Check engine light

[macinyart]

94 YJ 4.0 with 60+K miles.
Owner supplied oil changes, air cleaner, oil filter.
All gauges at optimal reading.
Runs smoothly..no problems at all.
I am wondering what the consensus is on why the light should come on? Does it mean the computer thinks (?) the oil hasn't been changed? Is there even an O2 sensor on this model? Suggestions. I would rather save my money for the next mod rather than pay the dealership to diagnose and reset something that doesn't really need to be reset? Thanks.

I made an attempt to display the codes and this is what I got;
12
21
55
My understanding is that 12 is the normal startup of the trouble code or : Battery input to PCM disconnected during last 50 key starts and code 21 references: Problem in the oxygen feedback circuit. and that 55 is the code that signifies the end of trouble code display. Does this help? Thanks.

[jporvaz]

Jack, I can't help as much as you'd like me to, however I might suggest taking it to Autozone. They'll scan the computer for you and tell you what the codes are and possibly what they mean, for free.

[RalphA]

Whenever our check engine light came on it had to do with the emission control system.

[macinyart]

Here is the resolution of the problem. I found that the O2 sensor in my 94 YJ should have 5-7 ohms resistance across the A and B poles. There are four, I checked across all them in sequence. Most had no connection at all, but one combination had 0 resistance. I figured this made this an "open" and declared the sensor dead. In addition, I disconnected the O2 sensor and got the same error codes. Thanks to John's suggestion about AutoZone which opened up a door to more thought and to George's suggestion that our local Checker carried the Bosch replacement part, I called the local Checker and sure enough they had the part. So I hopped on down there, bought the Bosch which already had the anti seize compound on the threads and installed it, started up Jeep and woo hoo...no engine check light. Success, thanks to help from friends and study. It is all good, we are back up with a quiet panel. Thanks guys.

[jporvaz]

Now if I can just fix my rear quarter panel and window as easily.....:D

[rockwerks]

Now if I can just fix my rear quarter panel and window as easily.....:D

you can its called duct tape and plastic sheeting.

[BobnJeep]

I finally found a locking gas cap, which under unknown optimal conditions, allows me to have a dash panel which only has the foglight indicator on (Dangdit signal switch needs to be replaced).

I'm Back! Thanks Joe!

[jeepcj5nut]

Excellent find and thanks for putting up your results!! This really helps a lot of people when they are searching for answers later!!!

The only part I will add to is that 0 ohms is a short, not an open. An open is infinate resistance.

macinyart

"Here is the resolution of the problem. I found that the O2 sensor in my 94 YJ should have 5-7 ohms resistance across the A and B poles. There are four, I checked across all them in sequence. Most had no connection at all, but one combination had 0 resistance. I figured this made this an "open" and declared the sensor dead."


Again, Good job!!!

[Pac8541]

So Jack, what is the Bosch replacement part # for the O2 sensor you replaced?

[macinyart]

Excellent find and thanks for putting up your results!! This really helps a lot of people when they are searching for answers later!!!

The only part I will add to is that 0 ohms is a short, not an open. An open is infinate resistance.

macinyart

"Here is the resolution of the problem. I found that the O2 sensor in my 94 YJ should have 5-7 ohms resistance across the A and B poles. There are four, I checked across all them in sequence. Most had no connection at all, but one combination had 0 resistance. I figured this made this an "open" and declared the sensor dead."


Again, Good job!!!

You are right about the infinite resistance being open and the 0 resistance is a short. I may have been sleeping, drunk or otherwise distracted when I wrote that....anyway I knew that. :confused:

So, just fortunately, I had not emptied out the trash can in the garage and found the box for the O2 sensor.
The only numbers I found on the box:
15704 and 0258005704009

It came with the antiseize already applied to the threads. :)
Hope this helps.
Again, this job was for a 94 YJ.