My 1984 280 SEL Euro 110 six cyl failed the emissions test. Before the test I changed oil and air filter and filled up with 93 oct. the plugs are a few months old. The car passed emissions 2 years ago. The readings are:
HC 143.9 pass is 157
CO .10 pass is 1.00
NO 3312.3 pass is 1121.0
CO2 13.87 pass is ?

The NOX is about 3 times higher than passing.
The car has 185,000 mi and does use oil through the valve seals.
I searched for info and found that retarding timing and checking EGR valve may help but not sure if that will reduce the levels enough. Will a bad cat. converter cause these high levels? Any help will be appreciated.
Thanks, Bob