DTC P0445 Evaporative Emission Control System Purge Control Valve Circuit Shorted
DTC P0300 Engine Misfire Detected
DTC P0449 Evaporative Emission (EVAP) Vent Solenoid Control Circuit
DTC P0561 System Voltage Unstable
We recommend clearing all the trouble codes and starting fresh with this Chevy Tahoe's trouble code failure diagnosis. The misfire detection (P0300) is more then likely unrelated to the EVAP trouble codes. You should conduct a complete tune-up (replace plugs, check coil packs and coil pack circuits), and check to see if the Check Engine light turns on again for the same P0300 trouble code.
The remainder of the codes might be related. A short within the EVAP circuit could trigger trouble codes P0445, P0449, and P0561 simultaneously. We recommend beginning with a simply voltage and amperage load & battery test. if all checks well move on the EVAP circuit and meticulously inspect the EVAP solenoid circuit wiring and harness.