I’m fighting a check engine light in my spare time for a cat that is underperforming (or perhaps a O2 sensor). What I have noticed is if I reset the codes the light comes back on about the time when the monitors are set, sometimes before sometimes after but close. However if I disconnect the battery overnight which also clears the light the monitors set well before that ugly light comes back on.
My question is why this makes a difference. Thanks.
By monitors I assume you mean the sensors communicating with the PCM computer? How can you tell when they are resetting? They do not all reset at the same time. They usually go through a certain number of engine cycles or time before storing a code and setting the light on. An O2 or CAT sensor is not going to set a code until the engine warms up, I think. The difference may be clearing the code on a warm engine and having it quickly turn the light back on and clearing a code on a cold engine which takes longer to find the fault.
My engine has been modified by removing the bulky AIR air injection that runs for a few minutes at start up to heat up the CATs quickly. I have learned to ignore the engine light.
Make, model and year of vehicle would help. A piece of black tape over the dash light will cover up the annoying light. A scan cheap scan tool will also work to clear the code but it will come back on.
Resetting the codes would just reset the code entry in the car’s memory, but not necessarily reset all the memory in all the car’s computers. There could be modules which retain some state information after a code reset. Disconnecting the battery would cause all the monitors to lose all state information. With all the monitors truly cleared, it may take longer for the the cat sensors to get to a state where they throw the code. With just a code reset, some monitors may retain partial state info which allows them to get to the bad state quicker. Just a guess.
It’s somewhat analogous with your laptop. If you have a misbehaving program, sometimes you need to reboot the computer to actually fix the problem. Just restarting the program is not enough. This is because there is state information in the operating system or something which is conflicting with your program. Rebooting ensures the computer is in a clean state. The car’s codes are stored within a computer in your car, so resetting the computer by disconnecting the battery ensures the computer is truly in a reset state.
I’ve read that my old Honda takes an involved drive cycle after a battery disconnect to run all the tests that are checked to get my emissions sticker. Lots of “drive steady at X MPH” and “coast from Y to Z MPH.” I don’t know how much was actually necessary, and I don’t know if any of these tests are related to something that would trip the check engine light, but it took me about half an hour.
Not answering your specific question but maybe a helpful suggestion.
My wife’s old van had a check engine light that the dealership said was a bad catalytic converter, +$800.00 to replace.
Went to get a 2nd opinion and was told sometime the sensors (O2?) get “coated” and will give a false reading, especially if you use cheap gas. Was suggested to me to put some Chevron Techron fuel system cleaner in the fuel talk, double the amount on the first tank, for a few tanks and see what happens. Light went out in a month or so, stayed off for a little over a year and came back on. Did the same thing as before and light went out and stayed out after I regularly added the Techron every other tank until we traded the van.
Maybe something to try to get your light to stay off.
Explain what you mean by a “monitor."
Yeah, I know I missed the correct term for that, and still can’t recall what they are called, but the systems such as O2, Cat, Evap, etc that one had to drive for a while for them to be set.
OK.
I think the answer has already been posted, but to reiterate - the PCM (Powertrain Control Module) needs to re-learn the idiosyncrasies of your engine when the battery is disconnected. After that, it can determine if a particular sensor (like an O2 sensor) is giving an out-of-spec reading, or just tends to read high or low. So, the CEL (Check Engine Light) gets set much quicker if all you do is reset a particular stored code, since the PCM doesn’t have to go through the whole training process.
Note that you can’t just reset the code and immediately drive to get your emissions tested, because the PCM will report that the vehicle hasn’t gone through enough cycles yet.
Monitors are the subsystems that check various emission-related aspects of your car, such as if it’s burning oxygen correctly and if your catalytic converters are working properly. If you live in a state where you have to get your car inspected on a regular basis, it’s the value of these monitors that are being checked to ensure your car is not putting out too many pollutants.
I fixed this cheaply, with a piece of black electrical tape – place it over the check-engine light on the dashboard.
Mine has been on for some years now. My mechanic says fixing it would require replacing the catalytic converter & other sensors. And that the cost would be close to the full retail value of my 20+ year old vehicle.
So I ‘fixed’ it with black electrical tape.
From Click and Clack’s Automotive Haiku:
“Check engine” light on.
Unscrew the dash. Stab with pen.
“Check engine” light off.
That, and this year I discovered that if you don’t do certain types of driving it doesn’t matter how long it’s been since the battery was replaced, you still won’t pass the emissions test.
You see, I haven’t had a reason to drive my pickup on the freeway for years. So it never sent about 40-45. Certainly hadn’t done any speedy driving since getting my new battery last September. Failed the emissions test this April. The tech told me to drive it 10-15 minutes on the freeway 3 or 4 times then come back and try again.
It worked. I was sort of cranked to have to burn the gas to do that, but that’s what it took. Also took realizing that our local interstates are so torn up that you simply can’t coast at freeway speeds right now, too much construction. So I went zooming down our local limited access highway and some country roads at “freeway” speed to get it done.
So… apparently some of the “cycles” involve driving 50+ mph for awhile without touching the brake.
I got a new cat for one of my vehicles a few years back (another 20+ year old vehicle). $1100 all told. But it was either that or lose it because if I can’t pass emissions I can’t legally drive it around here. It was cheaper than getting another vehicle, even a used one. Do you intend to keep the vehicle a significant number of years or are you going to replace it soon?
Fact is, the old vehicles I own do cost some money in replacement parts and upkeep. I don’t panic when the check engine light comes on, but I do want to know what’s going on with it.
Except that one time the check engine light didn’t just come on, it was blinking. Tried to pull over immediately but the power steering has also disappeared. Among other things.