Can you ever feel the truck surge in power with a very crisp "Timing" sound? This is usually a sign of a stuck IPR
coming "unstuck" and the ICP
jumping up quickly advancing your effective timing, giving the engine a very raspy sound until the IPR
duty cycle is gotten back in check.
I had a sticky IPR
and although it never set the SES
, I had the "Over/Under" ICP
code everytime until I took it out and cleaned it.
There is a very good chance you might have ICP higher
than desired if you've got a sticky IPR
. If the truck is setting the code during say, steady rpm, cruising situations, like rolling down the interstate, then I'd look into the IPR
1211 is just as easily set by high ICP
as low. It's an "out-of-bounds" error. It can be from either direction, although usually from the lower of the two.