st185 downstream 02?

i have a 90 alltrac just swapped it with jdm 3s runs great but occasionally loads up and fouls plugs gotta pull and clean them to get it to start. the oem 3sgte that was in there had pigtail for another o2 sensor. i have a down pipe on it and i have that hole plugged right now. is the jdm computer looking for a downstream? im using usdm harness of jdm 3sgte from same year

The gen II 3SGTE never had a downstream O2 either US or Japan.
Japan did have an EGT sensor after the cat to monitor catalyst efficiency but that shouldn't cause fouled plugs if it's not there. It's a 1 wire connector.
A picture of the connector would help in identifying it.

You might want to make sure the cold start injector isn't leaking.
Also make sure your O2 signal wire isn't damaged.
You're running a JDM ECU? Is your O2 a single wire [JDM] or 4 wire [USDM]?

cold start not leaking deff. its a single wire on a jdm ecu my next step is to check continuity from ecu to sensor make sure its pinned right cuz im using a usdm harness