Car dash warning lights: function and interpretation.
April 13, 2008 9:36 AM   Subscribe

Car dash warning lights - what sets them off, and how soon should you act?

I have a friend who was frantically running around on Friday trying to find a mechanic before the weekend because the brake warning light went on in his car. This led him to the conviction that he would certainly die a horrible brake-related death if he didn't get it seen to immediately; if it was me, I'd take it as a heads-up and resolve to get the brakes looked at in due course (although that little light can certainly instill some insecurities).

I'm assuming that warnings like "Service Engine Soon" are time or mileage based, but how do warnings like those for brakes or oil work? Do they measure pad wear? Are they accurate? And do they mean, "caution - you're about to die" or "your pads are 25% worn so please return your Benz so that we can charge you to replace pads and rotors that aren't that worn but we want to fleece you for as much as we can, and on the upside you'll probably interpret this as us looking after you..."?
posted by forallmankind to Travel & Transportation (9 answers total)
Possible causes:

Brake pads are low (but not STOP DRIVING NOW) low. More like '20% low' like you say.
Brake fluid level is low (you can check this yourself).
Handbrake/parking brake is slightly on/switch is broken (less likely).

He should check the fluid immediately, as this is the only one of major concern, and then (if that is ok) taking it in anytime in the next 2 weeks/500 miles would be no problem at all.
posted by Brockles at 9:40 AM on April 13, 2008

Well... a "brake" warning light does indeed need to be looked at right away. It could be something as simple as low brake fluid, which might not be urgent, but on the other hand could be very serious if there was a sudden breach in the system, leaving you with no brake pressure. So, yes, that light is worthy of immediate action.

Similarly, the "oil" light means you need to stop the engine right now and check your oil level. Same with the "temperature" light.

Those lights are not the same as the "check engine" light, which usually is not anything urgent. My Cadillac also has a "service soon" light, which comes one when it senses something weird that might need attention, but is not urgent.
posted by Fuzzy Skinner at 9:43 AM on April 13, 2008

I know two people who can tell you how figuring that the red oil light meant "look at in due course" instead resulted in replacing the entire engine. (Actually, I know at least three people who have destroyed engines this way, and possibly a fourth. So your reading of warning lights as merely advisory is pretty common.)

I think you have to figure out what the consequences are of ignoring the light. Low battery? Worst that will probably happen in that the car won't start. The consequences to no brakes, no oil, and no coolant are considerably worse, and shouldn't be ignored. The oil light is triggered by oil pressure lower than a certain threshold (it's binary, unlike an analog oil pressure gauge), usually caused by insufficient amount of oil or a failed pump. Brake warning lights used to be triggered by low brake fluid (sometimes caused by very worn pads), but with modern ABS systems might be different, I don't know.
posted by Forktine at 9:49 AM on April 13, 2008

My mechanic always said that a yellow warning light meant get home or to a repair shop safely, but in the immediate future. A red light meant stop the vehicle as fast as is safe and take care of the problem then and there or have it towed. Oil Warning = STOP! Not in the middle of a highway, for safety's sake, but you may not actually make it to the next service station either.

Of course with cars you get to know the yellow lights and what you can ignore. I favored electrical tape over the always on yellow 'Check Engine' light of my 20 year old car, my best friend uses a picture of his wife. (Ahhhhh, that's sweet.) For newer cars with problems that can be troubleshooted and fixed, don't ignore them.
posted by Science! at 10:51 AM on April 13, 2008

This is of course totally irrelevant, but in my car the SRS (airbag) service light starts to flash whenever my oil is running slightly low. What will happen when the airbag really needs to be serviced I have no idea...

If the car didn't come with a user manual, they're usually fairly easy to come by. Worth having for the reassurance of knowing exactly what the manufacturer intended each light to mean. Generally speaking, any warning light is a sign that something needs investigating a.s.a.p., even if it's only the warning light itself...
posted by le morte de bea arthur at 10:55 AM on April 13, 2008

Out of similar worries, I bought a code-reader. It's not very expensive, and lets you know what the computer is complaining about.
posted by a robot made out of meat at 11:13 AM on April 13, 2008

Best answer: Your reaction was most likely the correct one. On most Mercedes, the red "BRAKE" indicator will typically illuminate if the fluid level is low, or if the parking brake is set. If it's a yellow light that flashes intermittently, then it's the brake wear indicator, and your friend is not going to have a terrible brake-related accident.

The light most likely started flashing rapidly when he applied pressure (i.e., while stopping) and slowly became more prominent (solid while stopping, illuminating occasionally while driving), which indicates that his pads are approaching the end of their life. He still has a little time left before he must take the car in, although any Mercedes technician would probably recommend that it be done soon.

The brake wear indicator light operates on a simple principle. Depending on the model and how the car has been maintained, there are sensors attached to one or more brake pads. Once the pads have worn down, the sensors (which are basically a metallic piece attached to a wire) begin making contact with the rotor. That contact supplies ground and completes a circuit, thus illuminating the brake wear indicator lamp.

As far as the "A" and "B" service indicators are concerned, those are based on a number of variables that include mileage and the actual condition of the oil as measured by a sensor. Meanwhile, the "check engine" light is illuminated when diagnostic trouble codes are set, and the oil level warning light is illuminated when a sensor detects that the oil level has fallen below a certain level.
posted by scoria at 11:29 AM on April 13, 2008

Then again, I had a "Maintenance Required" warning light pop up on my Prius. I freaked and immediately cancelled other plans so I could get it to the dealer. They then informed me that it was just a light telling me I should come in for my scheduled maintenance. Now I don't know what lights to believe.
posted by lpsguy at 6:46 AM on April 14, 2008

Little off topic, but it does show all the FUD car manufacturers are spreading about. My Cadillac gave me a "Check Engine" light last week because, get this, the power adjusting pedals malfunctioned momentarily. I realize there is no room for a "Check Power Adjusting Pedals" light, but come on now. Save the "Check Engine" light for when I should actually check the engine, and give me a "Check Accessories" light instead.
posted by enobeet at 2:07 PM on April 14, 2008

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