The EGR status is monitored by the ECM, that's why the following fault codes will be logged in case of the correspondent failure(extract from http://cdbl.free.fr/Nanocom/V1-V2/Manuels/Anglais/Nanocom%20user%20manual%20V1.2.pdf
APPENDIX TABLE 1: COMPLETE FAULTS CODE LIST , page 44-45):
1-6 EGR inlet throttle short circuit (current)
1-7 EGR vacuum modulator short circuit (current)
7-6 EGR inlet throttle short circuit (logged)
7-7 EGR vacuum modulator short circuit (logged)
9-6 EGR inlett throttle open load (logged)
9-7 EGR vacuum modulator open load (logged)
20-4 EGR valve stuck open (logged)
20-5 EGR valve stuck closed (logged)
24-4 EGR valve stuck open (current)
24-5 EGR valve stuck closed (current)
IMO the ECM will "know" about short or open circuits as for any other monitored circuit and throw the highlighted codes based on ECT, MAF and IAT readings which must be in conjunction with the EGR modulation percentual status.
on mine, with the original ECM(now i have a remapped one) when i first only unplugged and clogged the vacuum pipe on the EGR valve for test i've got the "EGR valve stuck closed" code without touching the modulator that's why i ended up at that conclusion
Discovery Td5 (2000), manual, tuned
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