3 codes pulled and need to know how to better diagnose them. on 1990 Nissan Pickup (D21)

I went to a mechanic and he pulled 3 codes. EGR valve, Air flow meter, air temp. sensor. He quoted me $1000 to diagnose each individual issue and fix. Is there a way I can diagnose each problem so I can fix them under a cheap budget?

Simple take it to another shop....and get referals of a good shop first.
1 more answer
Chances are, you will need to pay some one to tell you what is actually wrong with your vehicle.these days, not too many mechanics ( if any at all) do this free of charge any more. You could also buy a code reader and a repair manual and try and tackle it on your own.that would be the cheapest way to go, but unless you do know anything about this type of work,I would not suggest this.find some one who might be able to work with you on a payment plan,and tell them what you know.But,for that "Mechanic" to tell you 1000.$ for each,that's just plain ROBBERY!!!