O2 Sensor Identifications - and intermittent CAT code troubleshooting

[CONCH]

So I had a CEL light with code P0420-00 (Catalyst System below efficiency threshold bank 1) pop up in early June. Cleared the codes and ran some fuel & CAT cleaner thru for a couple tank fulls. CEL/Code popped again about 3 was later. Cleared again and took a look with the GAP iiD. Tried monitoring the O2 sensors (pic). The iiD only shows a sensor 1 and sensor 2 for each bank. But review of the manual shows there are actually 3 sensors in the LR4 SCV6 exhaust system. So now I'm not sure which 2 were being monitored, and how the third could be listed and/or monitored.

While anything is possible, I'm skeptical that the CAT itself is actually bad at 110K miles. The manual TS'ing does indicate there can be other reasons such as exhaust leakage, misfire (haven't noticed any and it runs very smoothly), oil consumption (hasn't leaked or used any since dealer service about 2K miles ago) or air leaks. There was a reported changing of one O2 sensor by 1st owners service dealer but the records dont indicate which one. My primary question is to try and ascertain which of the two O2 sensors of the 3 the GAP tool is referring to -items 3, 4 or 6 in the EX Diagram. Im thinking 4 & 6 based on the highlighted text, but I could be wrong. Anyone know?

 

[jlglr4]

Sensor 1 should be the pre-cat sensor. It’s the most important o2 sensor because it governs the fuel mixture. I don’t know if sensor 2 is the mid or post cat sensor.

 

[scapistron]

Sensor 2 is mid cat. Sensor 3 is post cat, this is where system efficiency is measured.

You've got choices based on where you live. If you live in an emissions free state, nothing is a reasonable choice. You'll just have to check every now and again that you don't have any other codes.

New 02 sensor 3 might fix it. Spark plug anti fouler might work too, it didn't for me. This pulls the 3rd 02 sensor out of the exhaust stream a bit.

Also, check for a leak. Some of the v8s have had exhaust manifold bolts shear off.

 

[Land Rover Joe]

Sensor 2 is mid cat. Sensor 3 is post cat, this is where system efficiency is measured.

Also, check for a leak. Some of the v8s have had exhaust manifold bolts shear off.

I didn't have any leaks or faults but had four broken exhaust manifold bolts...which were a real pain to extract (sent them into a machine shop).

 

[CONCH]

I didn't have any leaks or faults but had four broken exhaust manifold bolts...which were a real pain to extract (sent them into a machine shop).

Thanks Joe (and Scapistron) for feedback. I did find all 3 on iiD screen after further work. Im hoping I dont have sheared manifold bolts - went thru that with my H2 Hummer. The JLR Dealer recently replaced the high pressure fuel pumps. Don't know if they would have seen the manifold at that time or had to do anything that may have caused this recent issue. More testing yesterday with the iiD seems to show that all 6 CAT sensors seem to be functioning equally, so I'm suspecting it can be something else like a misfire. I have a bank 1 #3 O2 sensor on order but in the interim want to look at any other potential possibilities. Could you hear the exhaust manifold leak? I know on my H2 I could literally hear the ex man leakage at startup which went away after warmup due to expansion.

 

[CONCH]

Sensor 1 should be the pre-cat sensor. It’s the most important o2 sensor because it governs the fuel mixture. I don’t know if sensor 2 is the mid or post cat sensor.

Turns out it is the Mid

 

[CONCH]

Sensor 2 is mid cat. Sensor 3 is post cat, this is where system efficiency is measured.

You've got choices based on where you live. If you live in an emissions free state, nothing is a reasonable choice. You'll just have to check every now and again that you don't have any other codes.

New 02 sensor 3 might fix it. Spark plug anti fouler might work too, it didn't for me. This pulls the 3rd 02 sensor out of the exhaust stream a bit.

Also, check for a leak. Some of the v8s have had exhaust manifold bolts shear off.

Spark plug Anti-Fouler? Is that like a Seafoam/Lucas or other Mfg fuel system cleaner? Or pulling the plug(s) and putting them in a plug cleaner machine?

 

[scapistron]

Spark plug Anti-Fouler

Intentionally misnamed device. It goes between the 02 sensor and the exhaust.

 

[CONCH]

Intentionally misnamed device. It goes between the 02 sensor and the exhaust.

Ahhh, got it. Just found a video of someone doing that. ....

 

[buckbluerover]

P0420