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U1255 code in many Modules

1543 Views 17 Replies 4 Participants Last post by  basscatt
1998 Seville STS, 4.6 engine, 80k miles

I was passing a truck under heavy throttle and the engine died like I turned the key off.
Mountain road on a curve with no power steering and no power brakes...very hairy!

The DTC code U1255 "lost communications" shows on all of the following... DDM , DIM, IPC, RFA, RIM, VTD, MSM. The PCM had no codes. At the same time "service stability system" showed on dash.
The "lost communications" possible causes listed are...
-Loose control module connectors
-Faulty Control Module
-Control modules poor electrical connection
-Control Modules harness open or shorted

The engine will not start (no spark), and there is an electric humming from the back of engine (without key on) that is draining the battery.

Where do I start to solve this? Any help would be greatly appreciated!!
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Thanks very much! I didn't really know where to start. I REALLY appreciate your help!
The DTC code U1255 "lost communications" ......................
the actual definition - there IS a difference -
U1255 - Class 2 Communication Malfunction (Serial Data Line Malfunction)

check the motor mounts -
I suspect that the front mount is broken -
thus allowing excessive engine movement - especially on acceleration -

and when you hammered the throttle - on a winding mountain road - passing a truck -
you tore some wires loose - broke a ground - etc -
Hi Basscatt
Here is something new... there is a small pump on top of the motor that pressurizes the injection that is running all the time until the battery is flat. This is with everything turned off. Any ideas?
absolutely no idea what you are referring to
Threads have been merged.

Here is picture of the "pump" that runs constantly

Here is a picture of the "pump that runs at all times. Picture did not attach the first time


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Here is picture of the "pump" that runs constantly
That is the Fuel Pressure Regulator, on the fuel (injector) rail, fed by the high pressure fuel pump in the bottom of the fuel tank.
The vacuum line sends engine load to the FPR which then adjusts fuel pressure/flow to the injectors by altering the fuel bypass to tank rate of flow. It is not a pump, but may hum due to pump pulsation - the thing works with 41 - 50 psi of high flow fuel.

EDIT: The fuel pump runs for a second at KEY: ON to prime the system. It then stays off until the key is turned to START - then it runs during cranking. As the engine catches and comes up to idle speed the pump latches ON and runs the entire time the engine is running.

I strongly recommend that you subscribe the car to - the online GM/Cadillac service manuals - so you have some idea of what you're looking at and of what we're informing you.
the fuel pressure regulator is running with the key off and runs the battery flat.
I found out today that about 75% of the relays are hot when the key is off. Many things have power and draining the battery cwith the key out. Took it to repair shop today. It seems everything has power but the engine .
PLEASE get the service manuals so you know what you're looking at.

There is no power - no power - to the Fuel Pressure Regulator. It is a control device for fuel pressure created by the fuel pump in the bottom of the fuel tank.

Here's your fuel pump assembly .............

View attachment 571098
Thank you, I found that out today I appreciate the help!
Thank you everyone for your help! I will update once all the problems resolved. I also found out today the motor mount is busted, I checked out all the cables and connectors but could not spot a problem. Again, thank you all for your help!
I can't remember ever running across a fuel pump that constantly runs -
but that sure seems to be the issue -

check the fuse box under the back seat -

FUSE #1 is for the fuel pump - pull it and see if that stops the sound you hear under the hood -
if YES - plug the fuse back in

RELAY #39 is the fuel pump relay -
verify that the sound under the hood is still there -
then use the plastic handle of a screwdriver - or similar - and GENTLY tap on the relay -
if the sound stops - you found the problem - a stuck relay -
replace the relay -
Hello Basscatt,
Thank you so much for your help! You were right! The whole problem was caused by the motor mount breaking which caused a bad ground to the motor. The car sat in a shop for 2 1/2 weeks before anyone could work on it but was fixed with your help in 2 hrs!
You are The Master of Diagnostics!!
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