The Check Engine Light can illuminate with codes due to the lack of EGR (exhaust gas recirculation) flow caused by the intake manifold passage plugging up with carbon and oil. The EGR valve has to be removed and the passage cleaned thoroughly. Partial restriction of this passage can cause Smog test failures for the NOx emissions without causing a drivability symptom.
Car Problem Reports
Mercury Cougar Check Engine Light on Due to Clogged EGR Passage in Intake Manifold
Mercury Cougar Problem
Exhaust & Emissions Affected: 2.5L V6
Average Mileage: 157,834 mi (122,338 mi - 185,000 mi)