I checked the thermistors (CTSs) at round 40F too. I filled a sour cream tub with ice and water; cut a hole in the lid and threaded the CTS in so it reaches into the water.
I took the readings at around 35F-40F. They all seem to be changing with the temps, within reason as per the temp/resistance chart. So I am going out to put things back together.
If codes 26 and 56 remain I am thinking it is a wire or connector problem (I've done everything else I can think of) with the purge valve and EGR valve.
I'll get back.
thanks
