stop engine light

2005 w900 with cat c15 acert engine...mxs00594 serial number. Starting doing something strange today. Got up and drove 70 miles this morning to my unloading destination. Cut truck off when I got there, untarped and unstrapped load. Got in and fired it up to cool off. About 10-15 minutes later the cel comes on then the red stop engine light does. About 15 seconds later it shuts off by itself . Restart it. And same thing. So I get out look around nothing abnormal. All guages read normal. What the heck!. So i start it again, this time it stays running, no lights, so I pick up my next load drive about 350 miles to my unloading destination for Thursday morning. Cut the truck off. Walk to the pilot up the street. Take a shower, grab a bite to eat. Go back to truck and crank it up to cool interior off to go to sleep. About 15 minutes later it goes to beeping and lights flashing and shuts down, so I turn key off and post this. Anybody gave any suggestions? Only code I am getting is 55

 

Thats what it was. Added coolant and all is good. Dad left the cap off when he checked the ph level