My car runs OK (using Mugen ECU) but has been apart many times and I am unsure that the O2 sensors are plugged properly. There is conflicting information on the site regarding which sensor has male and which one has female connectors.

When I have to use the stock ECU to pass smog it complains the sensor is slow. When I ordered a new one that is supposedly upstream (Denso 234-4124) it has a male connector which is different than the way my car is currently plugged. Could both mine be plugged into the wrong receivers? Is this one reason my mileage sucks?

So for a definitive guide....

The upstream (in the header) sensor is what sex?
The downstream (in the cat) sensor is what sex?

 

Rear O2 sensor:

http://ecx.images-amazon.com/images/...L._SL1500_.jpg


Front air/fuel ratio sensor:

http://ecx.images-amazon.com/images/...L._SL1500_.jpg


Connectors, in situ:

https://i.imgur.com/SmULu81.jpg

 

Thanks for the great photo guide. It is clear mine have been plugged in backwards for some years.

 

You're welcome. Odd you never got a CEL error.

 

Most of the time I was running the Mugen ECU which does not do CELs. Every other year I passed smog by putting the stock ECU back and you could drive many miles before it showed a 133 'slow o2' error.

 

I was really surprised after I installed the new Denso sensor properly connected to the right places and found power and throttle response seemed to suffer (this is with the Mugen ECU). Today I said screw it, it was better the way it was and just swapped the sensors from the header and cat and I am back in business with performance restored. I don't know why my car is this way but I will leave it hooked up backwards and suffer the drop in mileage and the hassle when I need to get it smog tested.

 

Wrong sensors in wrong ports? Both look similar. 22mm hex heads. Maybe, the new ECU was programmed with sensor wiring reversed?

Just thinking out loud.

 

The Mugen ECU is from the days before there were any options and Mugen cut a special deal to reflash the stock unit for competition. It modifies timing, fuel, and a few things to increase torque. I doubt any port assignments were touched. The little documentation I have is in Japanese... I bought it used over ten years ago and am happy with it but ignorant as far as details.

I won't speculate why but can say it clearly runs best hooked up backwards from stock.

 

Yeah. As long as the car runs well. The plugs look good. Gas mileage is ok... Let sleeping dogs lie.