When I pop my truck in either R or D the idle gets very low and it starts to sputter/shake. I also noticed that if I leave my truck to idle for more than 5 minutes the check engine light turns on which is the bad o2 sensor after the cat but I don't think that's my problem. When driving, it does goes away but I want to know as to why my truck is giving me a very low idle when shifting to R and D. This is intermittent - when I tried to diagnose it, it wasn't doing it. Any tips to resolve this?