Fault codes P1391 & P1406

My 97 Rodeo just failed the NYVIP inspection. Estimates to repair range from $675.00 to $950.00 and they all say something different except the Oxigen sensors. Any Help? Is this the costumary price?
OBDII Diagnostic Trouble Codes:
P1391 and P1406 Manufactor Specific Code.

P1391= "Glow Plug Circuit Low Input (Bank #1)"
Hmmm... That's for a diesel engine?!?

P1406= "Exhaust Gas Recirculation (EGR) Position Sensor Performance"
That's an easy fix. Remove, clean or replace the differential pressure sensor for your vacuum actuated EGR valve. Or maybe the EGR valve itself is stuck, clogged or has a pinhole leak in the vac diaphragm.

G/luck
Joel

P1391= "Glow Plug Circuit Low Input (Bank #1)"
Hmmm... That's for a diesel engine?!?

P1406= "Exhaust Gas Recirculation (EGR) Position Sensor Performance"
That's an easy fix. Remove, clean or replace the differential pressure sensor for your vacuum actuated EGR valve. Or maybe the EGR valve itself is stuck, clogged or has a pinhole leak in the vac diaphragm.

G/luck
Joel

Thanks Joel
I removed the EGR and found it to stuck. Folowing the advice posted here on EGRs, I cleaned it an it is working fine. The 1406 code is gone.
The CEL is lit on the 1391 code.
On another site, I found codes for the Isuzos and it indicates that1391 is "G Sensor Performance". Which sensor is this? Where is it Located? What does it do?

Thanks again