Check Engine Light Scams

Has anyone heard that if your car’s check engine light is on, no matter what the readout says is the problem, just take your car to a mechanic, after filling the gas tank, and he will be able to shut the light off without doing ANYTHING ELSE, just because the tank is full, and it will never come back on again?

I will be short and direct, no I have never heard this.

I have a code reader that I use to erase the DTCs and turn the light off. I do it all the time. However, I can do this with or without a full tank of gas. Makes no difference what-so-ever. And, once the computer has a chance to collect data from the various sensors, the light will come back on if the problem still exists. Sometimes right away, sometimes not for a couple of weeks. Depends on the problem.

BTW, did your mechanic tell you what the code was? It is generally in a format like Pxxxx, with the x’s being numbers.

Those who think that the check engine light is some kind of scam don’t have any understanding of how autos work these days. And as BustedKnuckes said, anyone w/ an OBDII reader can clear the codes regardless of how much gas there is. But thinking that this “fixes” something is like thinking that you can “fix” it by removing the bulb or putting black tape over the light.

The car’s computer monitors most of the major systems for performance. When something is found that is out of spec it stores a code in the computer and turns on the light. Those codes are very real diagnostic aids for fixing very real problems.

BTW: you can clear your own codes without any mechanic at all by disconnecting the battery for a minute or so.

I am an ex smog check technician. The gentleman who is erasing check engine lights “all the time” is ignoring the fact that there is an alert of some sort. It could be something silly. Ford had issues with gas caps incorrectly screwed on (yup, people can’t gas up a car right) and that set off the light. What it definitely does mean is that your computer is not recieving a correct signal from one of the many sensors or actuators that cause your engine to run properly.
Your biggest problem is finding a mechanic who is competent and trustworthy. The best way to find one is to find a shop where the owner is actually on site and personally supervising diagnostic work.

Yep. The “tank is empty” light won’t come on as long as the tank is full. Works every time!

By saying I erase codes all the time, as a technician, I read the code, determine the problem, if it exists, and reset the code. Some codes pop up due to iffy sampling and environmental conditions that trigger a code, but do not continue or return. The light may eventually go out, but not after so many key cycles without the condition reoccurring. By resetting the light first, it helps determine if it is a real problem or a ‘phantom’ happenstance.

Case in point: My Ford Explorer had the CEL come on for the first time in 5 years about 2 months ago. The codes were P0171 and P0174, both for system too lean on banks 1 and 2. Typically, these are for air leaks in the central plenum area most commonly associated with bad vacuum lines. I checked all the vacuum lines and found no apparent issues. I even checked the fuel pressure, in case the pressure was low, and leaning out the engine. No problems apparent, so I cleared the code. 2 months later, and the truck even drove an extended drive to a vacation. Total miles added were 2500. Still no CEL.

Another case: One customer’s mini-van had CEL on. Read the code, and it was P0420 - Catalyst System Below Threshold. After I told him that a new cat would be about $800, he asked if there was an alternative. I told him that a tank of bad gas can sometimes trigger this code, and we can try to clear it first to see if it returns. Cleared the code, and after 6 months, CEL has not returned. Just finished an oil change on same van, and no CEL.

I never said CELs were to be ignored. But, they are not all gospel, either. A good technician can find a fix a problem. But they also need to know if it really exists.

A competent mechanic will not use shade tree methods like black electrical tape to cover the CEL.
He will either remove the bulb and deposit it in the circular file or snip the ECM wire that illuminates the CEL bulb. Problem solved permanently with no worries about the light showing around the edges of the tape, no gas tanks to worry about, no repeated clearing of the CEL, etc.

Ahem :wink:

I’m chuckling ;-))

“and it will never come back on again”

Since the CEL is there to alert you to problems with your engine that affect performance, gas mileage, and emissions, why would anyone think that it is desirable to have a CEL that “never came back on again”? That would be one of the best examples of bone-headed shortsightedness that I can imagine.

Anyway, as ok4450 sagely implied, the mechanic who caused the OP’s CEL to “never come back on again” is an incompetent who either removed the bulb or cut the ECM wire. If the OP continues to patronize a mechanic like this, his car will have a MUCH shorter life, will likely use much more gas during its life, and will emit far more pollutants during that period of time.

Well maybe we >>have<< heard of a CEL scam now: the mechanic disables the CEL and assures the customer that it is fixed and “will never come on again”. And then when the the customer has a BIG problem because s/he did not have a CEL warning, the customer goes running back to the mechanic the get the “new” problem fixed – at a much bigger cost.

The code was P401, I think. The fellow at Adv Auto Parts said it was for a sensor that attaches to the engine. He said Don at Mine a Key would know how to fix it. Don looked it up in his book and said it was the EGR valve. Don is the one who said, when I took it in yesterday for its oil change (8 months, not even 3000 miles yet), that he just took a course in CELs and the expert lecturer told him that you cannot work on the CEL problem unless the tank is full. Don also said that I should not go over 3 months before changing the oil, no matter how few miles.
As background, the car has had the CEL go on as a pasttime. Last year it had a routine: on for 40 miles, off for 300. When its routine changed, I took it to the shop where a friend of mine works. His boss, an ASE tech., changed the EGR sensor, which had been changed twice already, and the light stayed off for 5 months…a new record! The strange thing is, the mileage is better when the light is on for a full tank. I freaked out when it went on after 5 months, but now my thought is to not worry about it until it needs its next inspection–in December. But I think that the “gas tank full” plan was full of hooey.

Oh, and Don said that sometimes the real problem is in the hoses and lines going to the EGR valve. Another mechanic (that I used to trust with my 1984 Topaz) told me that last year. My friend who works at a shop looked, and there was nothing blocking the lines. One guy even said he would need to take the valve cover off and take it to a metal shop to properly clear the lines to the EGR valve. It didn’t need it.

He was full of hooey on more than one level. That advice about oil changes is also completely bogus. Don’t let Don touch your car again for any reason.

You simply need to find a trustworthy mechanic. That is easier said than done, but that is the only solution to your problems.

A while back I was looking at used minivans and came across one with a CEL that was likely disabled - I never bothered to check out for sure why it didn’t work (there was an evap system code stored). I was guessing the seller did it, but now I am wondering is there are mechanics out there doing it as well.

“One guy even said he would need to take the valve cover off and take it to a metal shop to properly clear the lines to the EGR valve”


It appears that Monty54 has a knack for finding mechanics who are either incompetent or just plain crooks. Valve cover and EGR??? Those two words don’t normally go in the same sentence.

Monty–Is it possible that you are confusing EGR and PCV valves?

Monty might be much better off if he stopped asking his friends for advice and paid a trustworthy mechanic. Monty, the free advice you are getting is worth what you paid.

Nope, he said the large metal cover that used to be the valve cover gasket on older cars, would come off and be cleared out at a metal shop, and that would clean out everything that goes to the EGR valve.

One problem is, the mechanic who told me the metal had to be cleaned out is in the list of recommended mechanics on the Car Talk listing.

I’m sorry, but this makes no sense to me. Either you mis-heard what he was explaining, or…despite his presence on the “recommended” list…he is someone to be avoided. You have to bear in mind that this mechanic himself (or his cousin, or his brother, or a friend) may have placed him on that list.