Hello fellow Jaguar caretakers,

I know this topic has been covered before as I have read many of the existing threads. Unfortunately, despite this, I'm still exceedingly puzzled by this particular issue.

The ABS had been out for about three weeks prior to any issues with going into Failsafe mode. Therefore the stability control and abs light are likely unrelated.

For the past two weeks I haven't been able to get my 1999 Vanden Plas out of her space in the garage. Upon startup, it immediately goes into Failsafe Engine Mode. When this happens, I have been getting the p1121 code.

To remedy the issue, I replaced the old TPS with a NEW TPS. I did a hard reset of the car, turned her on, ran fine (about 1100-1200 rpm). So I adjusted the TPS, did a hard reset, tried again. Purred exceptionally. Right at 750 rpm as it should.

I let it get a little warm (5-10 min) gave it some throttle gentle and then abrupt throttle variation. It responded quickly, and without issue. Now is when i started to see the light at the end of the tunnel.

I put her in gear, let it sit idling for a bit. Moved through all shift points, still going strong. Left on brake, back in D, gave it 1/4 throttle... Held for a half second then fell on it's ***...

This gave me Engine Stall, Failsafe Engine Mode, Transmission Fault (transmission was replaced 15k miles ago), and P1121.

With a brand new TPS, why would it throw p1121 only when under load while in gear?

I'm lost at this point. HELP!!!

 

P1121 is the throttle pedal position sensor, or TPPS. Not the throttle position sensor or TPS. That's why it's still misbehaving...

I assume you've an AJ27 engine? They have an external TPS, internal TPPS. I struggled with this code, and random fails. Gradually got so bad I just swapped out the throttle body.

You can actually monitor the TPPS signal via OBD - looking at the throttle position in % by very slowly depressing the throttle pedal you can see the rise and fall on the OBD reader.
By watching this you can see the signal breakdown without running the car to confirm this part is failing.

They have duel PCB tracks that wear to the point of loss of signal to the ECU at which point it doesnt know the throttle position and goes into failsafe.

Fix = new throttle body.

The ABS sounds like C1095 and it's either a year related module (with same program code and a trip to a specialist to link it to the car via software), or solder the ABS pump pickups in the module.

 

Thanks for the p1121 clarification. There seems to be a LOT of confusion on that particular code.

Yes, I have the 4.0L AJ-27. Great motor!

Before I jump straight to a new TB, I have read that there has been some solid success with having the TB rebuilt.

As for the ABS, it's been on my ever growing to-do list. I'll be fishing it out and re-soldering as soon as time permits.

Is it common for the P1121 fault to also throw the transmission fault code?

 

I've had the 'scroll of death' when a bad connection somewhere on the can-bus plays up one of the module connections is likely to trigger a gearbox fault - in this case it could be the throttle body connection.
Your TB might be suffering from a missed recall Jaguar did by updating the pins to gold plate for better connection, if you do a quick search on this it might fix your problem.

What is the mileage on the car as the TB's start to play up around the 150k mark I've noticed.

On a rebuild, I'm not sure if the TPPS can be replaced, but they rebuild TB's so I'd guess this would be an effective fix, along with a used low miles TB - A new one I think is now unavailable from Jaguar.

ASI have done a good number of forum members TB's so this is where I'd suggest you send it - a quick turn around so I understand.

You've already done the research on initial setup of the TPS and manual correction so this shouldn't be an issue on the rebuilt unit.

 

Sorry for the slow response.

It has roughly 125k. So it's getting up there.

I hadn't checked the TPPS connection because I mistakenly thought the P1121 code was for the TPS.

I had a coolant leak about 2 months ago that must have gotten the connector wet. I simply cleaned the TPPS connection (which does have gold pins!!) and did yet another hard reset.

She's been good for 2 days now. I guess time will tell if this is a permanent "fix" or if I'll be revisiting the same issue in a month's time.

Thankfully, the scroll of disappointment is no longer flashing the Jag's shortcomings at me!

 

The transmission connector under the car is a bayonet that comes off like a air hose connector so don't try too hard . They can stick with rusty palls and can be soaked overnight and later tapped lightly with the blade of a flat screwdriver toward the wires direction . Just in case you are not familiar with them . The individual throttle position sensor can't be changed individually ? The wire colors to identify which throttle position sensor feed what is on page 40 lower right corner as # 1 has the blue wire and the # 2 has the green wire in the PDF below : If you put a meter on them they should read the same as a resistance check with no high bibs to open circuit as you range it through the throttle position

http://www.jagrepair.com/images/Auto.../jagxj1999.pdf

The ABS as a simple test of the sensors before removing and cleaning the signal clouding over debris before moving on to the C1095 fix can be read at the module as wire pairs and should read about the same resistance .