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Ron Z.
Ron Z., - Subaru Tech -
Category: Subaru
Satisfied Customers: 17790
Experience:  18+yrs experience. State Inspector and Subaru Diagnostics
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My check engine light keeps coming on. i remove the negative

Customer Question

hi sir, my check engine light keeps coming on. i remove the negative terminal to reset it a couple of times, light comes off but then after a day or two it comes on again. but it runs fine. will i be hurting the engine, if i continue to drive it?
JA: Does the light stay on, or does it blink?
Customer: it stays on untill i reset it
JA: What is the model/year of your Subaru?
Customer: 98 impreza
JA: Are you fixing your Impreza yourself? What have you tried so far?
Customer: i just undo the negative terminal
JA: Anything else you want the mechanic to know before I connect you?
Customer: sometimes i unplug one of the fan connectors and it reset itself
Submitted: 1 month ago.
Category: Subaru
Expert:  Ron Z. replied 1 month ago.

Hi Ed! Welcome to JustAnswer. I'm Ron Z and I'll be helping you with your question today.

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. You can have these p-codes read FREE (except in California) at any local "big chain" part store (ie. AutoZone, PepBoys, Advanced, etc).

When you remove the NEG cable, all this does is reset the on-board computer. It takes a "Drive Cycle" for the computer to 'reboot' and re-check all the systems. This is why the light stays off for a while. Problem not solved, just put off temporarily.

As for driving the vehicle- As long as there are no drivabilty issues or shifting issues, then it is perfectly safe to drive the vehicle. You should however, have the codes retrieved at least to verify. If the problem does lie in the EVAP system, it is strictly for emissions control/recovery and will in no way effect engine performance in any way. If the problem lies outside the EVAP system, it may be a good idea to have it serviced as soon as possible, as prolonged driving with a problem with the engine or transmission can make matters worse.

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Expert:  Ron Z. replied 1 month ago.

Hi Ed! I noticed this thread was still open and you did not leave a rating , so I wanted to check in and see if there was anything else I could do for you on this issue? Did you have any other questions? Was there something I missed that you wanted addressed? If you have no further questions and simply forgot to leave a rating, please take a second and use the stars at the top of the page to rate our Q&A session. Thank-you!

Expert:  Ron Z. replied 1 month ago.


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