I "Think" this may be the engine ambient temperature sensor on your unit.
Your engine is the Cummins ISB-XT 360hp?
Here is an aftermarket for about half price.
For clarification you said the engine ambient temperature sensor. But your mechanic was looking for it in the front grille area. He's looking for the air ambient temperature sensor that gives you a outside temp reading on your dash to let you know what the outside temperature is.
The engine ambient temperature sensor measures air temperature and pressure going into the engine and should be located on the engine manifold. This sensor will throw a fault. The other air ambient that gives you the outside temp should not throw a fault. There are two ambient air sensors, one for the gauge in the cab and the other going into the engine intake manifold that measures air temperature and pressure.
I've heard that you may have to find the engine ambient air and pressure sensor from the bottom of the engine on the intake manifold.
Oh, Welcome to Thor Forums.