Check to make sure whoever replaced the mainboard/CPU also applied thermal compound under the heatsink.  If that wasn't done, the system can overheat almost instantly.  If there is thermal compound under the heatsink, chances are the mainboard is toast.

Interesting, so ran a diagnostic before tearing down the machine.  Diagnostics came back fine. I did notice however that the CPU temp was 45c and fan speed was zero.  

Upon tearing machine down, I found plenty of thermal compound, looks like one of those thermal strips which are really thick. Didn't find anything obstructing the fan and fan cable was attached to the MB.

I am still wondering if there is a trip sensor that needs resetting somewhere.

You can clear the CMOS settings by holding the power button with the battery and AC adapter disconnected.  Did the fan spin up and pass the test?

I didn't hear the fan spin up, thus my reasoning for opening and tearing down.  I will build it back up and retest.  I will also clear the CMOS.

Out of curiosity, other then clearing the CMOS how do I clear the event logs?  I want to make sure that any failure is a new event, to eliminate the worry that there is a temp trigger somewhere that needs resetting.

Clearing the CMOS should clear the event logs.  There may be a "clear" setting in UEFI setup as well - on some models.

Okay I am increasingly convinced the MB is toast.  This implies that Dell did not replace the MB as they said they did, which really ticks me off.  

Reset CMOS, plugged everything back in and attempted to restart machine.  The power button led lights up as does the system led below the touchpad.  The power button led stays lit for 10 seconds then switches of then back on again in similar pattern.  

If I unplug and hold pwr/d button the laptop cycles through the screen colors.

Other then that no response, no post, no boot.

any other thoughts anyone? wish I could figure out how to test and determine the root cause.