no heat

Has anyone else had trouble with their heater? Some time this summer, the heat quit working. I put it in Auto mode and turn the temp up above the outside air temp and I can't get any heat to come out. I can turn the setting down and get A/C. I crawled under the dash and felt the bypass tube coming through the firewall and it felt plenty warm, so I'm tempted to rule out a blocked heater core. I disconnected the mixing door control arm and tried it at both ends of it's travel and it made no difference. I have not been able to find a setting that causes said mixing door actuator to move either. I was wondering if the temperature sensor could be bad? But it seems that moving the mixing door(if that's what I think I was playing with on the passenger side) would override any effect that a bad temp sensor would have. My service manual only suggests a blocked heater core. I was thinking of flushing the cooling system and making sure nothing was wrong there just in case. Any feedback would be very welcome.