Bad fuel sets off EGR DTC?
 

Could poor octane fuel set off a trouble code for the EGR system? I have
a !997 C230. The car was having problems with detonation noises. I was
burning high octane Amoco and discounted poor fuel as the cause. The
knock would go away at lower engine temps. This car has an electric
fan and engine temps would only get high in stop and go traffic. At these higher temps the pinging occurred. Changing out the electric fan control module did not help. Eventually the check engine light came on with an
EGR system code. Aha I said - perhaps I wasn't getting the proper combustion damping from the EGR, causing a tendency to ping. So I've ordered a new EGR valve and egr solenoid assembly. While waiting I thought
to change my source of fuel (I always gas up at the same Amoco) and
surprise, surprise the pinging went away and then double surprise the
check engine light went out. So back to my question could bad fuel spoof
the diagnostic codes?

EGR helps reduce combustion temps!!! Imagine that reduced EGR flow was causing higher combustion temps....you need to repair (clean) the EGR system & see what happens?