Check Engine Light only on in summer
 

Hello. I have a 1997 Ford Explorer Sport. For the last few years my Check Engine LIght has been on but only during the summer months. Every late fall it will go out and then in about April when it starts warming up it comes back on. I have had a few sensors replaced but it still happens. Last year I took it to a garage when th elight was on and they couldnt find any codes to indicate a problem. Since the light just came on again in the last few weeks I took the Explorer to AutoZone and they hooked the computer up and said I have a leak in the emission system. Does anyone know what this could mean - especially how it only is on during the warmer months. (Note: we had a warm spell in early March where the CEL came on but then the temperature went back down in the 20s-30s and the CEL went out again. It came on a few weeks ago when the temp started getting in the high 40s-50s)

Any assistance owuld be appreciated.
Thanks