Finding where the spur is being generated is not an easy task, and may not be in the HPA’s of the A or B transmitters.
We had a spur once in Portland when 103.3 came on the air, caused by an A+B-C where none of the transmitters were co located, being 2 to 10 miles apart. (106.7 + 103.3 – 97.1 = 112.9) Modulation detected was 106.7’s but the mixing was taking place in the 97.1 HPA. 103.3 had to pay for a filter on 97.1. It took engineers from all three stations and some guys at PDX airport to pin it down.