Bottom line up front: Do you all think resetting the ECU might help?

Here's the story:

Original engine, with Hi-perf cats, burned quite a bit of oil, but I rarely got one of these codes for the 30,000 miles I drove that engine.

Replacement engine (not burning oil), still with hi-perf cats, starts getting the codes.

Both lower O2 sensors have been replaced - no difference (might be worse actually) - still have the codes almost every day. The cats only have about 50K miles on them.

Interesting note: I tried burning the cats clean with an overdose of B-12 fuel treatment in a quarter full tank and about twenty minutes of VERY spirited, high rev accelerations. I had no codes during this play time (and no speeding tickets, thank God!). Of course, the next day they were back after normal driving.

Yeh, yeh, I know:...... all I need to do is keep my right foot planted and the codes will stay away. ;-)

Okay Y'awl - what say ye? Reset the ECU? Any other ideas?

THANKS

 

I would try a mil eliminator for a Mustang and splice it onto a jaguar connector or you could try to find out the transfer function on the rear O2s and wire in a circuit that will always indicate good. The rear O2 sensors are only for diagnostics anyway, your cats are performing like they are, if you are concerned with emissions replace them with stock cats.

My guess (and it is only a guess) is the oil you were burning made the front sensors read rich and caused the car to run with a lean AFR resulting in a low (good) post-cat O2 reading. Was the old engine pinging during acceleration? In any case the cars I've had that burn oil (none of them jags) rarely throw a P0420 including a Suburban I ran for 50k miles with hollowed out cats.

I would reset the computer to be rid of the light but it will likely come back.

 

Thanks Auburn. What's a mil eliminator?

I'd rather not disable the system entirely. Can't figure out why, with the cleaner engine, "now" I get the codes. I could see clearly through the cats as well, so I know they're not physically degraded.

 

If they are aftermarket cats they probably just aren't as effective as the Jag cats and as effective as the car thinks they should be.

MIL stands for "malfunction illuminator light" eliminator. It is a wire that plugs in between the pigtail and the rear O2 sensors. It changes the transfer function on the O2s so the cats always read good. I would try a Mustang based on the idea that the Jag O2s probably use the same transfer function as a Ford.

It won't do anything to your computer, nor will it change your emissions at all. All it does is change the resistance in the rear O2 sensor circuit so the computer thinks the catalytic converters are working great (even if you have no cats). P0420 is there to alert you (and the inspection station) that your cats are not working properly and you need to replace them. With a MIL eliminator you will have the exact same emissions, performance etc but you won't have that "warning" when the cats go bad ... of course that warning (or in this case the false alarm) is what you are trying to be rid of right now. I think the only other way you are going to solve it is replace the cats with new/different ones.

The rear O2 sensors are not used for anything other than monitoring catalytic converter performance anyway the rest of your closed loop system will function as normal. Note DO NOT put these on the front O2 sensors.

One final comment/question - your rear O2 senser is installed after the aftermarket catalytic converter correct?

Here is one set:

SR Performance Mustang Plug-in MIL Eliminators (96-04 All) - Free Shipping

 

Thanks Auburn. I checked out the site - looked good, but of course the connector plugs are different, and wouldn't you know it: I threw away the old O2 sensors that I could have used for the plugs on at least one side of the eliminator.

Anyone have any ideas where I could find a set of male and female connectors for this project?

 

Why not try new O2 sensors in front of the cats?

The reasons stated for these codes include.

HO2 Sensor disconnected
HO2 Sensor to ECM wiring fault
HO2 Sensor heater to ECM wiring fault
HO2 Sensor heater failure
Upstream HO2 Sensor failure
Downstream HO2 Sensor failure
Catalyst failure

So it's not unreasonable to expect that the front sensors will eventually need replacement.

Have you checked the fuel trims, are they high numbers?