I know most here use MS/aftermarket ECUs so members using stock ECU and potentially dealing with this issue may be slim but the knowledge base here is better than other places I can ask.
My NB1 was getting P0421 for a long while. I have an NB2 manifold ready to go on. I have the mechanical o2 sensors from an older project that I can try but I've read it is hit or miss.
I tried the 1uF capacitor and 1M ohm resistor on the secondary o2 sensor based on a few diagrams. IIRC I placed the 1M ohm resistor AFTER the capacitor (capacitor is closest to sensor itself) since it is trying to limit the signal FROM the sensor rather than that from the ECU.
Prior to installing the cap and resistor, secondary o2 readings would reach up to 0.8V and stay there longer than the primary o2 voltages would which definitely says the P0421 is valid, but after the cap and resistor, the voltages (via ScanGauge) do not lolly-gag above 0.5V anymore and the inversions are more regulated as they should. However, it is still throwing P0140 (which FSM states is from 100 seconds sitting with unchanged voltages).
Secondary o2 reads 16-17 ohm resistance which is inline with my working front o2 (DENSO) and another BOSCH I purchased recently in case an o2 was bad.
Does anyone know if the resistor is supposed to be CLOSER to the sensor instead?
I've referenced these threads:
Rear o2 resistor trick p0139/p0140
Intermittent P0140 code - MX-5 Miata Forum
P0037 and P0140 confirmation
In trying to find the other diagrams people have posted, I found this which does not include a capacitor. I can't find the original threads/images I referenced anymore.
Any help would be appreciated.