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  #1  
Old 08-31-2010, 11:26 AM
Fotheringay-Phipps Fotheringay-Phipps is offline
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Resetting the Check Engine Light for Inspection

I am aware that this is generally futile. The underlying conditions that caused the CEL still exist and the computer constantly tests for them and the light will go on when it finds the same problem. And until the computer cycles through its test it will fail emissions testing based on the fact alone that the computer is not yet ready. My question is whether there might sometimes be a window of opportunity in which it can pass.

The question is whether all the conditions that cause the CEL are always present but just that it takes the ECU time to find them, or whether (some of) these conditions vary, and the ECU might sometimes briefly test the system and find no issues, such that the car can pass inspection during that brief time before the condition (or symptom) returns and triggers the light.

I imagine if your oxygen sensors or catalytic convertor are dead, you have no chance. But I'm thinking more about things like leaks in the EVAP system, detected by system pressure or vacuum tests, and I'm wondering if these can sometimes test OK even if there is an underlying issue.

Difference is that if so, you can just keep on resetting the code every day and then if you see one time that it's well past the point at which it would normally have gone on and it's still out, why then you just zip over to the nearest inspection station and pass the test and see ya next year.

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  #2  
Old 08-31-2010, 11:45 AM
engineer_comp_geek engineer_comp_geek is offline
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Quote:
Originally Posted by Fotheringay-Phipps View Post
My question is whether there might sometimes be a window of opportunity in which it can pass.
There can be. Some conditions will not be immediately detected by the engine computer, though as you noted there are many that will. My pickup truck was running rough a few years ago and it took the engine computer over a week to finally spit out an error code and light the CEL because of it (it turned out to be a bad EGR valve, which the computer did happen to finally diagnose correctly).

Depending on your state, resetting the codes might get you past inspection and it might not. In PA (where I live) they basically just connect to the OBD-II port and look for fault codes. Other states are more thorough, and actually measure the gases coming out of the tailpipe in addition to checking for fault codes.
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  #3  
Old 08-31-2010, 12:08 PM
Fotheringay-Phipps Fotheringay-Phipps is offline
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Originally Posted by engineer_comp_geek View Post
Depending on your state, resetting the codes might get you past inspection and it might not. In PA (where I live) they basically just connect to the OBD-II port and look for fault codes.
That doesn't follow.

As above, until the ECU completed its testing cycle, connecting to the OBD port will fail you, because the ECU will return a "not ready" response. See e.g.: http://www.boston.com/cars/researcha...he_car_doctor/. My question is whether there is sometimes a window of opportunity, as above.
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  #4  
Old 08-31-2010, 12:13 PM
GreasyJack GreasyJack is offline
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This is sort of the reason why the computers are designed to go through a certain number of drive cycles before the computer completely resets. So in theory, there shouldn't be a situation like you describe. If the computer isn't going to catch a problem after a drive cycle or two, it's never going to.

Of course, you can get all sorts of intermittent problems, or problems which only become severe enough to cause a trouble code under certain conditions, so there certainly are some situations where clearing the computer can allow you to squeak through if you get lucky or can avoid the conditions that cause the problem. But it's not a situation of clearing the computer and retesting before the computer has time to catch the problem again.
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  #5  
Old 08-31-2010, 03:39 PM
silk1976 silk1976 is offline
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I had an issue in my old car where the CEL would come on every so often. At first, I replaced the gas cap because the gasket was visibly aged and cracked. That helped for a while, but a few months later, it came on again. Code P0440 - small evap leak. As the car was already circling around the drain, I didn't want to spend a lot of money troubleshooting, so I kept going through a process of resetting the codes, which would keep the light off for a couple/few weeks.

Eventually, I tracked the problem down to the gas cap.. AGAIN. If I clicked it closed < 5 times, the light came on before too long. If I clicked it at least 5 times, it tended to stay off. It took me a year or more to figure it all out.

So when it came to an inspection, I looked up the driving cycle to put the computer into a ready state, cleared the codes, filled up with gas, and followed the cycle. As soon as the computer switched to 'ready' I brought it to get inspected.

Of course, 2 months later the transmission started slipping and I got a new car, but thats my story of resetting codes and going through the driving cycle to sneak past inspection.

Last edited by silk1976; 08-31-2010 at 03:40 PM..
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  #6  
Old 08-31-2010, 04:37 PM
pericynthion pericynthion is offline
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Worked for me in CA with some kind of EGR problem. I don't know if the inspection shop connected to the OBD-II port, but they did sample the exhaust and it was well within limits (though a little above average for unburned hydrocarbons)
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  #7  
Old 08-31-2010, 05:41 PM
samclem samclem is offline
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Ohio here. I've had my CEL on for about a year. I know what the problem is--catalytic converter needs replacing. 2003 Camry.

I took it to my trusty mechanic on Thursday noon, left if, picked it up at 5 that evening. He said, yep, you need a new $900 Toyota Catalytic converter. I said, sometime when I get the money. The check engine light was off when I picked up the car. I got up on Friday AM and drove it on the highway for 15 mins on the way to the Inspection. Light was still off. Was through the inspection in 10 mins, passed.

Just an anecdote.
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  #8  
Old 09-01-2010, 07:55 AM
Fotheringay-Phipps Fotheringay-Phipps is offline
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Well here's how it turned out for me.

I had the CEL on for a while, and I poked around under the back of the car and found a hole on the top of the gas tank. Sunday afternoon I plugged it up with some JB Weld and reset the code. On Monday evening, after about 85 miles or so of driving, the light went on. Almost for the heck of it, I reset it again, but then I started to wonder: if it could get 85 miles before the light goes on (before I plugged the hole it never got anything close to that when it was reset) maybe there's a window of opportunity in which it could pass inspection. Hence this thread.

On Tuesday the car was driven about 200 miles and the light was still off, so this morning I took it to the inspection station. Passed.

[I'm still not completely sure what to make of it. I'm wondering if it has to do with how much gas is in the tank.]
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  #9  
Old 09-01-2010, 01:14 PM
Yarster Yarster is offline
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I'm in CA and had a rough idle problem that would happen very intermittently that would cause the CEL to come on about once a month (and then go off again after a prolonged period) on a 2003 Infiniti FX 35. I needed a smog check and took it in when the intermittent problem wasn't happening and passed. It stil does the problem intermittently, but the car otherwise runs fine, so until I notice something else, I'm sticking with it. I suspect this might be a minor knock/ping problem anyway since they recommend a mid-grade gas for the car, but everything I've read on line (and with high gas prices) has told me that most FX 35 users are using the cheaper 87 octane (rather than 89) for their cars with no problems.
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  #10  
Old 09-01-2010, 01:37 PM
Sister Vigilante Sister Vigilante is offline
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How do you reset the code on a CEL? Mine turns on and off periodically.
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  #11  
Old 09-01-2010, 02:11 PM
pyromyte pyromyte is offline
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Originally Posted by Sister Vigilante View Post
How do you reset the code on a CEL? Mine turns on and off periodically.
It depends on the make and model of your car, but there are a couple ways to reset the CEL:
  1. Generally with most newer cars you will need an OBD (on-board diagnostics) scanner. Its a little handheld electronic device with a cable that plugs into your car under the dashboard or in the engine compartment (depending on what kind of car and how old it is).
  2. Disconnect the cable from the negative terminal of your battery, let the car sit for 10 minutes or so, and re-connect the cable. This doesn't always work, but depending on your car, it might.. and it's free. (careful, this may erase your radio station presets, or require a security code to be entered on the stereo).

Neither of these two methods fixes the problem, it just turns off the light. It will come back on as soon as the conditions that triggered it are met again. Also, with method 2, disconnecting the battery, diagnostic data can be erased and could hinder your mechanics' ability to find the cause.

Generally speaking, if the light is illuminating intermittently, I would suspect an oxygen sensor that may be beginning to fail. But the OBD scanner can tell you for sure (and I think most auto parts stores will scan for free).

The computer is constantly scanning all the sensors, and sometimes one returns a funky value to the computer. If this happens a certain number of times in a certain period of time, it turns on the CEL.

So a sensor that is just beginning to fail will only give funny readings during certain operation conditions, and depending on the way the car is being driven can cause the CEL to illuminate intermittently.

As for inspections, the methods vary from jurisdiction to jurisdiction, but with many cars, it is possible to tell from the OBDII scanner whether the car has been through enough driving cycles to give accurate data. I have heard (but can't find a link to back me up) that they can ask you to come back later to be checked again in such cases.

Last edited by pyromyte; 09-01-2010 at 02:12 PM..
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  #12  
Old 09-01-2010, 03:00 PM
t-bonham@scc.net t-bonham@scc.net is offline
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Originally Posted by samclem View Post
Ohio here. I've had my CEL on for about a year. I know what the problem is...
The one in mother's car has been on for several years. And we know the problem in that case, too. The gas cap isn't tight enough. But if it is tightened completely, my 85-year-old mother can't get it off to put gas into the car. So we just leave it looser, and ignore that light.

When they call them idiot lights, I think they are referring to the people who designed them!
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  #13  
Old 09-01-2010, 03:32 PM
Fotheringay-Phipps Fotheringay-Phipps is offline
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Quote:
Originally Posted by pyromyte View Post
Quote:
Originally Posted by Sister Vigilante View Post
How do you reset the code on a CEL? Mine turns on and off periodically.
It depends on the make and model of your car, but there are a couple ways to reset the CEL:
  1. Generally with most newer cars you will need an OBD (on-board diagnostics) scanner. Its a little handheld electronic device with a cable that plugs into your car under the dashboard or in the engine compartment (depending on what kind of car and how old it is).
  2. Disconnect the cable from the negative terminal of your battery, let the car sit for 10 minutes or so, and re-connect the cable. This doesn't always work, but depending on your car, it might.. and it's free. (careful, this may erase your radio station presets, or require a security code to be entered on the stereo).
You can generally reset it by removing and reinserting the EFI fuse. (I'm not sure if this works for all models.)
Quote:
As for inspections, the methods vary from jurisdiction to jurisdiction, but with many cars, it is possible to tell from the OBDII scanner whether the car has been through enough driving cycles to give accurate data. I have heard (but can't find a link to back me up) that they can ask you to come back later to be checked again in such cases.
See my link earlier in this thread.
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  #14  
Old 09-01-2010, 03:45 PM
Bearflag70 Bearflag70 is offline
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My CEL has been on for some time now. I think it was a loose gas cap. I tightened the cap, but the light didn't turn off. I hear this is normal as it must be reset.

I am now due for a smog check. Should I go somewhere to check the CEL before I go to the smog station? I don't want to spend a lot of dough just to reset a damn light.
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  #15  
Old 09-01-2010, 04:41 PM
Bearflag70 Bearflag70 is offline
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Originally Posted by Bearflag70 View Post
My CEL has been on for some time now. I think it was a loose gas cap. I tightened the cap, but the light didn't turn off. I hear this is normal as it must be reset.
Crap. The auto dealer said if tightening the gas cap fixed the problem, then the light would have gone off in about 2 weeks. It's been well over 2 weeks. Diagnostic at dealership $135.

Last edited by Bearflag70; 09-01-2010 at 04:41 PM..
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  #16  
Old 09-01-2010, 04:53 PM
Bearflag70 Bearflag70 is offline
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Originally Posted by Bearflag70 View Post
Diagnostic at dealership $135.
Local mechanic: $50
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  #17  
Old 09-01-2010, 05:06 PM
El Nene El Nene is offline
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Originally Posted by samclem View Post
Ohio here. I've had my CEL on for about a year. I know what the problem is--catalytic converter needs replacing. 2003 Camry.

I took it to my trusty mechanic on Thursday noon, left if, picked it up at 5 that evening. He said, yep, you need a new $900 Toyota Catalytic converter. I said, sometime when I get the money. The check engine light was off when I picked up the car. I got up on Friday AM and drove it on the highway for 15 mins on the way to the Inspection. Light was still off. Was through the inspection in 10 mins, passed.

Just an anecdote.
I did the same thing three years in a row with my 97 Corolla. I would've done it a fourth time but the light wouldn't stay off long enough so I had to get it fixed.
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  #18  
Old 09-01-2010, 09:40 PM
Anachronism Anachronism is offline
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Quote:
Originally Posted by Bearflag70 View Post
My CEL has been on for some time now. I think it was a loose gas cap. I tightened the cap, but the light didn't turn off. I hear this is normal as it must be reset.

I am now due for a smog check. Should I go somewhere to check the CEL before I go to the smog station? I don't want to spend a lot of dough just to reset a damn light.
Most auto parts stores will check the code and reset it for free. Try not to go when they are busy.
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  #19  
Old 09-01-2010, 11:20 PM
GreasyJack GreasyJack is offline
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Quote:
Originally Posted by Anachronism View Post
Quote:
Originally Posted by Bearflag70 View Post
My CEL has been on for some time now. I think it was a loose gas cap. I tightened the cap, but the light didn't turn off. I hear this is normal as it must be reset.

I am now due for a smog check. Should I go somewhere to check the CEL before I go to the smog station? I don't want to spend a lot of dough just to reset a damn light.
Most auto parts stores will check the code and reset it for free. Try not to go when they are busy.
Assuming the "bearflag" refers to the State of California, then nope. For some reason they don't let auto parts stores read codes out there. But you can get a cheap OBD-II code reader for under 50 bucks that will read and reset the codes.
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  #20  
Old 09-03-2010, 12:43 AM
Bearflag70 Bearflag70 is offline
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I found out it's a bad O2 sensor. $200.
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  #21  
Old 09-03-2010, 08:29 AM
Fotheringay-Phipps Fotheringay-Phipps is offline
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My CEL went back on yesterday afternoon, 345 miles after having reset it, and about 100 miles after passing inspection. Here in NJ, we only need to get inspected every 2 years.

I'll probably look into it some more, as it's worth having things operating properly, especially as in modern cars the components are interrelated - the computer adjusts things based on signals that it's getting. But I can do this at my leisure.
Quote:
Originally Posted by GreasyJack View Post
But you can get a cheap OBD-II code reader for under 50 bucks that will read and reset the codes.
What's your take on these connectors that hook up the car to a laptop and read all sorts of other measurements (coolant temperature, O2 sensor data etc.) in addition to the code? I can see why a mechanic who does a lot of these doesn't want to be bothered dragging laptops around the shop, but ISTM that these are a better deal for the casual DIYer.

But of course, that's only if they work. I myself bought one of these ELM327 clones off eBay for under $20 earlier this week (it hasn't come yet, as it's being shipped from China so I can't tell what it does).
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  #22  
Old 09-03-2010, 07:00 PM
GreasyJack GreasyJack is offline
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Originally Posted by Fotheringay-Phipps View Post
What's your take on these connectors that hook up the car to a laptop and read all sorts of other measurements (coolant temperature, O2 sensor data etc.) in addition to the code? I can see why a mechanic who does a lot of these doesn't want to be bothered dragging laptops around the shop, but ISTM that these are a better deal for the casual DIYer.
I never really worked on cars in any serious way into the OBD-II era, so perhaps things are different now, but my impression is that they're not significantly more useful for diagnostics than a regular code reader. Since car makers try pretty hard to make it so any odd sensor ranges that would cause problems will trigger a trouble code, problems that are diagnosable with raw sensor readings but which won't trigger a trouble code are fairly rare.

They are pretty interesting and for a DIY-er or general car enthusiast they're probably valuable as an education tool, so for instance if you know what the normal behavior of a sensor is, you might be more able to comprehend the implications of a trouble code saying that sensor is out of range. If the $20 one works, it's probably worth that just for the neat-o factor!
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  #23  
Old 09-04-2010, 03:38 PM
zwede zwede is offline
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Originally Posted by Fotheringay-Phipps View Post
What's your take on these connectors that hook up the car to a laptop and read all sorts of other measurements (coolant temperature, O2 sensor data etc.) in addition to the code?
I have one and it works well. You do get some more stuff compared to the simple readers. For instance, I can watch sensor readings in real-time. If I suspect a coolant temp sensor that is not setting a code (yet), I can start the car and watch the sensor readings as it warms up.

I can also watch the block learn at different rpms which tells me there is probably a small vacuum leak (again, not enough to set a code, but is still causing rough running).

It has other features I haven't used yet, like it can make a graph of the O2 sensor voltage transitions which apparently will tell you that a O2 sensor is getting weak although it hasn't failed yet.

I also like that it works on just about any car, it does a bunch of different protocols. The cheapos often only work on a few brands.
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