Possible causes

– Faulty EVAP Vent Solenoid
– EVAP Vent Solenoid harness is open or shorted
– EVAP Vent Solenoid circuit poor electrical connection

When is the code detected?

The Powertrain Control Module (PCM) has detected the EVAP Vent Solenoid circuit was low

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P1646 CADILLAC Description

The Powertrain Control Module (PCM) contains chips called Output Driver Modules that provide 7 switchable ground outputs for operating solenoids, relays, telltales, and other devices. Each of these ground outputs has an internal fault line that feeds back to the PCMs microprocessor. When the output is OFF, the output driver should receive the 12 volts fed to the solenoid, relay, etc. When the output is ON, it should receive 0 volt since it is grounding the output. If it receives 0 volt when the output is OFF or 12 volts when the output is ON, it sets the fault line low.