Is there a diagnostic mode to check a check engine light in a TJ? Its a new one.

http://www.webejeepin.com/Check-Engine.htm

I found this and thought I'd pass it along here.

On my 01, 4.0L I can get the codes via the odometer.

From the on position without starting the engine, turn the key off/on/off/on/off/on (3 times) and the codes are displayed on the Odo one at a time and "done" is displayed when there are no more codes stored.

I've heard some say this does not work, but it works for me...

YMMV.

but since its new, just take it to the dealer...

Originally posted by Dan-H


but since its new, just take it to the dealer...

I would if I didn't feel the codes were generated by some of my own home fab work. :rolleyes:

Just trying to figure out where I screwed up. :mad:

the most common TJ code is the downstream O2 sensor. no big deal, pull the bulb. I did. :D

Originally posted by NoJoke


I would if I didn't feel the codes were generated by some of my own home fab work. :rolleyes:

Just trying to figure out where I screwed up. :mad:


I had the same problem a few months ago... Dealer wouldn't help cuz I had "engine mods". I figured it out though & it's been running fine.

What did you change & what are your symptoms?

Originally posted by P&T jeeps
the most common TJ code is the downstream O2 sensor. no big deal, pull the bulb. I did. :D

You can pull the bulb, but unfortunately the sensor breaks easily... Upstream gets fouled by engine mods.

If either 02 sensor is broken... The Heep won't be running right.

Originally posted by Kilby



I had the same problem a few months ago... Dealer wouldn't help cuz I had "engine mods". I figured it out though & it's been running fine.

What did you change & what are your symptoms?

Ummm, I knocked the battery temp sensor off when I was installing my Kilby dual battery tray (I think). I did the on/off/on/off/on check and "done" showed up and the engine light was gone.

Very nice product Brad! Thanks.

Did you ever figure out your problem (on the rubi I assume)?

I was also installing a tranny cooler/ engine cooler and originally thought that these things were creating the trip code....now I think it was my own carelessness with the sensor.

I'm not Brad.





But we do have the same last name.
:D

Originally posted by P&T jeeps
the most common TJ code is the downstream O2 sensor. no big deal, pull the bulb. I did. :D

That is just plain DUMB.
I'd prefer to be warned the next time an injector fails, my TPS goes bad or my oil pan is empty.
Ignorance is bliss I suppose in your case though ... :rolleyes:

Originally posted by NoJoke


Ummm, I knocked the battery temp sensor off when I was installing my Kilby dual battery tray (I think). I did the on/off/on/off/on check and "done" showed up and the engine light was gone.



I don't remember where I read it, either the FSM or somewhere online, but if you have an "event" that causes the check engine light to come on, even if you fix the event the light will stay on for X number of "trips", and I think the definition of trip may be different depending on the problem, and the number of trips may also vary. In some cases it is a start, and in others its a warm up cycle.

anyway, if you don't have a FSM, I'd get one. It will save you some headache sooner or later.

That and go make friends with a service writer that wheels.

Originally posted by Dan-H


I don't remember where I read it, either the FSM or somewhere online, but if you have an "event" that causes the check engine light to come on, even if you fix the event the light will stay on for X number of "trips", and I think the definition of trip may be different depending on the problem, and the number of trips may also vary. In some cases it is a start, and in others its a warm up cycle.

anyway, if you don't have a FSM, I'd get one. It will save you some headache sooner or later.

That and go make friends with a service writer that wheels.

This is true... i was told by a Jeep mechanic that the PCM will reset after "30 start cycles". Mine did. ;)

if one can't tell that the upstream is out by the way it is running that's bad news. :rolleyes:

as for the other stuff, bliss is the word. :D

You can buy the OBDII scanner from Kragen, I think it is like $120 on sale. You can read codes and best of all, you can erase codes and turn the Check engine light off. It is nice when you are too lazy to fix the problem, but you don't want to stare at the Check Engine light. Eventually the light will come back on, but I just erase the code like twice a month.

Originally posted by wgjeep
It is nice when you are too lazy to fix the problem, but you don't want to stare at the Check Engine light. Eventually the light will come back on, but I just erase the code like twice a month.


:rolleyes:
There is such a thing as BOOTY FAB engine repair!!

Just take it to an auto zone if you have one in your area. They do free diagnostic code checks. so should the dealer if it's a new one.

I found that when you don't have a gas cap on my TJ the engine light comes on. I keep running that fawker over. :flipoff2: :rolleyes: :rasta: :laughing:

Originally posted by Drunk Guy
I found that when you don't have a gas cap on my TJ the engine light comes on. I keep running that fawker over. :flipoff2: :rolleyes: :rasta: :laughing:
Yep....they must maintain a vacuum. Oh.....get one of those things that holds it on. I kept loosing mine to.:)

my check engine light is on but when i check the codes it just goes through the diagnostic check but no codes come up. every once in a while when my tj (99 4.0 ) starts running real rough at an idle then it goes away any idea what this is and how come the check eng. light comes on but no codes are showing up?

Originally posted by mori
and how come the check eng. light comes on but no codes are showing up?

Some model year TJs didn't have the self-diagnostic display... according to rumor.

You can still take it to ____________ (insert name of local car parts store).

on mine you hold the odometer button down as you turn the key on then it goes through a check but in the end no codes are showing up but the light stays on and then every now and again it starts running real rough and then it will run fine

Originally posted by mori
on mine you hold the odometer button down as you turn the key on then it goes through a check but in the end no codes are showing up but the light stays on and then every now and again it starts running real rough and then it will run fine

what year? im having similar problems with a 2000

The hold the Odo button test is the self test of the console.

I've been told some 00s won't display codes with the key on/off/on/off/on (3 times) but my 01 does.

and the gascap off throws several "leak in Evap" codes.