I know this is probably stupid simple but can anyone tell me what wire to splice into so that when I install my wideband o2sensor I wont throw an engine code? I have found lots of support on where to splice for a NA mx5 but not a clear answer for a NB mx5.
1999 Mx5 running stock ecu. Thank you for the support!

 

The 0-5V wire is AEM's the output wire. You select the output from the gauge. You're going to be splicing into the signal wire from the front O2 sensor.

Regardless, I've never gotten the AEM narrowband output to work with the stock ECU. Also your stock ECU will throw a code because if you're replacing the O2 sensor entirely, the ECU will no longer see the heater.

http://www.mellens.net/mazda/mazda_m...999_wiring.pdf

Looks to be the blue wire coming off the stock O2 sensor.

 

i was under the impression that if you splice into the signal wire for the stock o2 sensor you wont throw an engine code. is there any way to wire into the heater? Pretty sure the o2 sensor they send with the gauge has a heating element too.

 

That's how it should work, but personally my NB1 ECU threw a code when running my AEM in narrowband mode. YMMV
Also you can only splice into the existing wiring if you remove your stock front O2 sensor. Otherwise the AEM controller and stock sensor will be fighting.

The heater used in the wideband is controlled by the AEM controller. I'm unsure/wouldn't advise if you tried cutting the AEM's sensor harness to wire it into the OEM ECU's heater circuit.
You could just cut the signal wire from the front O2 sensor, and keep it hanging somewhere in the engine bay. That should prevent the ECU from throwing an O2 heater code.
You can also use a resistor to fake the OEM O2 sensor's heater. You'd have to measure the resistance that the stock sensor has between its heater+ and heater ground pins. Then calculate whatever wattage that should be using your car's running voltage. ( V^2/R). Then you could select a resistor of the appropriate resistance and wattage.

I was in a similar situation, and I just took my car to exhaust shops and asked how much they'd want to weld in another O2 sensor bung in the manifold. Some wanted like 100$, others wanted 40$. The AEM kit should have come with a mild steel bung. The OEM ECU won't benefit that much from having a newer style sensor talking to it.
Is there a reason why you want to monitor AFRs? Are you using a piggy back ECU? Just curious what the stock ECU uses for AFRs?