Is it possible a broken exhaust was preventing the service engine soon light...

Is the "Service Engine Soon" light the same as the "Check Engine Light" on the ZJ? I thought the service soon light was different and based on mileage for specific maintenance intervals.

Maybe it is check engine- I never realized they were different things- in my mind they were one the same.

BirdOPrey5 said:

Maybe it is check engine- I never realized they were different things- in my mind they were one the same.

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They may be. It just sticks in my mind that some vehicles in the 80's and 90's had a service interval light that was different from the CEL. Could be wrong though.

wct097 said:

They may be. It just sticks in my mind that some vehicles in the 80's and 90's had a service interval light that was different from the CEL. Could be wrong though.

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Yes, they did. However, "Service engine soon" and "check engine" are typically both referring to the malfunction indicator light.

My 03 Grand Prix company car had a separate idiot light for changing the oil.

Mom wasn't getting anything for 3 times... so she did 6 times like it says on this video and got the following on the odometer:

12, 21, 45, 17, 55

How does it run otherwise?

With it kicking out code 21, I'd put an oxygen sensor in it, clear the codes, and drive it.

Ralph said:

How does it run otherwise?

With it kicking out code 21, I'd put an oxygen sensor in it, clear the codes, and drive it.

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Seems to run OK otherwise... Anyway to figure out what 45 is?

When you say put an oxygen sensor in it, you mean, replace the one that is there?

BirdOPrey5 said:

Seems to run OK otherwise... Anyway to figure out what 45 is?

When you say put an oxygen sensor in it, you mean, replace the one that is there?

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Not sure what 45 is... according to allpar: 1980s Chrysler, Plymouth, and Dodge computer fault codes it's 45 Turbo boost limit exceeded (engine was shut down by logic module)

But, yes, I mean replace the oxygen sensor. Can't remember if ZJ has one or two.

Chrysler Computer Codes, 1980s-1990s

According to the above code 45 means: Turbo engines boost above predefined range

If the 17 code is legit, you might be getting an O2 sensor code as a result of the engine not getting up to temp. Would be odd, for sure, but possible.

It can run okay with a bad O2 sensor, it will just guzzle fuel.

there will be an upstream and downstream O2, check for pinched/burnt wires before just replacing anything. plus a real scanner should indicate the O2 sensor in question.

Actually it kind of makes sense- I see another explanation for 17 as "Coolant sensor not reporting more than 160° after 8 minutes of engine running."

The jeep is rarely driven more than a few blocks at a time. The repair place is about 4 minutes away and the furthest it has been since the repair. It probably never gets that hot.

wct097 said:

If the 17 code is legit, you might be getting an O2 sensor code as a result of the engine not getting up to temp. Would be odd, for sure, but possible.

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Definitely possibile, but O2 sensors are exceedingly common Chrysler failures. I've done at least one on each chryco vehicle I've owned. If it's throwing the 21 code, I'd replace it if it hasn't been done in awhile regardless of any other codes.

John said:

It can run okay with a bad O2 sensor, it will just guzzle fuel.

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A bad O2 sensor can be completely asymptomatic other than tossing a CEL.

Ralph said:

Definitely possibile, but O2 sensors are exceedingly common Chrysler failures. I've done at least one on each chryco vehicle I've owned. If it's throwing the 21 code, I'd replace it if it hasn't been done in awhile regardless of any other codes.

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Agreed. They're relatively easy to replace. Ditto on checking for burnt wires, but even then, it may be easier just to replace. I think I've replaced the ones on my TJ about 4-5x in the ~230k miles I've driven it.

If the engine never gets up to temp in regular driving due to short trips, I'd also be tempted to just clear the code, then start it and let it get completely up to temp a couple times to see if it persists.

I also always pull the O2 sensor and bench test it before I replace it since the old Chryco shit doesn't always accurately denote which sensor is bad. Bench test = propane torch and multimeter. I usually hit it with a MAPP torch first to burn out any fouling.

wct097 said:

I also always pull the O2 sensor and bench test it before I replace it since the old Chryco shit doesn't always accurately denote which sensor is bad. Bench test = propane torch and multimeter. I usually hit it with a MAPP torch first to burn out any fouling.

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Not a bad idea. As I mentioned before, I have an OBD suite on my laptop which can monitor the voltage from O2 sensors. I've used it before for specifically this purpose on a Neon. The downstream sensor was outputting a nice constant voltage within spec. The upstream was super erratic. Told the owner "replace the upstream one." Handy as hell

[edit] Side note, I had an o2 sensor replaced on my ZJ last year. They had to cut it and the threaded, welded nut off the exhaust system. They welded on a new nut.

I just have Torque and a BT scan tool. I don't think my TJ gives me voltage from the O2 sensors. Could be wrong though.

wct097 said:

I just have Torque and a BT scan tool. I don't think my TJ gives me voltage from the O2 sensors. Could be wrong though.

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I havent found a mobile app that will drill down to O2 sensor voltage. I cant think of any reason your TJ wouldn't give you that reading.

It has voltages for some things. I didn't drive the TJ today, otherwise I'd go check.