1993 Toyota Camry EGR code

I recently got my car smog tested and it failed because my check engine light wouldn't turn on. I was told my car was fine when I had it checked so I left it and the light eventually burnt out. When I replaced the light it still remained on and I had diagnostics run once again and I got an EGR code. I had the EGR valve replaced and the light went out. I went back a week later to get my smog re-test and it failed due to the check engine light coming on during the test. Why would the check engine light only come on during the test? It went out as soon as I drove away.