Check engine light reset - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 03-20-2004, 04:22 PM   #1
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Default Check engine light reset

I wanted to share my recent repair. Last week when I started engine Ran rough and check engine flashing and traction off light on. Had autozone read codes. Cyl 2 misfire. Spark on 2 ok, No idle change when I unplug 2 fuel inj but idle changes when other cyl are unplugged. I suspect could be fuel inj, computer or wiring between computer and inj 2. I remove 2 and 3 inj and swap them to see if problem stays at 2 or moves to 3. Now no problem at all car runs fine. Check engine light is on now but not flashing and trac off light is off. So i disconect bat to try to reset check engine light. no go. I drive car 1 mile and light goes off and stays off.
2 weeks later it reoccurs. Autzone scans to see if 2 or 3 and says 2 again. I disconnect 2 and idle changes. no change on 3 though which now has 2 injector so I concur that #2 injector was bad orginally since it now failed in the #3 position. Strange that the code was still reading the 2 failure from the first time it malfunctioned. Does computer store these inactive codes still? I tapped on the bad inj with wrench and it started working and check engine light stopped flashing. I reset engine light by turning ign on and off 8 times (not to start just to get dash lights on) and light went out. I found this on Autofan.com. Think I still need a new inj. I put some fuel inj cleaner in tank don't think it will work though. Lets here what you have to say? 65,000 mi
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Old 03-20-2004, 07:17 PM   #2
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I thought the only way to clear OBD-II codes was with a scantool.
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Old 03-20-2004, 07:20 PM   #3
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When the check engine light is flashing on an OBD2 vehicle it means that damage is occuring to your catalytic converter. The most common thing is of course a clyinder misfire. You were actually doing a cylinder balance test when you were unplugging the injectors and noting the engine speed difference. You should hook up a tach and write down each cylinder drop when you unplug the injector. When you find the one that is misfiring (no drop) you need to ascertain whether it is the injector, PCM or ignition. Use a screwdriver and hold it to the injector and to your ear (make sure you do it to one that you is good). Does it click? No... At this point you will need a digital VOM (Volt Ohm Meter) It needs to be digital because it has to be a high impedance meter. The analogue ones run the risk of damaging the PCM. There are 2 wires that go to the injector, one power and one ground. The switching from the PCM is done through the Injector Drivers which are simply transistors. These drivers simply make a connection to ground when it wants the injector to fire. So, power is always on (with ignition to run) and the actuating of the injector is done on the ground wire. Check for power with the DVOM on one of the wires (can't remember the colour..) but one should have 5volts (or maybe 12...again can't remember off hand, but one or the other) Is there power? No, check for a problem with the wire of out of the PCM. Yes?... Use a test light and connect the clip to battery positive and the other end to the unplugged end of the connector ground. When the engine is running is it flashing? If it is flashing then the drivers are working properly. If it is on constant then you have a problem with the driver and it is either a short to ground of the wire or the PCM itself. If it is off then check for an open in the wire between the connector and the PCM. If all of that is working fine then check the injector itself. Compare the resistance between a known good one and the suspect one. But actually if the circuit itself is working fine then it is obviously the injector.

First thing though is to ascertain if it is the fuel or the ignition.

Hope this helps
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Old 03-20-2004, 07:23 PM   #4
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To clear a code quickly it must be reset by a scantool. To clear it by itself it must go through a set number of drive cycles without having that problem again. A drive cycle is an amount of time with a minimum engine temperature and a minimum vehicle speed to make one drive cycle. ie. a trip out on the highway. This could take up to 20 drive cycles with no problem. Usually the tech will just clear the code after he fixes the problem. Simply disconnecting the battery will not clear the code like an OBD1.
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Old 03-21-2004, 12:11 AM   #5
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As said above- a scan tool is needed to clear any codes from the computer
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Old 03-21-2004, 12:59 AM   #6
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So eventhough the car is running fine now and no check engine light is on the cylinder #2 misfire code will stay in the computer until it is erased with a scan tool?
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Old 03-21-2004, 01:06 AM   #7
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Or is it erased now since it has no check engine light on. As far as getting the light to go off I found that on a Pontiac sunfire site about cycling ignition to clear non active codes. There example was the loose gas cap that will set the check engine light. After it was tightened cycle ignition 8 times to turn light off. It worked just like the guy said but I think that it turned light but still is in historic codes and has to be erased or driven out like you said.
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Old 03-21-2004, 09:46 AM   #8
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Quote:
Originally Posted by jebstuart
So eventhough the car is running fine now and no check engine light is on the cylinder #2 misfire code will stay in the computer until it is erased with a scan tool?
In simple terms the DTC resides in history, however after specified number of warm up cycles without the DTC being set, it will clear out, as stated previously by Bonnie Canuck.
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