I'll try to keep it short.
93 Accord with 215,000 miles. Failed the emissions test. NOx blew 4.43. (2.5 is limit) Took it to a registered shop to fix. Estimate for $460 for a good tune up. Plugs, Wires, Dist. Cap & rotor, Air and fuel filters, oil and filter. I asked if I could do the things I can, and then bring it back...sure. When I took it back the shop said I had to take it and have it retested. Did that, failed again. The shop says it may take 3-4 hours to diagnose and so far they want to clean the EGR estimate (2-3 hours), latest update, says well over $1500 to replace the entire intake manifold do to plugs inside.
Any advise would be greatly appreciated.

Considering taking car off-island to a Honda Dealer

back story...i live on an island with speed limits of 35-45, so carbon builds up. the car is driven only about 6,000 miles a year. Mistakenly thought synthetic oil would be better and have been using it for several years. Also used RXP Kicker (gas additive) after tune-up to clean system. After the tune-up the car is running very well, no lag, seems better MPG...140 miles on 1/4 tank.