I’ve a confusing picture related to my 2005 x350 Vanden Plas. It has 105k miles and I’ve owned it since March 2024. I’ve replaced every filter and every fluid; the spark plugs were replaced and likewise the coils. I just finished replacing the hoses and brakes. None of that work was “needed,” but with that many miles, I just did it. Historically, the car runs smoothly and without any notable sounds (no squeaks or rattles or clicking, or whatever).

I periodically get a DTC U3FF6 pointing at the Rear Electronic Module (REM) without the Check Engine Light (CEL) and without any indication of a consequence related to that DTC.

On two occasions I had the battery disconnected for a long period (weeks) due to the work I was doing on the car and in both cases the car had lots of trouble starting…sputtering, coughing, and running very rough until it quit running completely. In both cases, after many starting attempts, it kicked into a perfect purr and then runs that way from then on with no trouble starting.

During a third occasion of two-weeks of downtime without the battery being connected, upon starting it went through the rough starting processes, etc., but also presented me a restricted performance light along with the CEL. At that time, there was a wide variety of DTC’s from all over the place…no P codes. Once the engine settled down and it was running smoothly, I was able to clear all the DTC’s. I went for a long freeway drive without consequence, but the restricted performance light and the CEL remained lit without any DTC! I didn't notice any difference in performance. After two days of driving without any problems both lights went out…they were just gone upon start up.

Today after a one hour freeway drive and then being off for a half hour, I had to go through all the troubles of getting it to run (sputtering, coughing, struggling, etc) ~ the restricted performance light even came on briefly and then went away. After several failed starting attempts, the code reader showed NO DTC while I sat there waiting for the car to relax and compose itself (well, me, too.) Then it just started up. No lights and no DTC.

I’ve been checking for leaking hoses and shorted wires and wire bundles. I cleaned grounds: G1, G9, G22, G23, G24, G26, G29, G30. The battery is showing 13.8V with engine running and 12.9V when the engine is not running. (I was expecting something closer to 16V w/engine on.)

I removed the carpet cover over the REM & related wire bundles and everything is pristine ~ very clean as well as the plugs to the REM all seem seated. Wires look new.

I’m thinking that the fuel pump isn’t getting enough power when the battery has been offline for a while. At this time it is showing a U3FF6 and is running smoothly without CEL and without restricted performance light.

I would appreciate any ideas or thoughts about how to approach this. It would be helpful to have a direction WHERE to start looking and/or what I should be thinking of. Thanks for any help you can provide.

—marvin

 

Collective wisdom says replace battery!
You can find plenty reffrences to batteries seemingly working fine, but causing bunch of random errors.
I had exerienced radomly dash lights up like a christmass tree at speed on freeway. Battery seemingly worked fine:charging and holding charge, Never had problem with starting (2 years). I did replaced it 4 months ago, it was dated 2015. Knock on wood, no more issues.

 

Thanks. I've been reading that Home Depot sells a battery that works well with Jags...that our alternators put out 16V, which harms regular batteries.

 

I have a an power adapter 12V to USB pluged in my sigarette lighter socket. It has an display that reads voltage from the socket. I have never seen 16V reding on it. 14.5V max. regardless of battery. I think just after start generator commanded to output max current and voltage reading drops to 13.8-14.2 in few seconds after start.

 

I agree with Big Koshka; Battery would be prime suspect. Also, since it is hitting the REM, you might want to check all the grounds in the vicinity of REM (in trunk on passenger side behind trunk liner).

From the manual regarding voltages: The battery charging voltage is determined by the temperature of the generator. In cold conditions,
starting the vehicle from cold the battery voltage will be between 14.2 volts and 15.1 volts and will
reduce as the engine warms up. In hot conditions starting the vehicle when the engine is already
warm the battery voltage will be between 13.5 volts and 14.3 volts.

In my experience from digital DVM built in, cold start is 14.5, then trails off to 14.1/13.9 as battery charges after 20 minutes driving. Any good AGM battery will work, my personal preference is Bosch. However you can save a few bucks at Sams Club with a Duracell AGM Automotive Battery, Group Size 49 H8. I had one of these in previous XJ8 and it did great.

 

Interesting. Thanks for your thoughts on this. I really thought someone was going to point to the fuel pump or some electrical problem with it communicating with the battery...but the fuel pump seems to be operating just fine once the engine is fully started...so the battery makes sense ~ problems happen during the startup process, not when operating on the freeway. Does anyone have an interpretation of the U3FF6? The car doesn't seem to be randomly throwing codes; it is just that one on a consistent basis. BTW, last week I did go through all the grounds around the battery and those associated with the REM ~ corrosion was minimal and easy to clear and connections were tight. I appreciate this help.

 

Check the screw connection of the 3 megafuses in the trunk, follow the battery plus cable to get there.

And try a hard reset: Disconnect both terminals from the battery poles and connect them together for about half a minute. This causes potential equalization between the control units and on the data bus.

Fritz

 

For what it's worth, I concur that this sounds like a dying battery causing all sorts of mischief. If the voltage at the front cigarette lighter is below 12V when you turn the ignition on without starting, you need a battery. The original battery for my car was a T8 but these are very hard to find these days. An H9 is a very good alternative. I have used standard lead acid and AGM batteries in my car and both have worked fine. Also, anytime you are going to be down for more than a few weeks without driving its wise to put the battery on a maintainer if you can. I have used the SAME "Battery Tender Plus" 1.25 amp maintainer for more than 30 years on my various hobby cars and motorcycles with great success.

You can work on the U code after you are up with a good functioning battery.

Good Luck
Jeff

 

Fritz, Thank you for pointing toward a diagnosis. I had already completed a hard reset; one about 3 weeks ago and another last week. At the moment I am not going to do that again because I want the U3FF6 to remain until I explore a bit further; however, what should I look for after a hard reset related to this? I tested the tightness of the screws in megafuses and they were rock solid and look factory fresh. I disconnected the battery from a maintainer and it is currently 12.8V and throughout the megafuses I found 12.8V. I've been wiggling everything trying to find something loose.

 

Jeff,

The battery has always been on a maintainer when disconnected from the car...mine is a 2A trickle. Disconnected from the maintainer, the battery is currently 12.8V. I tried the cigarette lighter per your description. It read 12.0V to 12.1V.
I appreciated your discussion about what battery to get. The battery in there is an Interstate MTP-49...H8 (not 9) with 910 cranking amps. It certainly has at least 2 years on it ~ but don't know the age. So, I'm wondering what is the correct descriptor of the battery I should be shopping for. I don't have strong feelings about going AGM, but certainly willing. The Forum has discussions about Excide at Home Depot (!) and Bosch. Interstates are available all over, but the other two require special orders according to the folks helping me. Your thoughts? --marvin

 

Since running the "cigarette lighter" test, I was wondering what was available to the starter once the key was turned on, but the engine not cranking. It is 12.1V! I'm not sure that is enough for an effective start process...and that is with a fully charged battery.

 

So, the spec is 680 cold cranking amps and 90Ah. The battery I have is just over those descriptors: 710 and 100.

 

What tool says U3FF6?

Oh - use a different tool