[302Z28]

My '92 Metro failed the emission test with an Hc of 560. They said 220 is the limit, and that a lot of unburned gas was going out the tailpipe. The car has 296K on the motor, but is in good shape (175# on all 3 cyls). No check engine lights - car seems to run good. Daughter says she only gets about 37 MPG. I figured the drop in MPG is due to the teenage lead foot. Over a year ago, I was getting over 50 MPG. This is the first time the car has been emission tested. I'm thinking about changing the -02 sensor, but question why if indeed it is bad, no CEL light? Plugs, cap, rotor, wires are new. New injector has less that 30K on it.
Any thoughts would be appreciated.
Thanks,