Hey! My Jag XJ40 4,0 1991 gives code 44. When I start the car CEL doesn`t come. When I start to drive and stop in the light then the engine starts jerking and feels like all cylinders not working. If I touch the accelerator pedal, then the engine will be work normally. And CEL comes.
What do you think about it?
Peeter, FF44 is a code for oxygen sensor circuit indicating rich or lean condition. First check all the ground points under the hood and make sure they’re not corroded and the securing nuts and bolts are tight.
Thanks Joe, Ill do that tomorrow.
You can also read grooveman’s post: check engine light solved…YES! He’s got a '89 MY car and he was dealing with the FF4 code. That’s oxygen sensor circuit, system full rich. It’s a bit longer read but worth it.
Checked grounding points, they seem to be ok. Ordered new lambda sond. Maybe this is a solution.
Peeter …
Over the past 12 months I’ve unfortunately become very knowledgeable with this system 
On my '89 model the O2 sensor has two connectors. One connector has a single wire for the signal. The other connector has two wires for the heater. If your O2 sensor is the same then disconnect the two wire connector and check for at least 12v (battery voltage) on the harness side with the engine running. If you don’t have 12v or more then leave one lead of the multimeter on the positive wire and ground the other lead of the multimeter to the battery or some other area of the engine. If you now read 12v then you have a grounding problem.
If your O2 sensor has 4 wires then the 4th wire is the ground for the signal. The signal ground on the 3 wire sensor is through the threads when you screw it in.
Now put your multi meter on resistance and you should see 5 to 6 ohms on the sensor side of the heater connector.
The Haynes repair manual (you do have a copy don’t you) has an excellent troubleshooting section on this problem starting on page 6-5 “Oxygen sensor”.
Checked two-wire connector with a multimeter and there is 13,6V. And I don`t have Haynes manual.
Pretty sure you can download most of it
http://www.jaguardriver.nu/manualer/XJ40/XJ6_Service_Manual.pdf
Thanks abercanadian!
Peeter …
Since you’ve ordered a new O2 sensor wait till you install it before doing any further troubleshooting, there’s a good chance that will solve your problem. Make sure the threads on the exhaust are nice and clean before you screw the sensor in to ensure you have a good ground.
If you own an XJ40 you MUST have a Haynes manual, but Larry just solved that problem for you. I would still purchase a hard copy for bed time reading. There are a few things in the manual that aren’t quite accurate but overall it’s bang on. Especially good for troubleshooting.
Hey Larry … very cool on the Haynes download !!! 
Thanks, I have a paper copy I use mainly though!!
Hey, today replaced oxygen sensor and car work`s again normally! Now must clear the code 44 from memory and go to technical inspection. Thanks to all of you!
Peeter …
Great news on the new oxygen sensor fixing your problems.
Here’s what the Haynes manual says about clearing codes …
Grooveman, thanks!
Glad I could help 