'99 Sierra 1500, 4.3l. Problems began this time last year. Several codes came up, including p0446 and a couple of others (misfire & vacuum leak stuff). At that time, replaced plugs, wires, distributor cap & rotor, vapor canister, vc purge solenoid valve (per TSB), gas cap, & intake manifold gaskets. All codes cleared, except for p0446. Scanner indicated 2 codes, p0446 & p0446 pending. Was planning to get another emissions waiver for this year (as we're at a loss as to what to do next), so went to the inspection station & got the truck failed a couple of days ago. Then yesterday, the MIL went out. Was ready to rush back to station to get emissions passed, but decided to scan first. Scanner now gives p0446, but the p0446 pending is gone. Also, MIL is lit (stays on, not like the brief flash of all dash indicator lights) with key in on position, but goes off when the truck is cranked. Previous freeze frame data indicated p0420, & now that pending DTC is gone, freeze frame data indicated p0171 Very confused. Any ideas on where to go from here?