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1996 Miata OBDII Emissions Fail

The short of the story is: OBDII emissions test fails. No Codes.

* Oxygen Sensor - Not Complete

* Oxygen Sensor Heater - Passed

* EGR System - Not Complete

* Catalyst - Not Complete

I need one more to complete (2 out of 4 must).

The Long story. First Emissions test failed, Shop told me to drive it for 60-80 miles and bring it back to be tested. Drove for around 400 miles, brought it back with the same result.

I have borrowed an OBDII testing cable from my uncle. I’m able to see the stats and whatnot, but I nor him have seen anything out of the ordinary. All sensors seem to be functioning. I have done drive cycle tests(to the best of my ability. http://got…rive-cycle) still no change in readiness of the Sensor tests.

Does anyone have any experience with this sort of problem? Care to share your wisdom?



My best guess currently is to check the fuses for the ECM. It appears as though the ECM is resetting it’s memory every time you turn the key, which will naturally reset the emissions monitors. The monitors require readings over a certain period of time and driving conditions to become ‘emissions-ready’. If the memory is constantly being reset, the monitors cannot keep enough data to become ‘emissions-ready’. If all the fuses look good, you may need a good mechanic diagnose the ECM for memory problems.

Thanks for the tip, Just checked the fuse, it’s fine. Also checked ECU power, It does have power with ignition off (found wiring diagram, it was the blue/red wire in the connector box nearest the passenger door).

Going to run by my Uncles house, he has some fancy scan tools. I’ll for sure post what we find out.