If you used any aftermarket parts get rid of them , especially the MAF.

The ECU or ECM controls the AC system on a 99, so if there are problems it can manifest it self with wired fan behaviour

P01620 = EFI-514 Engine cooling fan (FC), low-speed. Signal too high. Permanent fault Diagnostic trouble code (DTC) information EFI-514 EFI-514 Engine cooling fan (FC) low-speed, signal Condition Diagnostic trouble code (DTC) EFI-514 is stored if there is a short-circuit to ground/supply voltage or ...

I'd double check that fan relay - especially if it isn't Volvo. Sometimes the connections to it get rather corroded. I know at one point I had a similar error, literally just had to clean the connections off with a wire brush.