replaced knock sensor, now check engine light doesn't work at all

[Linda1]

Took GMC Sierra 2004 1500 to mechanic to finally have valve cover gasket and knock sensor w/harness replaced.  The check engine light has been on for years because of the knock sensor issue.  Mechanic says they replaced the valve cover gasket and knock sensor.  The check engine light is now off.  However, this morning we noticed that the check engine light doesn't come on when you initially turn the key before actually starting it....nor does it come on during any of the engine starting process.  So, we disconnected a sensor to force the light to come on....and it didn't.  So the check engine light doesn't appear to work at all now (after staring at it for a decade!).  WHY?

[CamGTP]

Pull the cluster and replace the bulb if there is one.