2 system winter PM. Checked blower motor amps. Checked inducer motor amps. Inspected heat exchangers. Checked temperature differentials. Cleaned flame sensors. Downstairs unit would throw a code 33. Waited for 4 minute blower delay and it would turn on without any issues. Waited 15-20 minutes and it did not turn back off. Customer will call us if issue arises again and it doesnâ€™t turn back on.