[ischmitz]

You are right. A 1_15 means the DME has not seen the idle contact closed after the ignition turned on. This leaves the CEL on. It could be a simple mis-adjustment of the accelerator cable or microswitch. Before you even start to measure things there is an even quicker test:

Open the engine bay with the engine not running. Gently push the throttle to the closed position and listen. If you hear a faint click that means the micro-switch got engaged. The return spring is not strong enough to overcome the excessive tension of you thtottle cable. If that is the case simply slacken the cable. There is an adjustment where it comes out of the tunnel under the car. You need to crawl underneath.

If the throttle closes all the way against the stop and the switch does not engage properly you might have to adjust the position of the microswitch itself.

Also lube the throttle assembly slightly. Years of grime could take it sticky.

Last, there could be an electrical problem - such as a broken switch. I doubt that since your problems worsened over time. This is typical for a switch that is getting out of adjustment over time. And without the switch engaging idle is hunting and rough.

Good luck,
Ingo

BTW: The ICV is a 2-terminal device driven by a H-brigde. This allows the motor to run either forward or reverse. The DME switches polarity in quick fashion to achieve a certain opening angle. Both contacts are signal.