water pump and thermostats

What is your question?

I recently changed the water pump and thermostats , I accidently put the old coolant back and im yet to fix it, went to work a couple of times but today as I went shopping with the wife on my way back the engine light came back on ..what could it be??? old coolant? i have no idea on what to do next besides taking it in to autozone for a quick engine check ..any advice would be appreciated.

What makes this problem better or worse?

haven't notice anything weird yet it just happened.

How long has your 2007 GMC Canyon had this problem?

started today