The Check Engine Light can illuminate with codes due to the lack of EGR (exhaust gas recirculation) flow caused. This is commonly a result of a clogged passage in the intake manifold. To correct this problem, 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.Google+
Car Problem Reports
Mercury Mystique Check Engine Light Due to Plugged EGR Passage
Mercury Mystique Problem
Exhaust & Emissions Affected: 2.5L V6
Average Mileage: 110,047 mi (86,000 mi - 127,890 mi)
EGR Valve and pressure sensor failure, check engine light, takes several trys to start, acceleration is poor, bad MPG.
I had to have Precision Tune to clean the EGR valve passage and afterwards it passed the emissons test with a new EGR.
Visitor, 1998 Mercury Mystique, 101,301 mi
they egr passages were cleaned, egr valves replaced two time, and the check engine light is always on???
Visitor, 1997 Mercury Mystique, 127,890 mi
Ken's Tire in Livermore correctly diagnosed this problem. Carbon was packed so solid they initialy thought they were going to have to take off the intake manifold to clear it from the passage. Now I wonder if this was the real problem 60K ago when Pacific Ford in Long Beach charged me $3200 to replace the catalystic converters. I'm the original owner of the car. It's been pretty trouble free except for a replacement automatic transmission at 85K. I've been pretty regular with dealer maintenance until I moved to Livermore.