My 91 explorer has a check engine light on. I bought a code reader from Auto Zone that is supposed to work on the 91 explorer, however I can't find where to plug it in.

I guessed that the O2 sensor is the problem since it has never been changed, so I changed it and the explorer runs fine, but the engine light still comes on after driving for a few minutes.

How do I reset the chech engine light?

Where does the code reader plug into?

Thanks for the help.

 

Mine's a different year, but you may find it on the aft side of the air cleaner box. Look for a small rectangular cover that says "EEC Test".

If the problem is fixed, you can reset the light by disconnecting the battery for several minutes and then reconnecting. If the problem isn't fixed the light will remain.

 

You might want to scan it first before resetting it because a reset isn't going to keep the light from coming on if you haven't fixed the problem. Like said above the test plug should be in the area of the air cleaner.

 

Agreed, you should scan codes before resetting them. You can reset the codes with the scanner. This is better than disconnecting the battery to do the reset.

 

OK

I found the test plug and ran the series of tests. During the key on engine off test, we got an 11 code. The book says that this means all is OK. We did the next test with the engine running and we got 2 codes. 42 (rich O2) and 77 (operator effor).

We replaced the O2 sensor, the 91 only has 1, and we drove the car. no light for about 15 minutes and it turned back on. We tested it again and got the same code results. The tester only gives reset directions for the key on engine off test, for which we have no codes coming up. Any ideas?

Any ideas why we get this 77 code?

 

77 probably means you didn't pin the throttle correctly when you were supposed to. I'd try it again.

 

Correct, KOER 77 almost always means you neglected the goose test.

You didn't get a CM 42 at the end of the KOEO test?

I think the first thing I would do with a KOER and/or CM 42 would be to check fuel pressure and check for gasoline in the vacuum line to the fuel pressure regulator (indicates a bad FPR).

 

No 42 at the end of koeo, I just got an 11 four times, then nothing.

My testor didn't mention anything about pinning the throttle. What di I have to do?

I'll check the fuel pressure as you mentioned, but if the fuel pressure regulator was bad, wouldn't it run poorly? The truck is running great.

 

So, the tester either came with bad instructions, or just hard to follow or something. I would suggest a new set of instructions. There aren't a lot of good instructions on the net (and FTE doesn't like it when I post links to competing websites). Ken00 used to have a set of instructions taken straight from the factory repair manuals in the stickies at the tops of the BII and Ranger forums, but I'm not sure what FTE is doing in that regard right now. You might see if you can find it.

I once posted a link to a set of instructions I've posted at a competing website, but FTE blocks that link now. It was in this forum in the last couple of months, you might be able to find it.

If you put "Ford EEC-IV self-test" into your favorite search engine, you should be able to find several sets of instructions, some better than others.

Any self-respecting repair manual should also have a workable set of instructions.

Enough of that. I find it interesting that it only sees the fault during the KOER test. But you also said the CEL is coming on when driving, so there should be something stored in CM. I wonder if CM is getting erased somehow... Maybe a problem with KAM power supply or computer ground???

If the FPR was bad, one might expect it to run poorly, but maybe it's not really bad, just enough to trigger the code. I would start by checking fuel pressure.

 

I checked the vacuum line and there was no gas there. My gas milage has been getting worse and it has been running poorly cold, so I separated my self from $84 and bought the fuel pressure regulator.

Sure enough you all were correct, everything is working properly now. No codes and it is running well and idling very smooth. I have not check the milage yet, but it sure is better based on the slower movement of the guage.