What does the code PO645 mean - the A/C clutch relay circuit is open or shorted?

The computer is trying to complete the command to turn on the ac and it cannot as it does not see a signal to ground.

The circuit has to be checked as to why there is no signal to the pcm

Roy

Thank you very much, the air and/or heat hasn't been coming out of the vents very well. And a year ago it was looked at by a mechanic who said it just needed cleaned out, but the problem restarted about 6 months later. Thank you again.

Would that relay cause the air or heat to not come out the vents very well? Or maybe it just needs recleaned again?

No, that could be the blower motor itself.

Roy

Thank you.