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Old 02-17-2016, 12:38 PM   #1
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Default P0140 Secondary o2 sensor, inactivity

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:
https://www.miataturbo.net/general-m...9-p0140-49309/

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.
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Old 02-17-2016, 03:22 PM   #2
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EDIT: double post

Last edited by greeenteeee; 02-17-2016 at 06:08 PM.
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Old 02-17-2016, 03:22 PM   #3
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Just found this this morning, looks like I need to play with resistor values at the very least.

MX-5 Miata Forum - View Single Post - 02 simulator

Now I need to learn more about wiring resistors/parallel/series and how they affect the resistance ultimately. And I have tried before. =(

EDIT: 2016/02/21 replaced the 1M ohm resistor with 2x 1M ohm resistors in parallel, effectively 500k ohm resistance. So far no CEL, where as P0140 was sometimes recurring within 10 miles.

With 1M ohm resistor near sensor:



2x 1M ohm resistors in parallel:

Attached Thumbnails
P0140 Secondary o2 sensor, inactivity-24810148029_2b8fe7df8d_b.jpg   P0140 Secondary o2 sensor, inactivity-24810149079_f26b7e1608_b.jpg  

Last edited by greeenteeee; 02-22-2016 at 03:33 PM.
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