these are some folks that sell two versions of co testers. as i recall
(cause i don't have the info here at work), the cheaper one is about $120
and the more expensive one (with dwell tach and some other test goodies)
is about $200 or so.

Auto Expert Products         Gastester CO meters
2574 NW 29th Drive           Eezbleed pressure brake bleeding system
Boca Raton, FL 33434
(800) 795-6958

their tester just sticks up the tailpipe, though ... you don't have to
remove that plug on the exhaust.

joel