A hard reset will not damage the circuits.
For the o2 its volts you are looking for, normally it varies between 0v -1v from memory. Normally you want it to be mid point <>.5v I stand to be corrected on the actual volts may be 0-5v
And should that be a steady/level reading, while the sensor goes from “cold” to “warm” during the warming up process, or should it vary during it?
You really can’t tell with a multimeter if an oxygen sensor is starting to die with it in the car…unless the voltage readings are not fluctuating, then you know for sure its dead.
The voltage fluctuates around the mid point the feed back to the ECU is continually swinging between slightly rich to slightly lean.
Robin, when you say “hard reset”, are you referring to disconnecting the Negative terminal from the battery for about ten seconds?
Or are you saying disconnect both battery cables and touch them together? As if to discharge any microvoltages stored on any components?
Looking for more clarity regards what, how and why — hard reset
Thanks
Yes touch the neg and pos together for <> 30seconds
Okay. Good to know. Thanks Robin
O2 sensors get covered with carbon and very slowly lose their sensitivity. So your car will slowly run richer and richer as the sensor can’t detect as well. At first bad gas mileage, but eventually too rich and it can cause poor performance.
Whenever i buy a used car, i always replace O2 sensors as soon as i get car running ok. Previous over rich conditions can coke up the sensors with carbon, you just have no idea. Also good to start ownership with good sensors, as they are so important for ecu fuel map.
Its dead easy to replace them. Only buy Bosch replacements. Worth the extra money.
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Yeah, but you don’t know if the swings are accurately fluctuating. Older sensors start to get “lazy” and they don’t swing as quickly as is needed.
Furthermore, hooking up a multimeter will just show you random numbers since the swings should be constant. You would need an oscilloscope to figure out if the sensor want reporting data fast enough. You certainly wouldn’t know by looking at it, even with a stopwatch.
O.K., Gregmatic (and VK) … you have me convinced to give it a go and r/r them. The cost doesn’t look to be too exorbitant for new ones, anyway. 
It appears like the task requires a special “open slot” wrench, which it looks like AZ lets out under their tool loaner program. 
My big concern is how “fast” the old sensors are going to be in place, given they may not have been replaced in Superblue’s entire 100K+ miles. 
I don’t like the idea of maybe breaking something off in there trying to get them loose, which could be disastrous. 
What sort of lubricants (for want of a better term) do you feel confident is good at loosening them up first, and how long should I leave the stuff on them before making the attempt? (and when putting the new ones on, like with plugs, I will use anti-seize to make sure it isn’t as laborious the next time around, if necessary 
).
Also, I’m working on the assumption that (both ?) of the sensors can be r/red from above, w/o having to get under the car … (or removing too much stuff first topside) ? 
I believe the AJ 6 only has one O2 sensor.
You only have one and have been avoiding changing it?!?!
The AJ16 has FOUR of them
No idea where yours is on your six cylinder. I’m sure it’s from underneath, and usually involves feeding the wires through the fender well or engine compartment. So it’s not simple. But its dead easy if you can jack up your car, and are used to dirty jobs. If not, bring it to mechanic, should be super quick for them on a lift.
I usually spray a penetrating oil like PB blaster on the threads, let it sit a while, a big wrench may be needed to break it loose.
Definitely use antiseize when putting in new one.
The O2 sensor for the 4l is in the exhaust, just forward of the fire wall. To get at it, you will need the o2 sensor socket (readily available). I highly recommend that you chase the threads on the exhaust pipe before installing a new one. You will see the wires running down to it nest to the passenger side wheel well. You can replace it with a generic 3 wire and save about $100 dollars.
Old fuel, and fuel injectors come to mind. Recommend, at a minimum, add some seafoam to the fuel. Pulling the injectors is not too bad, and they can be cleaned at home with a small battery (1.5v) cycling the injector with spray cleaner being applied. At a minimum, the fuel filter is in the drivers side wheel well (remove tire and access thru the liner), and should be replaced.
I run about 21 mpg on my 94. It has 175k on the OD. Cleaned the head and valves, and installed seals at about 130k to deal with some fuel burning issues. Also replaced the distributor which solved a slight miss and hesitation I chased for a couple of years.
Good luck.
Yes, I just checked, and it looks like the AJ6, mercifully, had just one sensor (i.e. the pre-'95 4.0s) … In '95, it looks like there was more than one on the 4.0s (and of course more than one on the 6.0s) … It also gets rather confusing in that the parts list (per the Jaguar Classic site) makes mention of “Class A”, “Class B” and “Class C” emissions systems, as well as something about German cars having a possible additional sensor. wth?? 
I just checked the Jaguar factory workshop literature on the procedure (and, btw, it was kinda hard to find, as the emissions system directory for the 4.0s seems to omit it 
?) and it advises that the car needs to be “put on a ramp” for the job (supposedly) in order to access the sensor. Looks like it has 2 simple wires going to it (in a “Y” configuration) and, if you get the OEM Bosch version, it is already equipped with the same type of connectors for them (they almost look like mini extension cords + plugs). Ebay has one vendor on there selling them for under $30. The literature just says to unplug those two wires at their respective connectors, loosen the sensor with a 22 mm. “socket”, and then reverse that procedure, for the job. Sounds suspiciously simple, but then Jag instructions sometimes are. 
– you often only find out once you get into the job it is in reality a bit more “complicated” than shown. They do advise using anti-seize on the new one, btw.
I checked with the A-Z tool loaner program, and they have a very long sensor wrench angled at about 45 degrees or so near the socket, which I assume is to make reaching the sensors easier. ? They also loan out a full “kit” for such a purpose, which incs. sockets of varying sizes, some of which are in a “duck’s foot” configuration, presumably for that same reason. 
So, from those of you who have actually done the job on an AJ6, if I use one of those types of wrenches should I be able to access the sensor w/o putting the car “on a ramp”? 
When you say “chase the threads”, are you referring to using a thread cutter on them?
I do run a small bottle of fuel injector cleaner through her about once a month, as well as Techron fuel system cleaner every 2-3 months (although I’m having 2nd thoughts about that product of late, since it has doubled in price ), both with only a few gallons of fuel in her tank. I have also, on occasion, used that “Star Tron” enzyme product that supposedly helps break down any ethanol deposits in her tank.
I also get lower 20s MPG on her, but only if I’m driving strictly on the highway. Again, since I just putter around my locale, it’s more like 14-15 MPG. btw, what do you mean by “fuel burning issues”? That kinda scares me. I would like someday to have her exhaust valve stem seals r/red, but there are shortcuts to doing that to prevent actually removing the head for the process. In the meantime, I continue to put up with her namesake blue puff(s) of smoke at (usually overnight) start up. I’ve tried all the oil additives for same, to no noticeable difference in that regard. 
Actually, I wonder if that little bit of unburned oil getting where it shouldn’t is actually helping her engine’s “longevity” somehow. ? btw, I forgot - do the AJ6s have “fixed” or hydraulic valve lifters? If fixed, how often should they be adjusted, and has anyone here actually found it necessary to do so?
Here is the new sensor I bought:
Bosch Oxygen Sensor 13006 For Alfa Romeo Audi Ferrari Hyundai Jaguar Saab 80-98 | eBay
The valves are set by shims, the only time I hade to check the valve clearance was when the head gasket blew on my XJ40 (at 100,000miles)
I wouldn’t concern myself with the puff of smoke on start up. Unless your fuel is extremely dirty I would think you are throwing money down your exhaust pipes using injector cleaner that often.
the O2 sensor thread chaser I used is: “Lisle 12230 Oxygen Sensor Thread Chaser” - google it and it comes up on Amazon for $7. Also available at local auto parts stores. My reference to the oil burning was due to the exhaust valve stem seals - basically just O rings. I pulled the head on my 94 AJ6 to do the valves and install these seals about 8 years ago after having the issue with the blue smoke. It was not that bad of a job, I recall starting one Friday evening, and finished by Sunday around noon. However, I had acquired another 94 4l engine off a XJ40 doner, and had spent about a casual month working the valves on that head, so I just had to do the head swap. If you just go after it with the existing head, probably add a couple of days to lap the valves and install the new seals. It was not a bad job - Simple compared to a 87 xj6 I had pulled a few years back (which had the long wetted head studs which took about 8 ton of jacks and engine lift to brake free).
Regarding the sensor, with the socket tool I can reach it from above with long ratchet extensions and a swivel. I also can get at it from below, reaching around to the top of the exhaust pipe, with the front right hand side jacked and wheel off.
