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I have an '89 325is . Today I went in for an emissions test. It failed on the idle test only. The check engine light would come on intermittently and the code from the stomp test was 1222. I replaced the O2 sensor and cleared the code with the Peake R5 code reader. Took it out for a 20 minute drive and everything was great. I parked it and about 2 hours later, took it for a trip to Wally World. On my way, the light came on and stayed on. When I got to Walmart, I did a stomp test and the code read 1264, which is the EGO Heater (O2 Sensor Heater). The relay is fine and the O2 sensor is brand new. I have the Miller MAF conversion done and it's been on a few months and running good. The check engine light would come on if I was cruising in between 1.5K and 2K RPM's sometimes, after the conversion was done. Also I have poor fuel economy on the highway. I used to average around 27 MPG on a long trip doing about 80 the whole way. Now I can only average about 20 MPG. Anyway, my R5 code reader is now not working when I plug it into the OBD port. I'm not sure if it's the conversion, wiring, or if the ECU is crapping out and causing all of this. Any help would be greatly appreciated.

The MAF conversion has no effect on the OBD port. Have you tried going through the trouble shooting manual for the MAF? At the bottom of any page on our website you will see a section called support. Under that you will see MAF Manuals. There, you will find the Trouble Shooting guide.

MPG should not change unless your O2 sensor goes bad, which you seemed to have sorted out, or the MAF was installed improperly, or the MAF has failed. Running through the Trouble shooting guide will help determine if the later two possibilities are a concern.

Also, check of intake leaks. The AFM is good at masking vacuum leaks and because the MAF is so much more precise, it is less forgiving.