In the case of lincoln twncar 1995, smog tested with smog test only station, dealer service dept. on 6 services gets light off, but .2 TO 19 miles drive off the light back on. Can't pass smog, with light on, though with tailpipe monitored, itpasses emmissions. only no-pass is check engine light on.
What to do if vehicle passes, twice all emission, but the check engine light on? on 1995 Lincoln Town Car
by Visitor in Woodland Hills, CA on March 12, 2010
ANSWER by patrick mannion , March 12, 2010
Tail pipe emissions are the critical part of the state emission test but the Check Engine light shows a fault in the emission system. The computer in your car tests the emission system when certain criteria is met while driving or even when the ignition switch is turned on. A scan tool is needed to retrieve the codes stored in the computer in your car and the fault fixed.