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.

Thx for the info, if I run into these problems, I'll have something to refer to, check on, and wait before bugging you.

Sounds like your IB got flooded with ?'s. LOL.

Hey BlackWorkInc, haven't seen any recent outcomes or post on your quick disconnect SB. I personally was interested in this setup.

Na, its just been something I thought about doing and the slew of threads lately with so many gas cap answers just sort of made me a little looney.

"Sticky" me!!!!

Maybe YOUR gas cap is loose....

This is awesome. Thanks! STICKY!

If people want to monitor the truck and read dtc's the UltraGauge is a great cost effective option. $75 for digital screen for monitoring and allows you to read and reset the DTC's.

This happened to me and it was a faulty gas cap. Before you spend tons of money go to the local Toyota dealership and spend $30 for a new gas cap. I installed it and disconnected the battery and let it sit for about 10 minutes then I connected the battery back and everything was cleared and I haven't had any problems since. Try that and if that doesn't work then it might be the OBD.

This entire thread is the result of posts like the one above.

What I am trying to explain to people is that gas caps do not fail nearly as much as everyone seems to thing and that every MIL is not fixed by a gas cap (which by the way has an OBDII DTC that can direct you to that being a problem).

The above post shows the erroneous thinking I am trying to bring to the attention of the forum in general; just because you THINK a gas cap fixed it doesn't mean it actually did and that it works for everyone.

What I mean by that is, in your post all you did is put a new gas cap and cleared the MIL by doing a "poor man's reset" on the PCM. You didn't bother to diagnose the MIL on by pulling codes (which for the most part can be done for free), did not bother to verify what the codes were and THEN perform repairs and/or further diagnosis. What you did is took a guess, and hoped you were right; for all you know that MIL was on because of an intermittent transmission solenoid and/or maybe a borderline failing O2 sensor or Catalytic Converter. You don't know, because you didn't get the information needed to make and accurate judgment call; the DTCs, the Freeze Frame Data and any other useful information is now gone and that problem may resurface two weeks from now or 2 years from now. All you've succeeded in doing is creating a baseless and erroneous "logical association" with your 100% unsupported guess and the fact the light stayed off, which is what a lot of crap mechanics and a lot of crap dealers do to everyone already; why should we perpetuate that on the forum?

The entire purpose of this thread is to show you how to properly diagnose a vehicle, not haphazardly throw parts at it and hope it fixes itself. No one on this forum, including me has any business telling anyone what the "fix" for their MIL is until we have the proper information to even begin diagnosis. How would you feel if you went to a mechanic and was like, "hey, my truck runs rough and doesn't shift right..." and the technicans goes, "Gas cap, that'll be $200." You'd be pissed and insulted, there's a reason why we charge at least .5-1.0hrs to diagnose a check engine light rather than charge you $100 to slap a gas cap on and have you come back and pay more money.

Edit-
I apologize for the agressiveness of the post, I'm not attacking you personally; I am just trying to point out the reason this thread was made and why thinking like the above tends to cause problems in general.

Wow man I am not a mechanic but I went by advice with this same problem on multiple posts in this same forum. I am not a mechanic and I tried what I read in a couple of other posts. I was just trying to help out. That is all.

I understand that and I do apologize if it came across like I was attacking you out personally. My intent wasn't to beat you into the ground; because as you said "you were following the advice you read in multiple posts."

That's what I was really trying to get at, when people see dozens of people saying the same thing, you get a sense of false security in that statement, particularly when its just parroted and without substantial evidence or understanding to back it up. All I am trying to do with this thread is to explain to people the reason a check engine light comes on and how to properly start the process of figuring out why it is on.

You don't have to be a mechanic to pull codes, or even to try and understand them somewhat; but if you can get all the information possible, then you can make an informed decision on how to repair that vehicle properly.

Once again, I apologize if I made you feel that I was attacking you in any way as that was never my intention; I'm just trying to help people of varying skills and experience approach vehicle repair in an informed manner.

Hey man it is all good. I know we are all here to try and help each other out. I don't have any hard feelings regarding the post.

I “hope” the CK Eng Light and VSC TRAC lights are off and solved. “Before the recent info.”

1. I took the advice of getting the truck scanned for a code and it showed a small leak at the gas cap. I d/n think to ask the Tech for the code vs him tell and what it represented.

2. At this point I noticed no resistance of a seal on the cap and the good bad “Soft O-Rings” shows why. Plus, the old was not pushing the cap off enough to help remove the cap (attached).

3. The Tech said the light should reset later. I asked if the scanner would detect the leak as solved and it did (the light also went off).

4. To and from the Auto Store the back driver’s side window went down on its own. Is there a connection there?

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Thanks for the exhaustive explanation here. My 2009 4runner SR5 V6 with 113,800 miles suddenly (after being parked for a couple of months) lit up the check engine and VSC OFF indicators. The cause was a worn out gas cap. I kept tightening it but it kept getting loose.

Spent $8 on a new gas cap from a popular website but had to wait about three days (did not drive a lot of mi.) before the sensor(s) turned off the 'lights'. Thanks for sharing your knowledge here. Saved me grief, time, and money.

I have a 2003 Forerunner 6 cylinder Limited 4-wheel drive 198K miles
I had a dead battery so I replaced it and the tow truck guy said the check engine light would be off and to go take it to AAA to get it inspected. I went to AAA Service and they said I had two error codes that were incomplete and to drive it 50-70 miles, which I did. Still had not cleared but before they did, the check engine light came back on so they reset it. They said the scan shows a 420 code (driver's side catalytic converter) but no other code. So I spent the next couple of days driving it, getting it retested, and same 2 INC codes for Cat and Evap System. I am now being told, not by AAA, that in order to get the Evap System to clear, I have to do a specific drive cycle, certain amount of gas in the tank, etc and I have to eliminate this 2nd incomplete error code to pass the inspection.

So is there some place that has the proper methodology to get thru the INC for the Evap System? Again, nobody has said there is any error code associated with the Evap system. Did it show up since I had my battery replaced? Can a mechanic clear this code or does it just have to reset itself by this proper drive cycle/amount of gas/etc requirement?

I know I need to get 4 new catalytic converters and probably oxygen sensors but the estimates are high and I really just need to get my car to pass this inspection since my registration has expired and I cringe every time I see the police.

Thanks for your time.

Dusty Butler
Durham, NC

I have 20 OBD DTC readiness code and I already pulled the battery and plugged it back in. The truck doesn't have a CEL, but does anybody know if it will pass emission test? I'm thinking of replacing the O2 sensor or even the ECM too.