I have a 2006 XJR with 149,780 miles on it. I started noticing that when the engine is cold and speed is about 40 mph, RPM fluctuates from 1200 to about 1350 RPM. Once the temp reaches normal, the hunting goes away;

I checked for a code and got U0426. I have the list of OBD II codes from Jag-Lovers.org, but it does not include U codes.

Anyone know what U0426 is?

 

cold/warm up surging is most common on the s types with the ZF 6HP26 trans.
it seems to be popping up a bit in the x350/x358 forum more and more .

it is some times un curable with out a torque converter replacement .
but start with a fluid & pan & sleeve change . if no luck
then clear adaptions via ids/sdd .if still no luck re flash the TCM.
if still no luck it needs a new torque converter ,
or you can - like me - just select 4th un till its warm for the rest of the cars life . it seems to do the trick . at keeping me sane anyhow .
mine was one of those that needed the T/C replaced . but thats not happening unless the trans has to come out for any other reason .

 

oh year , my jtis only goes up to 2004 .
but a quick search says -
U0420 - Invalid Data Received From Power Steering Control Module

didn't know it had one . ether way its a can bus code .

 

My '04 XJ8 VDP does the same thing. After about 2-3 miles it goes away. Mine either stops or becomes un-noticeable if I get it above 60 MPH too.

 

yep thats exactly it ,
its noted in the S TYPE forum TSB'S in the sticky's .
it seems to otherwise have no ill effect on longevity of life on the trans . i have documents from my cars first owner at very low mileage and still under warrantee . and many more tries with the previous owner to me .
they date back to when my car was still low milage and still under warrantee .
and some how it was never fixed .
although with plenty more miles on the clock and counting it seems fine to just leave it like it . with that 5-7min warm
up in 4th , i can live with it , the trans is otherwise faultless ..


.

 

I can confirm Datsports advice re putting it into 4th until warm-up is complete, (warm-up for the tranny oil, not the engine). My last X350 did it from purchase at 30k miles, and was still doing it when I part-exed the car on 115k miles. My current X358 doesn't do it. All the niggles I had with the last car are not present except one. The last car suffered from a juddering steering wheel on braking from high speeds, and this one does it too. The suspension and brake design seems to emphasise the slightest disc imperfection.