Check emission

I have a problem with my car when I switch it on I get check emission. It come up sometime I switch it on. Like yesterday in the morning I had check emission then later in the evening it was normal and didn’t have the problem. Today I get check emission again. Does anyone know what this problem could be
 
Might be worth mentioning if you have any mods etc
There no mods nothing. It’s stock 197 f1. The service is done and everything new parts spark plugs and ignition coil. The light didn’t come up on the car only after 2 days it started to come up
 
I would suggest then getting the car scanned (preferably with CLiP) to see what errors are flagging / stored, then even clear and see what comes back.
As it could be so many things, lambdas, leaks etc.
 
I have the same issue. I had the O2 sensor changed and the long term fuel trim settled and did not come back for a few months and then came back.
I think I might have an air leak in the intake though as I get a whistle when accelerating
 
I have the same issue. I had the O2 sensor changed and the long term fuel trim settled and did not come back for a few months and then came back.
I think I might have an air leak in the intake though as I get a whistle when accelerating
Mine goes away when I feel up with 99 fuel