"When you open a can of worms, the only thing that you can put it into is a bigger can...". Yep.

Still throwing the P0420, did some more testing. Upstreams are changing, just not 'fast' enough.

I did a current test on the sensors, and neither heater is playing, so apparently they are both gone south. That probably explains the blown fuses, too.

I was going to change bank 2 sensor 1 first, figuring that I could get in there pretty easily. WRONG!

Pop off the covers, the airbox, and look in there. Easy enough to see, but there is no way to get a wrench in there with the air intake in place, and the upper radiator hose/pipe assembly looks like it needs to go, too. I suppose I could deal with that, but I can fairly well guarantee you if I touch that mess, something will start leaking.

Looks like the last mechanic under there was a butcher, too. There is a kludge in place of the PCV hose, using screw-type band clamps and heater hose, and it looks like they may have cut the intake side hose, stuck a tube or copper nipple in it, and clamped (using the same cheap clamps) it to the heater hose and the manifold.

Then, I looked at the coil wire loom. No insulation, no tape, no loom...just a zillion wires all hanging in space, albeit with just enough tape to keep the wires tight. A little bit of friction added to the heat above the exhaust...a disaster waiting to happen.

Sensor replacement to the wayside; secured, wrapped and covered the loom. Have to deal with the PCV for the moment, I'll order the correct hose tomorrow. I have no idea how long the sensors have been dumping a rich mixture into the cats...

I'm starting to think that I might be better off to get new cats (about $260 or so), get the other two sensors, throw in the upper and lower rad hoses, and just have all of it replaced, once and done. I'll deal with the CEL light for now, get all of the parts together, along with the oil pan gasket, and have the transfer fluid changed, exhaust gone over, pretty much everything, come springtime when the car's inspection comes around.

I want to change the rotors and pads now since there is a little scoring on the rotors (pads are good, so they changed them without turning the rotors...) just need to decide if I want blanks or drilled and slotted...and I need to rebuild the rotted away rockers...there is a poor design if ever there was one...but at least they aren't actually structural on our cars.

And so the fun begins!

 

These are good if you haven't already got one, top one

Oxygen Sensor Socket | AutoZone.com

 

Already have one, but thanks!

 

Wow - you are making me feel almost guilty about my only slightly neglected Jag.

You've got a bit of a rough road ahead of you but - good luck and success!

FWIW - drilled rotors are a purely cosmetic function on the road - if you like the look get them, but if you're looking for performance improvements you won't find them there - hell, we don't even use drilled rotors on our track cars - unless you get very high quality "drilled" - (actually cast holes, and chamfered to boot), they are only another vector for stress cracks and failure.

IMHO of course, and with respect.

 

wow... and it all started from a p0420 too...

I have to try tackling my p0420 one of these weekends as well. I'm going to check the voltage on the O2's later today and see what they look like with my scanner. I did get a chance to look at both fuses under the hood - and they look fine, so it wasn't that simple a fix yet

My problem last time I tried removing the airbox was that it does not come out at all ! I removed the 3 outside bolts, and the 2 inside, but the dang thing is hung up on something from underneath and I don't want to forcibly pull it out in case something gets rips off or broken... ugh. I don't even put back the 'jaguar' cover anymore b/c it's just a pia to get the rubber lip over the oil cap each time, so it just sits in the shed for now.

let us know you you're making out. looking forward to your updates !

 

Well, the car is 'new' for us. I tend to pick up cars inexpensively, do a good mechanical resto on them, and have a fairly inexpensive car that I can trust.


I have a 9-year-old who had a kidney transplant a few years ago, and we spend a lot of time fighting rejection. As a consequence, we spend o a lot of time driving between our home at the north end of the Pocono Mountains down to the Penn State Children's Hospital in Hershey, PA. The drive takes place over a stretch of highway that can be positively evil, so we tend to get 4-wheel or all-wheel drive and cars generally recognized as safe.


I was looking around for a Disco II that wasn't trashed, but somehow, their price went up lately, so I went on the prowl for one of the x-types, negotiated a good deal, and I'll go through the same process with this one.


Unfortunately, I seem to lose a lot of cars to late night collisions while the cars are parked in front of my house in my parallel driveway. Not sure how or why they happen, but they tend to end up with my car 60 or 70 feet from where it was, and the 'hitter' car a smoldering wreck on the street..and no other parking options.


I hope this car can avoid that fate, I rather like it.

 

Which fuses are you talking about? There are fuses that blow causing O2 sensors to fail? You lost me. I had a p0420 then it went out only to have code for O2 2 bank 1 come up - does a bad cat make sensors go bad? and what are these fuses?

 

The fuses are in the underhood box.

The sensors have heaters in them to bring them up to temperature quickly, and to give them a very linear output curve. Normally, they draw a few amperes.

The heater filament is very much like a light bulb filament, a coil of nichrome wire. It is running through, but not touching, a metal tube when working.

When it 'burns out', often the heating element breaks in two, like the element inside a light bulb, and the wire makes contact with the grounded tube, causing a short circuit and blowing the fuse.

Sometimes, as it ages, the filament gets 'sloppy', and a pothole or turning a corner sharply will allow the element to hit the tube without breaking. Still a short, and it will still blow the fuse. When the fuse blows, the power is cut off, the filament contracts, and the short is cleared.

You can then replace the fuse, and the sensor will work normally again, sometimes for a very long time, until it happens again. In either case, it is soon time to replace the sensor.

The post mortem on mine shows that after the element broke ans shorted out, the filament broke in a second place, so it was no longer touching the tube, so when I plugged the new fuse in, the fuse didn't blow, because there was no circuit attached anymore, just like a burned out bulb...you can flip that lightswitch a million times, but that bulb will never light again!

If you look at the sensor output, the voltage that the sensor puts out on the other terminals should quickly ramp up over a few steps, and keep repeating that cycle, a few times a second. When the heater is waffled, it takes several seconds for the cycle to happen, and the computer will throw a sensor code. Sometimes, the heater, rather than failing completely, will only partially short, not enough to blow the fuse, but enough that the element doesn't heat as much as it should. The cycle will then run in about a second, which the computer thinks is OK, but sees as a lean condition.

The computer says, "hey, I need more fuel!" and it dumps more fuel into the system (it leaves the injectors open for a few more thousandths of a second). The excess fuel passes through the engine, out through the exhaust. The catalytic does its job, and burns the unburned fuel, but now the downstream sensor sees that the cat is working harder than it should be, and says "hey, this catalytic converter isn't working efficiently!", which it communicates by throwing the 0420 (firewall side) or 0430 (radiator side) code.

In the beginning, that code is just a warning, but run the cat long enough with the extra fuel, and the excess heat will cook the catalyst honeycomb, and you WILL need a new cat. Head it off, and the cats should outlast the car.

Incidentally, heated sensors ARE consumables, they should be replaced every 30K miles or so!

There is more than you ever wanted to know about oxygen sensors!

 

Great write up and explanation.
not a lot of people know that. You learn something new every day.
I hope you and your family are doing well and wish you luck.