I have a 2000 Toyota Corolla. I took it to AutoZone because my check engine light was on, and they plugged in some electronic thing that read the problems. What computer / electronic system is keeping track of those problems? I'd like to understand it better.

Plus that check engine light has been reset like 50 times, and it just stays on for no reason. It's been on since I was in high school (like 8 years). If I could learn enough about the computer, maybe I can write some code to keep it from coming on.

  • The check engine light doesn't stay on for no reason as you say, it is an indicator of a fault within the engine. It tells you to CHECK the ENGINE. You can technically reprogram the computer MCU, but why would you want to do that instead of fixing the reason that makes the light go off? Especially since it is easier said than done, the ECU firmware is highly complicated and you would need to replicate all of it's functions to make the vehicle work. – I have no idea what I'm doing Jun 6 '16 at 12:52
  • Because we did fix the thing it said to fix (the thermostat). Even after the replacement, with a brand new thermostat, it was still telling us to replace it. We took it to a couple other mechanics, and they said the light shouldn't be going off. – user17827 Jun 6 '16 at 16:22
  • That is odd, but keep in mind that the ECU may try to trick you. Some faults can invoke false codes, as the testing procedures aren't perfect. I wonder how it checks for the thermostat fault. If it just sees that the coolant temperature never reaches working temperature and pops a thermostat code, this might actually be a coolant sensor that hasn't failed, but shows bogus readings. In that case changing the thermostat won't help and an incompetent mechanic will just shrug. However if everything is truly fine, this could mean a faulty ECU. You could swap it with an identical unit to check. – I have no idea what I'm doing Jun 7 '16 at 14:56

The controller is called the Powertrain Control Module (PCM). On this model it will be located near the glove box. It is made by Denso Corp. These units and the software that control the system are proprietary and tampering with any of the components or parts or software is against the law in most countries. In the US tampering with any part of the system is a federal crime with a stated fine of $25,000 per occurrence. Changes to the software can be made; the software then is to be re-certified to ensure that it meets tailpipe emission limits. That said there are many who have figured out how to get around the regulators.

The other option is to repair whatever has failed that is causing the trouble codes to set. The CEL does not set for no reason.

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    By all means, get the light "codes". P3xx (ignition) P17x bank lean? P420 catalytic converter efficiency ?? There are a myriad of codes that can be read out with the scan tool associated with the Check Engine Light, which is simply an indication "You have a problem" ... – SteveRacer Jun 5 '16 at 3:50

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