So I have been having the P0420 code on my '02 3.0L S-type. I changed out the coils and plugs (all six) about a month ago now. (See other threads for that info) The vehicle has about 98K miles on it now.

In the last month, we have gotten 3 more P0420 codes. It seems to be going up a hill (curiously the same hill each time).

I am wondering of it is really the Cat converter on the passenger side or if one of the senors has gone bad on it.

Does it seem like a reasonable test to swap the 02 sensor after the CatCon with the one from the left side. They appear to be the same part number.

So I am thinking before I go spend $50-150 on a new O2 sensor or big bucks on the CatCon I could swap out the post-converter sensor from L=>R and R<=L and see if the code moves with the sensors?

Does this seem like a reasonable thing to try? Any dangers or potential damage in doing this? I can get to the sensors, no problem. I have had my hands on them many times in the past...

If you KNOW let me know. The JTIS also has some senors tests to check them out. I guess I will run those while I have them out as well.

 

Just BTW: Changing out the plugs and coils was not futile. That process did get rid of all the engine "stutter" or "shudder" once the engine is warm. It runs MUCH better now. It is getting about 25mpg in combination hwy/city driving.

But I do not like this CEL P0420 and wish I could resolve that...

 

I have read a post here iirc that JTO noted thats the way to test if a sensor is bad or going bad. He'll have to chime in on that to be certain.

It seems to make sense in theory, so I would do it personally.

 

Just to be clear, the two sensors I am thinking to swap places with each other are these two after the catalytic converters...



Is this worth a try?

 

The JTIS lists these tests of the Catalyst Monitor Sensor... but I am not getting the monitor code the JTIS suggests for these tests???

F1: CHECK THE VOLTAGE AND SIGNAL GROUND SUPPLY TO THE CATALYST MONITOR SENSOR

1. Switch the ignition switch to the OFF position.
2. Raise and support the vehicle. For additional information, refer to
3. Disconnect the right hand catalyst monitor sensor electrical connector GB3.
4. Switch the ignition switch to the RUN position.
5. Measure the voltage between the right hand catalyst monitor sensor electrical connector GB3 pin 1, and pin 3.

Is the voltage greater than 10.5 volts? Yes goto F3 No goto F2

F2: CHECK THE CATALYST MONITOR SENSOR VOLTAGE SUPPLY CIRCUIT

1. Measure the voltage between the right hand catalyst monitor sensor electrical connector GB3 pin 1, (GR) and ground.

Is the voltage greater than 10.5 volts?
YES REPAIR the circuit between the right hand catalyst monitor sensor electrical connector GB3 pin 2, (BO) and the powertrain electrical connector GB1 pin 15, (BO). CLEAR DTC. TEST the system for normal operation.
NO REPAIR the circuit between the right hand catalyst monitor sensor electrical connector GB3 pin 1, (GR) and the spliced joint GBS1. CLEAR DTC. TEST the system for normal operation.

F3: CHECK THE CATALYST MONITOR SENSOR HEATER CONTINUITY

1. Switch the ignition switch to the OFF position.
2. Measure the resistance between the right hand catalyst monitor sensor electrical connector GB3 pin 1, and pin 2.

Is the resistance between 3 and 10 ohms?

YES Goto F4
No INSTALL a new right hand catalyst monitor sensor. CLEAR DTC. TEST the system for normal operation.

F4: CHECK THE CATALYST MONITOR SENSOR FOR FAILURE

1. Connect the right hand catalyst monitor sensor electrical connector GB3.
2. RUN the engine until the catalyst monitor sensor has warmed up and maintain a speed of approximately 2500 rpm.
3. Disconnect the right hand catalyst monitor sensor electrical connector GB3.
4. Measure the voltage between the right hand catalyst monitor sensor electrical connector GB3 pin 3, and pin 4.

Is the voltage between 600mV and 1.0 Volt?

YES Goto F5
NO INSTALL a new right hand catalyst monitor sensor. CLEAR DTC. TEST the system for normal operation.

F5: CHECK THE CATALYST MONITOR SENSOR FOR FAILURE

1. Connect the right hand catalyst monitor sensor electrical connector GB3.
2. REDUCE the engine speed to idle.
3. Disconnect the right hand catalyst monitor sensor electrical connector GB3.
4. Measure the voltage between the right hand catalyst monitor sensor electrical connector GB3 pin 3, and pin 4.

Has the voltage dropped to less than 400mV?

YES Goto F6
NO INSTALL a new right hand catalyst monitor sensor. CLEAR DTC. TEST the system for normal operation.


F6: CHECK THE CATALYST MONITOR SENSOR FOR FAILURE

1. Connect the catalyst monitor sensor electrical connector GB3.
2. INCREASE the engine speed to approximately 2500 rpm.
3. Disconnect the catalyst monitor sensor electrical connector GB3.

Has the voltage increased to between 600 mV to 1.0 Volt?

YES Goto F6
NO INSTALL a new catalyst monitor sensor. For additional information, refer to CLEAR DTC. TEST the system for normal operation.


F7: CHECK THE CATALYST MONITOR SENSOR FOR SHORT TO GROUND
1. Switch the ignition switch to the OFF position.
2. Disconnect the oxygen sensor electrical connector GB3.
3. Measure the resistance between the catalyst monitor sensor electrical connector GB3 pin 1, (GR) and the monitor sensor body, and between the catalyst monitor sensor electrical connector GB3 pin 3, (NU) and the monitor sensor body.

Is the resistance greater than 10 M ohms?

YES Goto F8
NO INSTALL a new catalyst monitor sensor. CLEAR DTC. TEST the system for normal operation.


F8: CHECK THE CATALYST MONITOR SENSOR SIGNAL GROUND CIRCUIT FOR OPEN

1. Measure the resistance between the catalyst monitor sensor electrical connector GB3 pin 3, (NU) and ground.

Is the resistance greater than 10,000 ohms?

YES REPAIR the circuit between the catalyst monitor sensor electrical connector GB3 pin 3, (NU) and the spliced joint GB53. CLEAR DTC. TEST the system for normal operation.
NO Goto F9


F9: CHECK THE CATALYST MONITOR SENSOR INPUT CIRCUIT FOR OPEN

1. Remove the powertrain control module (PCM). For additional information, refer to
2. Measure the resistance between the catalyst monitor sensor electrical connector GB3 pin 4, (WU) and the powertrain control module electrical connector GB1 pin 28, (WU).

Is the resistance greater than 10,000 ohms?

YES REPAIR the circuit between the catalyst monitor sensor electrical connector GB3 pin 4, (WU) and the powertrain control module electrical connector GB1 pin 28, (WU). INSTALL the powertrain control module (PCM). CLEAR DTC. TEST the system for normal operation.
NO Goto F10


F10: CHECK THE CATALYST MONITOR SENSOR CIRCUIT FOR SHORT TO GROUND

1. Measure the resistance between the catalyst monitor sensor electrical connector GB3 pin 4, (WU) and ground.

Is the resistance less than 10,000 ohms?

YES REPAIR the input circuit between the catalyst monitor sensor electrical connector GB3 pin 4, (WU) and the powertrain control module electrical connector GB1 pin 28, (WU). INSTALL the powertrain control module (PCM). CLEAR DTC. TEST the system for normal operation.
NO Goto F11


F11: CHECK THE CATALYST MONITOR HEATER OUTPUT CIRCUIT FOR OPEN

1. Measure the resistance between the catalyst monitor sensor electrical connector GB3 pin 2, (BO) and the powertrain control module electrical connector GB1 pin 15, (BO).

Is the resistance greater than 10,000 ohms?

YES REPAIR the output circuit between the catalyst monitor sensor electrical connector GB3 pin 2, (BO) and the powertrain control module electrical connector GB1 pin 15, (BO). INSTALL the powertrain control module (PCM). CLEAR DTC. TEST the system for normal operation.
NO Goto F11

F12: CHECK THE OXYGEN SENSOR HEATER CIRCUIT FOR SHORT TO GROUND

1. Measure the resistance between the catalyst monitor sensor electrical connector GB3 pin 2, (BO) and ground.

Is the resistance less than 10,000 ohms?

YES REPAIR the output circuit between the catalyst monitor sensor electrical connector GB3 pin 2, (BO) and the powertrain control module electrical connector GB1 pin 15, (BO).
NO INSTALL the powertrain control module (PCM). TEST the system for normal operation.


F13: CHECK THE CATALYST MONITOR SENSOR SIGNAL GROUND CIRCUIT FOR SHORT TO GROUND

1. Disconnect both catalyst monitor sensor electrical connectors GB3, GB4.
2. Measure the resistance between the catalyst monitor sensor electrical connector GB3 pin 3, (NU) and ground.Is the resistance less than 10,000 ohms?

YES REPAIR the signal ground circuit between the catalyst monitor sensor electrical connector GB3 pin 3, (NU) and the spliced joint GBS3. INSTALL the powertrain control module (PCM). CLEAR DTC. TEST the system for normal operation.
NO INSTALL a new powertrain control module (PCM). CLEAR DTC. TEST the system for normal operation.

 

seems like they have it setup to trouble shoot from the simplest case to worst case order.

like i said personally i would switch sensors and see if it throw a code for the other bank afterwards. if so its a faulty sensor.

if it doesnt, then i would go through with the jtis steps.

 

They can be very tight and may break during removal. Otherwise, swapping is OK.

Generally the after-cat ones don't often go bad because they're not subject to the horrid stuff the upstream ones suffer.

Far better would be to look at the waveforms of the post-cat sensors (and for that matter the pre-cat ones) and compare side-to-side. (As well as checking they're OK waveforms!) The usual graphs you see on web sites are the pre-cat ones; the post-cat ones should either not switch at all (just kinda small ripples) or switch far less often.

The freeze frame data may help you see what's occurring. Also any pending codes.

How are the fuel trims, especially under the conditions shown in the freeze frames?

Is the car burning any oil? You'll have put in the correct oil but those before you may well not have done and oil (additives) can poison cats (and/or O2 sensors, though again you'd expect the post-cat ones to be OK).

Uphill the engine load will be higher and so things are doing a bit more.

 

Well..... I swapped out the two rear Catalyst Monitor Sensors. They were not very tight at all. The most difficult part was getting the ziptie off the driver-side wires. Cannot get a deep-welled socket over them because the wire plug is too large. I ended up using a 7/8" open-end wrench cause I did not have a metric large enough. It worked fine.

They are supposed to be torqued to ~30ft-lb so I had to guess on that based on what it took to get them lose.

Anyway... (knock on whatever you knock on) I have not gotten any more codes after about 70 miles. There was a lot of soot built up on both sensors and I cleaned that off before I put them back in.

(I was too lazy to work through all those tests too... swapping seemed like the least effort... We shall see.)

If I still get a signal on that side, I'll move the front O2 sensor and retry... If it still recurs, I guess it is time for a new CatConv. So far so good.

 

Oh. The connectors seemed pretty dry. They get a lot of exposure to the elements down under there. So I "lathered" them up good in a nice thick layer of dielectric grease to make certain they remain clean and the connections are good and solid.

 

I just reread my post two messages up and realized it might be a little unclear... "swapped out" means I moved the passenger side monitor to the driver's side and vice versa.

 

nice, happy it worked out

 

Well, switching the back sensors did not solve the problem. I reset the CEL and it came on a few hundred miles later. I reset it several times and it continued to come on.

The front sensors, before the converters were VERY tight! I could not get them with the large Cresent wrench without fear of turning the head.

Three days ago now, I was in Harbor Freight looking for something else and found aO2 Sensor socket. (As I was laying under the car, I thought this is what I need... someone should design one.) They had this one.



And one that was more like a deep welled 22mm socket with part of the side cut way. Both were about $3.50 so it was an easy decision to pick one up. I bought the deep welled socket for O2 sensor removal. (Cannot find a pic on Hf website.) It worked like a charm. Put the socket on, put my torque wrench on (needed the leverage)! It popped right loose, with a good firm pull.

I swapped up the front sensors left to right and right to left. I was hoping that one of those sensors might have gone bad. But the light came on MUCH faster this time. With about 15-20 minutes of running. So I guess my passenger side Cat Conv. really is bad.

I'll be looking for that replacement Cat sometime after Christmas (unless Santa drops one on the doorstep on Christmas eve...)

 

Nice pic!

If it's an 02 car, that's about when jag changed the PCM and also changed the type of pre-cat O2 sensors (to linear aka wide-range ones). The newer type of O2 sensors don't switch all the time but instead they effectively measure how lean/rich (in O2) the exhaust gas is. They also take in a little air from outside the sensor in order to compare with the exhaust. Some take the air in via the wiring and some don't (presumably via the connector body) but I don't know which exact type jag fitted. (Or whether you have the old or new type.) Because they take in a little air they need to be clean. (I know, under the car, what an idea!)

An OBD tool such an ELM327 reports the type of sensor.

If you can graph the sensor waveforms you can tell if a sensor is plausibly working or is clearly broken. If broken, you'd replace it and then see if the cat code still occurs. And if you can graph the waveforms for the post-cat sensors you can tell whether the cat is working (after checking the sensor is plausibly working).

Cheap software for an ELM327 is EngineCheck. (The free stuff seems not to do graphs.)

 

Since I swapped both sensors left2right and right2left and the code stayed on the passenger side, I am pretty certain it is not the sensor itself.

Doesn't that make sense? (Pun somewhat intended!)

 

Yes, unlikely to be the sensor. Doesn't rule them out the way looking at live data would.

Fuel trims at idle, 2500rpm, and when driving tend to be rather helpful. Also check other sensors (IAT, ECT, BARO, MAF etc).

Are you ONLY getting P0420 (cat efficiency) now?

 

Bugdoc, I am not going to quote the whole thing. NEVER use a torque wrench to "break" loose bolts or nuts! Instead use a small pipe such as black pipe used for plumbing (Homie D or the other BIG box) that will slip over a "breaker bar" not rachet. Next time your there pick up a piece 2' long or so under $5.00. Sure beats breaking a $100.00 torque wrench.

Nice to see you back.

 

I know everyone would like to feel the cat inefficient code meens you need cheaper 02 sensors rather than more expensive cats. It DOES NOT, you need a cat. And yes when dealing with same part, same function parts..Swapping IS the best, fastest, and cheapest diag, but you figured that on your own. If I suspect an 02 sensor etc I swap side to side and see if the problem follows the part. 02 sensors fail so little but are erroneously changed so much that you CANNOT change them without and engineering act of congress and jumping through fire rings. I have never seen anyone ever fix a cat inefficient code with replacing 02 sensors! The ecu set the code by monitoring downstream 02 sensors 02 content, compared to upper 02. If the "cleaning" falls below what the EPA says is acceptable lets say 94% or less and you hit that % of cleaning or less, then you get a cat inefficiency code. Letting your engine live with mifires from coils, plugs, poor air filtration etc is what kills cats the fastest. I see many times people dont want to get rid of misfiring coils and just "live with it" and then are looking at damaged cats, won't pass emmissions testing and are paying a $1000 or more than if they would have addressed the issue quickly. On this same point, I never replace cats with a single cat code, I clear and see if it returns as you have already done before I condem them. Also keep in mind theyre covered under the 8/80 emmissions warranty

 

He started with a car he picked up that needed coils etc (misfire codes etc). Don't suppose the 8/80K still applies

 

Sorry I kind of dropped out of the thread (and forum) for a while. Christmas/New Years got a little crazy. We had an unexpected death in the family.

JagV8 is correct. I bought the car with ~75k miles on it and it started running rough within about 10k or so. The P0420 code started about that time. I changed out the coils/plugs at that point. It ran very smoothly after that, but the P0420 codes kept coming.

I have not changed the CATs yet. I likely will have to. But I just started using Lucas Upper Cylinder and Gas Treatment about three tanks ago and it has made the CEL go off. So far, it has stayed off. I'm not certain if a valve was not closing completely or if the Lucas is making the gas burn more completely or what is happening.

If the CEL comes back on, I guess I will end up buying the CATs. Brutal, you are right, I was hoping for a $80 O2 sensor. At least I wanted to knowif that was what the problem was before I spent megaBUCKs on new CATs.

My problem started at about 85K and I called the nearest Jag dealer to see if they could work with me, but it was no dice...

Anyway, she is purring nicely now, if that pesky P0420 CEL will remain off!

 

I know... I know... I have a 25" breaker bar I could have used. If I had needed to pull beyond what the torque wrench could take without clicking, I would have switched out. But having a ratchet up under there is nice in that tight spot.

I've done the pipe trick before too... In fact, I do have a couple of different length pieces of pipe laying around the workshop for that purpose. But it only took about 95ft-lbs to break it loose. Just more than I could get with a short ratchet. Not enough to break the torque wrench.

Thanks for the warning though. Someone might have read what I wrote and not been as cautious.