My daughter was driving the truck and was coming to a stop sign. She said that a Check Gauges light came on and the engine stalled. She tried to re-start it but it just rolled the engine over. It eventually started. My wife took the truck to the dealer. He found the P1521 code and said that this code would cause the engine to shut down. I am very skeptical.

Is it true that a P1521 can shut down the engine?

 

What year of truck, trim level, mileage, engine, etc. etc.?

Need these to help you properly.

 

The truck is a 2006 Ram 1500, SLT. It has ~144k km on it. It has the 5.7l HEMI

 

Nice truck, looking to get myself a 2005 or 2006 within the next couple of months. I will move this to the 3rd gen section, get you some help.

 

What viscosity oil are you using?

 

MDS Hemi engine is VERY oil viscosity specific to run properly. MDS will not function properly with any viscosity other than 5w-20 and engine damage may result. However, I've not heard of it completely shutting an engine down or even putting it in limp mode...

 

Big brother is watching.

 

Not true. Incorrect oil viscosity is stored as an internal code and would never show up as a DTC that would set off the CEL. The only one who can view that code are dealers with a Starscan tool. I myself have installed 5w-30 oil for better protection only because I have the mds disabled. The mechanic at the dealership pulled that code and and as I played dumb and asked him why that was, he said...nahh don't worry about it, it's a nothing code and you probably got some 5w-30 when you had your oil changed...happens all the time he said.
Well,, I change my own oil..haha and I even ran 5w-30 for roughly 1kmi before disabling mds and didn't have any trouble.

So the dealer didn't see any other codes? If anything could cause this and not throw codes...Bad ignition switch!!! I'd suspect this might be your culprit.

Your best bet is to make sure you have 5w-20 in there and see if it happens again.
Since i dont know the details on if it had to crank multiple times or if it started right away, I cannot give better advice.