2001 Vauxhall Corsa Hi Fitted New Timing Chain Used Correct Tools Hold Place

hi i have fitted a new timing chain i used the correct tools to hold it in place but since i have done it the exhaust pops once when i am slowing down it does not do it every time. But also the engine managment light cam on with coolant temprature sensor fault i have fitted a new one and that has sorted that but it is still popping i am wondering if i have missed something.