I have a Bryant plus 80 furnace with the following symptoms.

Blower motor starts on low speed.
Gas ignites
Flame burns for about 90 seconds
Relay in motherboard makes a clilcking sound.
Fan does NOT kick into high speed.
Furnace turns off
Mother board produces error code 33.
A reset on the blower door safety switch makes code 33 go away and displays error code 13 for three minutes then the process begins again.

Any ideas why the motor does not kick into high speed?
How do I get rid of error codes 33 and 13?
Why does the furnace turn off after relay clicks and fan fails to spin on high?