Question
We have had this problem with our grass turning yellow in areas right after green up. so we started using chem lawn co. hoping that they would be able to get it under control. this started about 4 yrs ago and it will green up nice and then start getting yellow in areas? chem lawn sent out one of  their pro's to take a look last year and he said for the first cut of the year to mow it short and add peat moss. It was supposed to change the ph levels/? I did that but now I see its coming back always in the same areas.

Answer
Cutting the lawn close at the first mowing of the year is only part of a larger program. The mower picks up a lot of the debris which should be bagged and put to compost. This is followed by a good raking to get rid of even more 'stuff'. Now the lawn is prepared (opened up) for the application of fertilizer and weed killer, in that order.
Correction of pH is a long term process; one has to measure the existing pH first, treat and then measure again to find out how effective the treatment has been.
But we digress. The commonest cause of the yellowing you describe is the shortage of nutrients because the soil ties them up and the plant roots cannot absorb them. You can easily test this by applying a liquid foliar fertilizer over the affected areas. Do not exceed the recommended rate. You can discuss this with Chemlawn and they should be only too pleased to try it at no extra cost to you.
Chlorotic areas on a lawn are often over the place where building materials were buried during construction of the house; but there are other causes.