Got a B16A2 from a '00 Civic Si in a '00 DX. It was purchased in a '96 DX, and the owner had jury-rigged an OBD2a harness to work on it. I've got it in the proper year chassis, with the proper P2T ECU, and I purchased a proper 99-00 Civic Si harness off eBay to get everything all square. https://www.ebay.com/itm/99-00-OEM-H...kAAOSwcF1bIdVx

The sole problem is the harness only has the upstream (primary) O2 connector. There is no connector for the downstream sensor. Everything I've read says there's supposed to be two, and it seems like a rather significant oversight for the reconditioning company to make. So what's going on? I haven't checked the pinout yet to see if the downstream wiring is missing, or had it scanned to see if it's throwing a cat code (the CEL is currently triggered by at least the upstream O2, I haven't replaced the sensor pigtail with a female connector yet to get it to plug in). While I work on that, I figured I'd toss this up here, since it can take a little while to get an actual response out of things like "just buy a GSR lol".

 

In the '00 Civic Si, the downstream O2 sensor is not part of the engine harness... it is a part of the chassis harness and is found along the center tunnel and passes through the tunnel to meet the exhaust system. Your '00 Civic DX has both sensors on the front of the engine and thus, the engine harness has both the primary and secondary O2 sensor plugs on it.

Since you didn't mention adding a cat under the car to your exhaust system, I am not sure how to instruct you to proceed. Please clarify your exhaust plan.

 

The plan was to plug-and-play now and fiddle later, but it looks like I'll be jumping right to it. I plan on deleting the cat (yeah, the exhaust has an Si cat where it belongs, and was plugged into the obd2a engine harness) and replacing it with a straight-through muffler like a Borla X1 to quiet things down. Based on what you're saying, it looks like my best bet is to wire in a downstream-O2 spoof or bypass straight out of the ECU plug. If you have that committed to memory and would like to save me the research, it would be appreciated.