These guys are using the parts canon approach. It's highly unlikely the PCM is your root problem.
The EVAP readiness test takes a while to run that's why the money light stays off.
Diagnose this and fix it. If a tech can't diagnose and repair a Purge and Seal GM EVAP system they're a poor mechanic.
P0446 is set when the car's computer believes (either correctly or incorrectly) that there's a problem with the vent valve.
I've had plugged up canisters, screwy vent valves, and bad fuel tank pressure sensors cause this error.
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1959 M35A2 LDT465-1D SOLD
1967 Dodge W200 B383, NP420/NP201 SOLD
1969 Dodge Polara 500 B383, A833 SOLD
1972 Ford F250 FE390, NP435/NP205 SOLD
1976 Chevy K20, 6.5L, NV4500/NP208 SOLD
1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8
RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.