It is very unusual for an engine to fail the NOx test. This means the mixture is burning too hot and creating to much NOx. In order to pass this test you need to slow down the burning rate. This can be done by using a high octane fuel or through the use of octane additive. Such problems can also occur with insufficient timing (retarded spark). Your vacuum advance may not be working or your spark is retarded. Modern day cars have both oxidizing (to reduce CO) and reducing catalysts (to reduce NOx). The Delorean catalyst was only an oxidizing catalytic converter. Bob Brandys 97? DeLorean Time Machine.