in addition to the info above - the monovalve has a switched ground and a steady +12v with the ignition on. Normal rules of thumb like checking for voltage won't fully diagnose a monovalve that doesn't close to shut off heat.

No continuity to ground via the control unit = heat all the time.

Switched continuity to ground = controlled heat

No +12v at the monovalve will also result in full heat, but you need the +12v AND the path to ground.