Ghost in the machine? Or does my check engine light mean something?
August 15, 2011 9:45 PM Subscribe
Why is my check engine light on at night?
Ok so this is something I've been wondering for a while. For a long time I was very concerned about it but the fact that my car hasn't died yet makes me more curious than anything. My check engine light is on...at night. I am sitting in my car right now, no keys in (it is nighttime) and it is on. No other lights are lit - nothing. In the day though I can't see it at all, it turns bright (along with other lights) when I start my car but then not visible at all. Like I said, it's been many months and no problems with the car so I'm not exactly concerned. I have a ford focus, 2003, and have been to the dealership, less than zero help there. What is this? I will monitor any responses and give additional info if needed.
Ok so this is something I've been wondering for a while. For a long time I was very concerned about it but the fact that my car hasn't died yet makes me more curious than anything. My check engine light is on...at night. I am sitting in my car right now, no keys in (it is nighttime) and it is on. No other lights are lit - nothing. In the day though I can't see it at all, it turns bright (along with other lights) when I start my car but then not visible at all. Like I said, it's been many months and no problems with the car so I'm not exactly concerned. I have a ford focus, 2003, and have been to the dealership, less than zero help there. What is this? I will monitor any responses and give additional info if needed.
Short of disconnecting the battery, there's usually a fuse you can pull to reset this, assuming it's a false positive. You can google the make and model of your car for specifics on the fuse location.
posted by drjimmy11 at 10:09 PM on August 15, 2011
posted by drjimmy11 at 10:09 PM on August 15, 2011
Curious. Just to reiterate the OP: I am sitting in my car right now, no keys in (it is nighttime) and it is on. No other lights are lit - nothing.
This isn't normal, in my experience. Even when I've got a code tripped, my check engine light is never lit when the car's off. Certainly not when the key is out of the ignition. Is this normal for a Ford Focus, MetaMechanics?
posted by mumkin at 11:44 PM on August 15, 2011
This isn't normal, in my experience. Even when I've got a code tripped, my check engine light is never lit when the car's off. Certainly not when the key is out of the ignition. Is this normal for a Ford Focus, MetaMechanics?
posted by mumkin at 11:44 PM on August 15, 2011
Right, warning lights aren't supposed to remain on after the ignition has been turned off. So this is not normal. Paranormal, if this only happens at night, or when passing the little white crosses on the side of the highway, or after meeting a lady trucker named "Large Marge."
I'm betting short circuit. Some likely short locations: the alternator (which contains the volt regulator and rectifier bridge), and the instrument panel (wiring problem, corroded circuit board, or the warning light itself). If you can get a look at the circuit board and it looks wonky, it might be as simple as a can of wd 40.
Disconnecting the battery and resetting the ECU might be a good first step though. Kind of like crossing yourself before stepping into the inner sanctum of some godforsaken shrine.
posted by L'oeuvre Child at 4:21 AM on August 16, 2011 [3 favorites]
I'm betting short circuit. Some likely short locations: the alternator (which contains the volt regulator and rectifier bridge), and the instrument panel (wiring problem, corroded circuit board, or the warning light itself). If you can get a look at the circuit board and it looks wonky, it might be as simple as a can of wd 40.
Disconnecting the battery and resetting the ECU might be a good first step though. Kind of like crossing yourself before stepping into the inner sanctum of some godforsaken shrine.
posted by L'oeuvre Child at 4:21 AM on August 16, 2011 [3 favorites]
I am sitting in my car right now, no keys in (it is nighttime) and it is on. No other lights are lit - nothing.
The fact that you are seeing a difference in brightness between ignition on and off suggests some bleed back from something in the wiring back-feeding the light. I don't think the light is on in any traditional sense (as in this is not indicating an engine fault), but something is sneaking into the circuit. Probably a blown diode in the loop somewhere.
This is not normal, but I am struggling to find a way that this is at all problematic other than the tiny current drain when the car is parked. I'd ignore it, personally.
posted by Brockles at 2:30 PM on August 16, 2011
The fact that you are seeing a difference in brightness between ignition on and off suggests some bleed back from something in the wiring back-feeding the light. I don't think the light is on in any traditional sense (as in this is not indicating an engine fault), but something is sneaking into the circuit. Probably a blown diode in the loop somewhere.
This is not normal, but I am struggling to find a way that this is at all problematic other than the tiny current drain when the car is parked. I'd ignore it, personally.
posted by Brockles at 2:30 PM on August 16, 2011
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this weekend, or whenever you're not going to need your car for an hour or two, simply disconnect your battery completely + & -. after an hour or two, reconnect your battery. this period should allow whatever code has been stuck in (basically the RAM) your ECM to clear out and be erased. It's possible that in a day/week/month that it may come back. At that point return to your dealer if you're still under warranty, or to your local shop if not. They should be able to read it again, and completely clear all codes. sometimes these things just get stuck in a weird feedback loop. hopefully that's all it is!
posted by Bohemia Mountain at 9:59 PM on August 15, 2011 [1 favorite]