Check Engine Codes

1997 AWD Astro Van w/ 85,000 miles. I have 3 check engine codes that have been bugging me, especially the first one I will list. If anyone has any info on what may be wrong please let me know. The 1st code read: PO300 (Multiple cylinder misfire detected). The 2nd is: PO140 (Sensor CKT no activity) - what the hell is a CKT censor? And the 3rd code was: PO135 (Heater cicuit malfunction). Here is what I experience when driving. When the van has been sitting for a while like overnight and I go to drive it it seems to sputter when I am first accelerating from a stop, but then seems to go away after the engine warms up or I get up to highway speed. The other thing I'd like to figure out is why none of my settings for the blower on my heat/ac work.....(no matter what setting the air only comes out of the dashboard vent/defrost vent/ vent closest to windsheild). If anyone out there has any info for me I would greatly appreciate it. Thanks for your time and consideration. Jay.

I don't know what a ckt is and I'm having problems with my Astro when it's cold. As far as the fan, there is a small black hose about 1/8 diameter that goes from the intake manifold (top of the engine, close to the driver's side, need to take off cover) that connects to the front passenger side close to the fan blower (right of the blower when looking at the van from the front). That hose controls the vacuum which controls the settings. Check to see if it is disconnected, broken or has a hole which will not let the vacuum build up.

I had the blower always blowing on defrost, cruise control would not work and I believe it wouldn't come out of park when I pushed the brake. I had to start in neutral. The problem was a broken vacuum hose off the intake. The heat/AC/vent switch is a vacuum switch. Don't mess with that until you have elimated everything else. I broke mine and had to get another one. Then I had to cut out part of my subpanel to get the new one in without breaking it. A broken vacuum line can cause the engine to run rough and hard starting. With a vacuum leak you lose a little vacuum to everything that uses it, so even an unrelated vacuum line may cause the problem. Good luck.