If a diagnostic light stays on, then the instrument cluster is getting a signal from somewhere in the vehicle that is letting the vehicle owner know that there is an issue with a particular aspect of a system. This is usually not the instrument cluster's fault. The IPC is simply the messenger.
If a diagnostic light doesn't turn on at all it could be a few things, however, most likely the IPC (instrument cluster) is at fault. Diagnostic lights are usually LEDs soldered to the circuit board of the cluster and rarely fail.
The main reasons why a diagnostic light like the 'Check Engine' isn't turning on is:
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The LED light was intentionally removed.
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Tape was installed over the LED light.
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There is a gasket maker or some other residue stuffed inside the faceplate that covers the LED light when installed.
If you have a diagnostic light not turning on, please let us know what light that is and we can do a bench test to see what issue is preventing it from turning on when you send in your instrument cluster for repairs.
Please note: It is important that you let us know you're having an issue with a diagnostic light prior to sending it in for repairs. While our techs do check and inspect the IPC (instrument cluster panel) a small area or component could be missed if not examined carefully for that particular diagnostic light.