O2 Heater Codes

[Mercury]

Hello!

I have an odd problem with my 99 TJ 4.0. Both of my O2 sensors went out a few months ago, standard codes, replaced both with NTK/NGK. A few days ago I noticed the idle being rough and smelling stronger exhaust than normal.

I pulled the codes and got heater circuit codes (screenshot attached). I pulled the fuel trim info (also attached). There's no CEL but these codes are in memory and pop right back after clearing.

I posted on another forum and was told it was likely a bad PCM. In an effort to avoid dropping $300 to get a replacement I'm hoping someone here might have another idea of what could be wrong or at least something I can test.

Do I have any other options?

 

[Jerry Bransford]

Check the O2 sensor wiring for any signs of damage from heat from the exhaust system. And Mark at @Wranglerfix can tell you if that sounds like a PCM problem, he's the guru of Jeep PCMs. www.wranglerfix.com

 

[Mercury]

Check the O2 sensor wiring for any signs of damage from heat from the exhaust system. And Mark at @Wranglerfix can tell you if that sounds like a PCM problem, he's the guru of Jeep PCMs. www.wranglerfix.com

Hey, Jerry!

I sent Mark (or at least his company) an email.

Alright, I'll go check the wiring. If it's something I can just run a little wiring and fix I'll be a happy camper.

 

[Mercury]

The wiring seems good overall. The only thing that looks suspect is this:

 

[B1Toad]

Here's how to trouble shoot the 02 sensor heating circuit on a 1997 4.0 https://www.scannerdanner.com/chapt...2-p0175-from-a-blown-o2-heater-fuse-jeep.html Actually this one was just a blown fuse but he has another one on how to test the circuit but I can't find it, it was also on a Cherokee 4.0.

Pay attention around 9:15 in the video where he explains that the ECM does not control the voltage to the 02 sensor heater circuit on the 97 and look at your wiring diagrams to see if 99 is the same as 97..

 

[B1Toad]

Basically the test consists of you finding the two wires that feed the heated 02 sensor and check for voltage at the connector and if it's not there follow it back to the fuse box. Notice that of the 4 wires, two are 12 volt + and - for the heater power, then there is a 02 signal wire which I think is tan/white and finally there is a sensor ground which I think is black/lt blue and it goes to the ECM. Since your ECM is already reporting a problem with the heated 02 sensor we know that it must have a connection to the sensor, so I would suspect the 12 volt circuit that feeds the heater. In other words, the ECM does not provide the power to heat the sensor but it knows when the sensor is not getting heated, so it can only be the heater circuit.

 

[Mercury]

Basically the test consists of you finding the two wires that feed the heated 02 sensor and check for voltage at the connector and if it's not there follow it back to the fuse box. Notice that of the 4 wires, two are 12 volt + and - for the heater power, then there is a 02 signal wire which I think is tan/white and finally there is a sensor ground which I think is black/lt blue and it goes to the ECM. Since your ECM is already reporting a problem with the heated 02 sensor we know that it must have a connection to the sensor, so I would suspect the 12 volt circuit that feeds the heater. In other words, the ECM does not provide the power to heat the sensor but it knows when the sensor is not getting heated, so it can only be the heater circuit.

Interesting! Do I test this with the engine running, key on accessory, or fully off?

 

[B1Toad]

Interesting! Do I test this with the engine running, key on accessory, or fully off?

Sorry I was out on a bike ride. Yes start the engine so automatic shutdown relay closes. See attached wiring diagram for my 2000 which I added notes to, not sure if your 99 is the same but likely. If you have California emissions none of this pertains, it is a lot different with relays included in the heater circuit.
Those splices you show look a little suspicious.

 

[B1Toad]

It's interesting to note that your #2 sensor threw a high voltage code but your #1 did not, just like in the video voltage was higher on the #2 sensor because it is further from the manifold and colder. They both threw heater malfunction codes but only the downstream one is cold enough to throw the high voltage code.

 

[Mercury]

That write up is really handy, thank you!

It's interesting to note that your #2 sensor threw a high voltage code but your #1 did not, just like in the video voltage was higher on the #2 sensor because it is further from the manifold and colder. They both threw heater malfunction codes but only the downstream one is cold enough to throw the high voltage code.

That page with the notes is really helpful, thank you!

I have a spoofer on the second one so it's possible it could be related, but I've used that spoofer for years without issue.

New update: I didn't realize there was a fuse for the O2 circuit. It was blown. I replaced it, ran the Jeep up to temp, and all but the sensor two voltage went away. Of course that isn't the solution, something caused it to blow, but I'm guessing there's a short. I'm going to replace the bad wiring (the part that's bad looking was touching the intake manifold) and see what happens.

I'll watch that video in a few and try to get the wiring and diagnostics all done tomorrow morning before the bad weather hits.

 

[B1Toad]

It's possible whoever spliced those wires didn't know about the fuse either.
edit: nevermind, I see you've owned it awhile and the issue is new.

 

[Mercury]

It's possible whoever spliced those wires didn't know about the fuse either.
edit: nevermind, I see you've owned it awhile and the issue is new.

It's ugly. Just butt connectors and tape that has now fallen off. I have heat wrap and silicone tape, should be able do that a lot better.

 

[Mercury]

Fixed the bad patch of wiring and the codes returned, except this time not bank 1. Irritating because I had hoped for an easy fix and planned to drive the Jeep today.

 

[B1Toad]

Did the fuse melt again? So this time it's only your upstream sensor, was the bad looking splice at the upstream or downstrean sensor? Keep in mind that orange and dark green wire that comes from the fuse has a splice in it that sends power to the other sensor. In the diagram it looks like power hits the downstream first then heads over to the upstream.

 

[Mercury]

Did the fuse melt again? So this time it's only your upstream sensor, was the bad looking splice at the upstream or downstrean sensor? Keep in mind that orange and dark green wire that comes from the fuse has a splice in it that sends power to the other sensor. In the diagram it looks like power hits the downstream first then heads over to the upstream.

Funny enough the fuse was fine. When I replace the blown fuse yesterday and ran the Jeep all the bank one codes went away, but the bank two high voltage code was still there. This time as soon as I started the Jeep the bank one heater code came on, and once it reached operating temperature the bank one high voltage came on, but not the bank two. Stopped the jeep, checked the fuse, fuse is fine.

 

[B1Toad]

Just to avoid confusion, you only have one bank, bank 1. You have two 02 sensors, HO2S-1 and H02S-2. H02S-1 is the one before the cat and H02S-2 is the one after the cat, and they are both on bank one because there is no bank two on our inline engines. The CA models with the extra cats under the hood may call them bank 1 and bank 2 but that I can't confirm.

 

[Mercury]

Just to avoid confusion, you only have one bank, bank 1. You have two 02 sensors, HO2S-1 and H02S-2. H02S-1 is the one before the cat and H02S-2 is the one after the cat, and they are both on bank one because there is no bank two on our inline engines. The CA models with the extra cats under the hood may call them bank 1 and bank 2 but that I can't confirm.

Definitely mixed that up.

It's just strange that when I replace the blown fuse the upstream sensor codes went away but the downstream high voltage code remained, then when I replaced those bad sections of wire the upstream codes came back and the downstream code went away.

 

[B1Toad]

You're going to have to unplug them, identify which wire is which and check for voltage with a meter.

 

[Mercury]

Basically the test consists of you finding the two wires that feed the heated 02 sensor and check for voltage at the connector and if it's not there follow it back to the fuse box. Notice that of the 4 wires, two are 12 volt + and - for the heater power, then there is a 02 signal wire which I think is tan/white and finally there is a sensor ground which I think is black/lt blue and it goes to the ECM. Since your ECM is already reporting a problem with the heated 02 sensor we know that it must have a connection to the sensor, so I would suspect the 12 volt circuit that feeds the heater. In other words, the ECM does not provide the power to heat the sensor but it knows when the sensor is not getting heated, so it can only be the heater circuit.

So, to confirm, I'm setting my DMM at voltage, turning the Jeep to on without starting, plugging the leads into the two ports that handle power for the heater circuit where it plugs into the O2 sensor, then testing?

If I mix up the wires will I damage anything?

Can I test the heater within the O2 sensor itself this way with a conductivity test?

 

[B1Toad]

The engine has to be running or the ASD relay will not close. Testing for voltage you should not damage anything and yes you can check the heater inside the sensor with the Ohms setting but I don't know what the value would be, but at least you can verify that it is not open. If there is no power on the two heater wires try checking from hot to the frame, then if you see power you'll know it's a bad ground, otherwise it's the hot side. Might be a good idea to verify your engine to frame ground anyway.