[tvan]

The Heat downstairs stopped working.

It is a Carrier 8000 weathermaker model 58WAV091-14 using natural gas heat and electric AC

The error code is 3 (short) 1 (long). - 31. Which according to the label on the outside of the unit 31 is one of these:

PRESSURE, DRAFT SAFEGUARD, AUX-LIMIT SWITCH (when used*) OR BLOCKED VENT SWITCH (when used) DID NOT CLOSE OR REOPENED (DOWNFLOW ONLY*) If open longer than five minutes, inducer shuts off for 15 minutes before retry. Check for: - Proper vent sizing - Low inducer voltage (115VAC) - Defective inducer motor - Defective Blower motor or capacitor - Restricted vent - Defective pressure switch - Excessive wind - Inadequate combustion air supply - Disconnected or obstructed pressure tubing If it opens after trial for ignition period, blower will come on for 90 second recycle delay

I did the component test the result was:

1) The inducer motor came on briefly (about a second)
2) the ignitor comes on
3) the blower motor comes on and runs slow then faster.

I unplugged the main harness from the circuit board and reran the Component test. This time the inducer ran for about 15 seconds and the other tests were the same as before. When I put it all together and turned the heat on at the thermostat the inducer motor started running, the igniter turned on and the burners lit. Then the fan came on a it ran for a few minutes and turned off with a code 31 flashing.

Now it goes through the same cycle and starts but either shuts down before it reaches the set temperature (showing a code 31). Sometimes it will run to temperature but always has more than one attempt to get the heat going properly.

What I have tried:

I checked for any kind of blockage, vacuumed the vent pipes and cleaned the inducer fan motor etc.
I reset the two switches (draft safeguard switch and the one above the burners.
I swapped out the pressure switch with the upstairs unit (also a Carrier same part number) but it made no difference.
I tested the continuity across the switches.
There is 26 V on the switches and some sensor plugged in to the firewall. The pressure switch has continuity once the inducer starts up.

I don't know what else to check for. I am kind of learning as I go along.

Any help or pointers will be greatly appreciated