My daughter has a 1989 GMC jimmy. and its using way too much gas. So id like to know if i can get the codes from the check engine light on a GMC, and if it can be done would you please tell me how? Thanks a lot.
Yes you can get the codes from the ALDL connector which is the diagnostic connector located under the dash on the drivers side. You need to get a paper clip or something metal to connect the A and B slots on the diagnostic connector which are the first and second wire slots on the top right of the connector, .
When you turn on the key the check engine light will stay on. This is normal. With the key in the on position you connect the A and B wires together; the light will flash once, then twice. It will do this three times. This is a code twelve which is the only code you want to see. This is normal. If a code is stored lets say it is a code 45, the light will flash four times then five times and will repeat this three times.
The code 12 will flash first then any code after that. If more codes are stored it will store them and display them in numerical order. When you know it is finished it will flash code 12 again.
If you really need to know if there is a problem you can probably go to an Auto Zone auto parts store and they will scan the computer for you. Check with them to see if they still do this for free.
You can still have poor fuel mileage and not have any codes stored. The most common problem with fuel mileage with this truck is a lazy oxygen sensor. I know this because I own the same truck. There still can be many other things like a dirty air filter, clogged fuel filter, dirty fuel injectors, a vacuum leak from a cracked or rotted hose or throttle body base gasket. The list can go on. See if any codes come up first then go from there.