The ECU detects that EGR flow is outside expected limits — either present when not commanded or absent when commanded. This is a general EGR system performance fault.
Common causes
EGR valve and passages clogged with carbon deposits (most common on high-mileage diesel and petrol engines)
Stuck EGR valve (open or closed)
Faulty DPFE or EGR differential pressure/position sensor
Vacuum leak on pneumatic EGR systems
Cracked EGR pipe
Symptoms
MIL illuminated
Detonation or knock at light throttle
Rough idle
High NOx at emissions test
Possible slight power loss
Severity
Moderate — primarily an emissions fault, but prolonged detonation can cause engine damage.
What to do
Inspect and clean EGR valve and passages
Test EGR valve operation with scan tool (command open and closed)