P1812 code

For several months I have been experiencing a slight bucking between 35-40 mph, and feeling like it's going to stall when I'm stopped at a traffic light. The first time I noticed was in June, I felt it buck and the check engine light blinked for about 40 seconds. I took it to several places and no one could come up with a code. Last week the check engine light blinked again and a friend suggested it might be a transmission problem so I took it to the shop and again, no code. Yesterday and today it started stalling as I was pulling to a stop. I took it to the shop again and the only code we got was P1812 - see manual. I looked this up online and read "P1812 Transmission 4-wheel drive mode selector circuit failure." I don't use the 4-wheel in it, but can this be the cause of my problem? I'm taking it back to the shop Sat. morning and I would like to be somewhat informed on what this means.

May we have the year? Just checked the manual and the test is for the 4X4 mode switch. If the mode switch checks out OK, the next thing is the 4X4 module. Good luck.

Sorry forgot to post that. :banghead: It's a 1999 4.0 SOHC.

Still the same. There is a test for the switch. That seems like the most common failure point in the system. The control module is a last resort. It could also be a wire somewhere in the system. Good luck and keep us informed.

A few days my "wanna be mechanic" friend was "looking for a vacuum leak. I was standing in front of the vehicle with the hood open and did hear something that sounded like a possible leak, but he said he didn't see anything. I took it to Gibraltar Transmissions today. They told me the motor for the 4 wheel drive is bad. I can't locate the part anywhere but the dealership for $383.23.
Could this be the cause of the rough idle and check engine light?
I would hate to go forward with this repair and not fix the problem. At the very least I'll have the shop check for a vacuum leak.

It is possible the motor is out. It should not cause a rough idle though. If the P1812 is the only code stored, then that is why the check engine light is on. You may want to have the codes read again to be sure there are no other codes in it.The test for the code is long and detailed. If they determined the motor has input to it and it is not responding, the motor is probably bad. For the vacuum leak, check the PCV hose closely. They are bag about getting holes in them. Good luck and if there is anything else we can do, let us know.