Thank you!! Very thorough and meticulous.
You may have missed your calling as a college Professor! This was fantastic!
So, this seems to be a constant source of confusion for many members as of late and since I have some time to kill, I figured I'd help clarify some things in an attempt to help members with this issue in the future.
On the 4th Generation 4Runners (this applies to a lot of Toyota SUVs, Minivans, and some cars as well) when an OBD-II DTC set and stored in the system, the MIL, TRAC, and VSC lights will always come on to indicate a fault in the OBDII System. If you have an ABS capable scantool you will see a DTC C1201-Engine Control System Malfunction, this DTC is always set when an OBDII DTC turns on the MIL; its the code that shuts off the VSC & TRAC systems and turns on the lights on the dash . Don't be worried about this DTC, when you correct the original DTC(s) that set the MIL on, the C1201 will erase along with the original codes with a scantool.
Before we delve into the how's and whys, lets first clarify some terminology;
MIL- The MIL or "Malfunction Indicator Lamp" is the SAE (Society of Automotive Engineers) term for the colloquially referred to "Check Engine Light (Sometimes abbreviated as "CEL") and is most often shown as a small engine block, sometimes "Check" is under the icon or in some other vehicles "Service Engine Soon" is used instead.
Examples of MIL Icons:
TRAC- TRAC is the abbreviation used by Toyota (and some other manufacturers) for Traction Control System. This is system is often used in combination with the VSC and ABS systems.
VSC- VSC is another acronym for Vehicle Stability Control and is often what you see when the vehicle is skidding/sliding as the vehicle tries to regain control; this is often in conjunction with the ABS and/or TRAC systems.
OBDII- This stands for OnBoard Diagnostics Generation II, this a standard of automotive self-diagnostic system(s) created in conjunction between SAE, EPA, and Vehicle Manufacturers. If you'd like to learn more about OBDII here's a helpful website; OBD-II - On-Board Diagnostic System
DLC- This is the 16 pin OBD-II mandated connector that all factory and aftermarket scantools communicate through and is mandated by OBD-II regulations to be mounted under the driver's side dash area. (On the 4the 4th generation 4Runner it is located underneath the access panel for the interior fuse block)
DTC- Another SAE term that stands for Diagnostic Trouble Code; these are designed to help aid diagnosis of a problem and identify a system that may have a problem, and example of a DTC would be a P0455- [Emissions, Gross Leak Detected] (A helpful break down of how to understand DTCs if you want OBD-II Codes Defined)
PCM- PowerTrain Control Module, the heart and brain of most modern vehicle's fuel injection, ignition, emissions, and sometimes transmission control systems. This puppy make your 4Runner go vroom vroom.
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Now that we have some of the terminology explained, lets get into the two most common issues people run into:
MIL OFF, TRAC On, VSC On (and maybe ABS On IF an ABS code is set also)- There is a Non-OBDII Failure in the ABS/TRAC/VSC system and it requires an ABS Capable Scantool to pull accurate codes from, this is NOT an engine or emission related issue. Most Autoparts stores should have a capable scantool to get codes, but you may have to go to an independent shop and/or the dealer. An example of a failure of this type would be a speed sensor or failure in ABS wiring, ect. (Note that in 2003~2004 4th Gen 4Runners, low battery voltage and/or clearing DTCs can sometimes undo the ZPC (Zero Point Calibration) value stored in the ABS Module, this will result in the TRAC & VSC warning lamps turning back on despite having "fixed" your MIL condition.)
MIL is Blinking- This means there is a catalyst damaging misfire occurring (this is an OBDII mandated requirement and is present on all OBDII vehicles), you need to stop the vehicle and turn it off; failure to do so can result in permanent damage to your catalyst(s).
MIL On, TRAC On, and VSC ON- An OBDII failure has occurred in the vehicle that affects Engine Performance and/or Emission Control Systems and does not need a factory-capable scantool to pull codes (this means you can use a code reader from an autoparts store and get accurate codes) An example of this type of failure would be a Misfire, transmission issue, EVAP issue, ect.
Now I want to go into detail a little on the above mentioned OBDII failure because MANY people on almost EVERY forum there is incorrectly associate the MIL with a "Loose Gas Cap" and it gets repeated over and over and over again like some sort of magical "get out of jail" card...
An MIL on condition means there is an OBDII Failure Present in your vehicle that affects Engine Performance and/or Emission Controls and a DTC has been stored in the PCM. A loose gas cap COULD be the cause of the issue, but it more often than not is not even close.
Helpful Hint- If you have a P0455 DTC and ONLY a P0455 DTC stored in your pcm, then the gas cap MIGHT be the issue. A P0455 indicates that the EVAP Control System has found a leak over .040" and could not pass its monitor. This means you can replace the gas cap and still have the same DTC set if the leak was not the gas cap but somewhere else. Please also note that it can take a couple drive cycles to reset the MIL (without using a scantool) if the issue was in fact a loose gas cap, this often means anywhere from a day to a week or more depending on driving habits.
Helpful Hint- TSB 0030-10 indicates a prematurely failing gas cap as the cause for DTCs P0441, P0455, and/or P0456. The easiest way to rule the gas cap (this isn't 100% accurate, but works the majority of the time) is to tighten the gas cap until it "clicks", the gently unscrew it. If the cap has no resistance and unscrews it requires replacement, if the cap takes some effort to unscrew it's most likely good.
If you do have an MIL On condition, I highly recommend you go to a local autoparts store and have them pull and write down the DTCs stored in the vehicle before you post on the forum. The first thing we need to help you out with an MIL On condition is what codes are stored in the PCM; no one can do anything for you until we know what we're dealing with. This will save you the trouble of waiting on us to tell you to get the codes pulled and will help you get accurate information in a timely manner rather than taking days to get to any real significant assistance or direction.
I will also mention that some people may have scantools that indicate "Current" and/or "History" DTCs, these mean that the DTC was set by an issue that is occurring right now (Current) or that the issue has occurred in the past and the vehicle recorded the DTC for later review (History.) Typically only Current DTCs will set a warning lamp and History DTCs will simply show up when you do a health check; its important to know this distinction as you could pull up a History DTC for a problem and end up wasting time and effort trying to diagnose an issue that is not even present. Another type of DTC category you may see is "Pending", a Pending DTC is part of the "Two-Trip Logic" that OBDII dictates and some other modules use as well. Two Trip Logic means that the problem has to occur twice before the vehicle will set a "Current" DTC and turn on the MIL, a "Pending" DTC means that it has only happened once and the vehicle will not set an MIL on condition until it trips a second time and the DTC matures to a Current DTC (for example a P0420 or P0430 "Current" DTC means that the ECM ran the catalytic converter tests at two different times and returned two failure results, where as a Pending P0420 and/or P0430 means that the ECM ran the test and got a failure once and is waiting to see if it happens again before it sets the MIL and matures it to a Current DTC). Some modules such as SRS and ABS/VSC/TRAC do not use Two Trip Logic and will instead set a warning lamp on the first sign of an issue, these systems are "Single Trip Logic".
But if it makes you feel better you're more than welcome to tighten the gas cap and wait a few days to a week to see if the lights go away if you want, we'll still be here anyways.
Post Note-
I realized I never explained why the VSC & TRAC systems are disabled when an MIL is turned on. The simple answer is because Toyota is conservative and decided that since 9/10 the MIL is turned on for an OBDII DTC (which, except for a small handful of EVAP related DTCs directly impacts engine performance) that it meant the engine is not working as designed and cannot be trusted to work correctly with the VSC & TRAC systems when demanded (VSC & TRAC modulate ABS and Engine Output to bring the vehicle under control at times.) So the easiest and conservative solution is that when an OBDII DTC is set (i.e. an MIL is turned on,) they disable the VSC & TRAC systems because half the system has an issue. A lot of manufacturers have started to do this in the last decade as these vehicles get more and more complicated.
You may have missed your calling as a college Professor! This was fantastic!
