API Gas sensor fail

The 165 I'm servicing seems to have encountered a gas sensor failure as it no longer shows up on the GPIB after running well for weeks.  I've found the sensor on the manifold wired in tandem with an external sensor.  there is no gas flow reaching the external sensor which has a reading of 30mV to it.  Any way to circumvent?  Similar sensor for 1997 vintage sciex models?