My dad's 1991 300ce has a fast idle issue and I can't seem to solve it.

I do know the issue is related to the idle control valve. I have unplugged the ICV and the idle immediately drops to the normal 500 RPM. (This set off the check engine light.) The OVP fuses look good as well.

I went ahead and replaced the Idle control valve and it helped reduce the idle RPMs from 1500 to 900, but it still won't drop to a normal RPM and is a little hard to start.

I ran about 3/4 can of seafoam through the intake and put 1/4 can in the tank just to make sure nothing was blocking the intake. It seems to have fixed a slightly rough idle, but the RPM remains at 900.

I will be replacing the temp sensor in the morning to see if that helps. The thermostat is relatively new so I'm not planning to replace that.

Anybody have ideas what to check if the temperature sensor doesn't work?