Got a Coolant Flush and then P1151 Check Engine Code

I would just bring it back to the shop and let them sort it out, you did pay for the service

 

The shop probably didn't even know there's a thermos in the system. That makes it very difficult to dump the rad & engine coolant and then try to get the air out. Air gets stuck in that thermos.
You must cycle the thermos pump to circulate the air trapped in it. Be careful with this situation because there is no temp gauge in our cars and you will have alot of air stuck in the top of the cylinder head as that's the highest point of the engine and it will not do a good job of cooling.
You could be over heating the engine and never even know it.

Use the search forums link up top and search

CHRS

and read all the posts. It will show you how to manually cycle that pump by jumping out the CHRS pump relay. This is why I never recommend doing a full coolant change. Just dumping the rad only a few times over a few months will do.
Remove the black plastic cover over the top of the rad you will get to the rad cap. Dont bother with the overflow tank its level is meaningless.
Open that rad cap up with engine cold over a few days and keep and eye on that fluid level it will move around alot.
You can leave that cover off the hood traps it when closed. Mine has been loose for about 12 years now. Its not going anywhere. makes it real easy to check the rad.

 

Old part failing from what lol? Ok your dealing with some clueless idiots here.
Run.

I would take the cover off the rad and open up the rad cap and see how low the coolant is before driving it. The overflow tank is meaningless for coolant level check it could look fine and the rad could be empty.