CRX VX w/HF Transmission but P28 ECU
 

So I finally got my car back from the shop with my d15z1 and hf transmission installed. Problem is that the so called Honda Pro told me that the car would not run right if he had installed the 5 wire 02 sensor and A00 ecu that I provided to him. On top of that, he didn't connect my EGR valve. There's no CEL at the moment but he told me that if he does hook up the A00 Ecu that it will be throwing a CEL code regarding the 02 sensor. So, my question is, if I hook up the A00 ecu to replace the P28 ecu without hooking up the 02 sensor, will the car still run well and obtain better MPG? At the moment, through two tanks of gas through Vegas I have averaged out 33mpg. I don't really care about the CEL or emissions just better MPG.

Sounds like this so-called 'Honda Pro' either had no idea how to connect the P07 or just didn't want to bother with it (since it IS more difficult to wire up than a P28). After all, using the P07 means connecting the EGR valve, EGR control box, and five wire oxygen sensor in addition to converting your car to OBDI wiring. The good news is that if you are not running a check engine light, you probably won't in the future. Not onnecting the EGR is not a problem - the P28 does not use one, so it isn't looking for one. BUT, your mileage will certainly be lower than it would with a properly set up P07 ECU. With this said, the best thing to do would be to install the P07 and connect the EGR and five wire oxygen sensor. BUT, if you just slap in the P07 as-is, your car would run in limp mode. This not only means check engine lights and poor emissions. It also means WORSE fuel economy than you would get running the P28. So my recommendation is to do things right. If this 'Pro' is not willing to do it for you, find someone else. Or do it yourself.

Okay so I decided to do it myself from your consideration reply. The vx engine did not originally come with the original 4 wire o2 sensor, but I figured from https://www.gassavers.org/showthread.php?t=4389&page=2 the following link it should be a pretty straightforward 7 wires from ecu to 5 wire o2 sensor and ground wire. Correct me if I'm wrong.

I had this setup with a P07 ECU and My MPG INCREASED when I switched to a DX trans. I also thought the D15Z1 would have enough torque to push the gearing of the HF but it does better with a DX trans by about 6 MPG.