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Ask Ron Z. Your Own Question
Ron Z.
Ron Z., - GMC Tech -
Category: GM
Satisfied Customers: 18863
Experience:  18+yrs experience. State Inspector & GM Diagnostics/Repair
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Service Engine Soon light on. I thought it meant I needed

Customer Question

Service Engine Soon light on. I thought it meant I needed oil change. Light still on
JA: Do you have the trouble code? Do you have a code reader or scan tool?
Customer: I don't have the trouble code. I also thought the auto parts store read the codes, but that practice has been discontinued.
JA: I'll pass this info along to the GM Mechanic so that he can get you a quick fix. What is the model/year of your GM?
Customer: 1998 GM Yukon
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: Well, I am trying to save $100 to have a diagnostic test.
JA: Anything else you think the mechanic should know?
Customer: I don't think so
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the GM Mechanic about your situation and then connect you two.
Submitted: 1 year ago.
Category: GM
Expert:  Ron Z. replied 1 year ago.

When the Check Engine light comes on, this is the on-board computer's way of telling you it sees a problem in one of the monitored systems. Unfortunately, there are literally 100's of parts and sensors monitored in various systems, and to take a "guess" as to which part/sensor in which system is faulty, is virtually impossible. However, if you are not experiencing any drivability issues such as skipping, stalling, shifting problems, etc, then more than likely, the problem is going to lie in the EVAP (emissions control/recovery) system. Very commonly, this could be a faulty, loose or missing fuel cap or even putting fuel in while the vehicle is running can set the light. Check the underside of the fuel cap for any cracks or signs of defects that may keep the cap from sealing correctly. If there are any doubts about the inspection of the cap, replace it. Keep in mind, you will need a cap that meets OE Specs. Those "universal" or "locking" fuel caps sold at after market parts stores do not meet OE specs, and will not seal the system correctly. Check the top of the fuel filler neck for any signs of damage or debris. Check under the vehicle, around the rear areas of the vehicle, looking for any vacuum lines that have dry-rot cracks, holes or loose/missing connections. If these all check out, then the best thing to do at this point is to have the Diagnostic Trouble Codes (aka "p-codes") read from the on-board computer. These p-codes are what is used as a "starting point" for the diagnosis and will tell which part/sensor, in which system, has the fault. There isn't a Mechanic on the planet that can tell you what is wrong with the vehicle just by "looking" at the Engine Light. Every single Mechanic's first course of action WILL be to obtain the p-codes. It's standard diagnostic procedure. Once you get these p-codes you can more accurately and efficiently diagnose the problem and then make the correct repair.

As for having the code read at the parts store- this "should" still be available, except in California. Try another location. Advance Auto, Auto Zone, and some NAPA locations still do this as a free service, in case they can sell you a part related to the code.

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