Where did you find the info that the engine is automatically reduced to 2 or 3 cylinders?

 

There are 2 Limp Modes,

1. Involving the Throttle
Mechanical, cable operated 'limp home' mode with restricted throttle opening.The limp home mechanism consists of the accelerator input shaft link lever and the two throttle shaft levers, all three levers being interlocked for limp home operation. If a failure in the throttle mechanism or control system results in the engine management system selecting the limp home mode, motor power is cut by de-energising the motor supply relay and/or by de-activating the PWM motor control signal. In limp home mode, the throttle valve is operated mechanically from the drivers pedal and throttle opening is restricted to a range from idle to a maximum of approximately 30 degrees.

2. The Transmission / TCM
The TCM constantly monitors the transmission for faults. In the event of a problem the TCM will adopt a 'limp home' mode in which only P R N D - (selector in D but only fourth gear is enabled) are available. The operator will be made aware of certain faults by an instrument panel warning.The electrical and diagnostic system has been designed such that system integrity is protected at all times, the safety concept being based on the following three points:The hydraulic system has 'fail-safe' characteristics regarding its electrical operation, such that should the power supply be lost to the electro-hydraulic actuators the transmission will initiate a limp-home mode.Recognition of critical shift operation by monitoring the last element in the signal path, ie the solenoid valve, and checking by means of redundant measured variables, ie engine speed, input speed and output speed.Each time the vehicle is started there is a check on the entire safety hardware and the associated program parts and signal paths. A malfunction in this part of the system, or triggering of the safety circuit, is communicated to the operator through the illumination of the transmission warning lamp.

 

That would be 2 or 3 cylinders of output power equivalent. I was sure the system did it differently but I had no idea how and thanks to jamdmeyers for explaining it. If the mechanism hadn't been sufficiently complicated I'm sure Jaguar would never have used it.

 

Now if you were talking Cadillac STS Northstar that's a different answer, That motor will run on 4 of the 8 cylinders when it's in limp mode, mostly to keep the engine cool and I'm not sure which ones it picks, prob 2 / bank fyi

 

So we got the car into the shop and did the thing with the module, soldering the suspect wires securely to the board. Put it back in and all the codes had magically disappeared. Made it about 3/4 mile and they all came back this time with a check engine lamp instead of the ABS lamp. Turned off the key and when I turned it back on, the codes were gone although the check engine light was still there. Drove back to the shop and took the mechanic on a short test drive. Made it about a mile and the problem returned. Left it in the shop with a P0121 code showing on the code reader.

The mechanic says he suspects the throttle body sensor but can't say for sure.

P.S. The module job took an hour and a half and required loosening the 3 outside brake lines so as to move them enough to get the module out.
Note 1: The 4 screws on the front of the module require a torx E-5 socket.
Note 2: Splitting the case requires removal of 4 push nuts located under the 4 Torx screws. Use a dental pick to lift them off the standoffs.
Note 3: Replacing the washer filler spout requires pushing the brake lines back pretty close to where they were.

Add these items to the work description over in the sedan forum.

I remain open to suggestions as to the cause of the Stability control failure/failsafe engine problem.

 

The P0121 is often a compatibility issue with the throttle position sensor and the plug this link should help.

Link JagRepair.com - Jaguar Repair Information Resource

 

A co-worker informs me that some models of Cadillac are prone to the identical problem I'm seeing and the Caddy dealer replaced the TPS unit at no charge which fixed the problem.

Jag will happily replace your entire throttle body, TPS and all at your expense but won't sell you the sensor. As it happens the sensor is also used on Aprillia motorcycles, specifically the '98-'03 RSV1000 models. P/N 198500-3220.
Update: Aprillia P/N AP0274030 Price $182 at Fay Mayers Honda motorcycles in Denver
Does anyone know of other sources for the sensor? Aliexpress, mentioned on another thread no longer has them.

 

It is an issue with the plug not the TPS.

 

My mechanic reports that he cleaned both plugs thoroughly and has put 5+ miles on the car without seeing the faults return.

Fingers crossed.

 

At this point I will report that the problem has not come back. He showed me his tool set which included a pair of tweezers with abrasive on the inside which can reach down inside the plugs and polish the pins. Clever tool. The kind of thing you'll need once in your lifetime but will not fix the problem without.

Summer is icumin in which means the Sno-Cat will be spending most of the next 6 months under a cover. All I have to do is remember to take it out once a month to keep the battery up.

At this point I believe I could be talked into trading it straight across for an '05 or '06 Subaru Baja which would be less stressful.

 

My stability light is stuck on and I still cruise at 90 on the freeway.