ok...well EGR will toss a code and most codes the first time they are tossed, won't kick on the light. Most codes take two consecutive fails to light up and 3 consecutive passes to turn the light off.

EGR runs many tests and it's actually a pretty smart thing based on it and the pcm. It'll know if it's valve isn't moving, isn't moving fully, seems stuck and if flow changes aren't detected when it's opened/closed. Downside is most people see a no flow when flow expected as an EGR and commonly it's a map sensor, that's how the system detects it.

Fuel level senders: this is common across almost all GM's of the 97-03 era. And before you believe that gallons used thing.. you may want to think about how it determines how much fuel is used. Does it rely on basic injector pulse data or level of sender?