can change the name to the “Boy, you really were an idiot” light. No, they are supposed to come on to alert you to a problem when you may still be able to prevent it from getting worse.
Why did your oil light not come on? There are several possible explanations. One is that the light isn’t working. Most people don’t know that the so-called “idiot” lights all run self-tests when you turn the key to the start position (or first start the car).
If the circuits are all working and the bulbs are good, you should see every light come on briefly when you start the car. If a light fails to display then, then it’s not going to come on when you need it. So that’s on you to get it fixed.
Another possibility is that the oil level wasn’t low enough to trigger a pressure warning. That oil light reflects your oil pressure rather than oil level (some cars have a separate oil level light). So, let’s say your car holds 5 quarts of oil, and you are a little over a quart low. You would see no oil on the dipstick because it would be below the “fill” mark. But you’d still have 4 quarts of oil, and your pressure might still be OK.
As for the battery, it’s very hard to tell when a battery is going to die. If the output of the charging system is adequate, that light will remain off. But that just tells you the battery is being adequately charged. When batteries die, it’s usually because they can no longer hold a charge. And that’s a hard thing to measure.
That’s why I recommend replacing batteries every five years or so, so you don’t get stranded. They typically don’t last much longer than that, in my experience. If your charging system is having issues, you may then see the battery light come on. That means your alternator is failing. And that’s fair warning to you to get it fixed, or you’ll get stuck before long.
So, it’s not always the lights, Mike. It can also be that we idiots who interpret them are at fault.
Got a question about cars? Write to Ray in care of King Features, 300 W. 57th St., 41st Floor, New York, NY 10019, or email by visiting the Car Talk website at www.cartalk.com.
