Air injection is only active on TWC (three-way catalyst) engines during warmup. On OC (oxidizing catalyst) equipped engines air injection is full time.

In all likelyhood this car has a TWC, and it will be indicated on the emission/tuneup decal in the engine compartment. All cars have these stickers, which give basic tuneup information and have codes for emission control equipment - federal law. Mercs have had TWCs since the early to mid-eighties, and virtually all cars have had TWCs since the late eighties.

Air injection has nothing to do with direct NOx control, however, early OC equipped cars ran richer than stoichiometric, which reduced NOx, and the additional engine out CO and HC was consumed by the injected O2 in the catalyst.

Duke