The ONLY sensor which isn't cross compatible is the oil pressure sensor. If it's a California car (originally sold in Cali) the ECU will also be expecting signal from a sensor in the EGR cooler which other MKIIIs don't have; As you don't have an error code for that sensor I assume that to not be a Cali vehicle or the swap was more complete.
Does your oil pressure gauge work?
How does he know it's JDM? It's not written anywhere and aside from the oil pressure sender and the way the coolant is routed there are no other differences.
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