2008 Gen 2 O2 Measurement Mystery

2008 Prius (172,000 kms) periodically triggered PO420, no exhaust leaks, good mileage, does consume some (synthetic) oil. PO420 became more frequent (every time I drove the car) so I changed the cat pair + O2 sensor (although ‘old’ O2 sensor had only 35,000 kms, and never provoked any O2 codes). Considered changing A/F sensor, but that’s WAY too difficult. As soon as cat was changed, car started triggering PO136 & PO139 (no other codes, good mileage, no detectable exhaust leaks). Dang!

Scanner reads voltage on new O2 sensor as 0.8V (within spec. according to shop manual). Resistance measured on ‘new’ O2 sensor (cold, Pins 1 & 2) is 13 ohms, compared to ‘old’ O2 sensor’s 16 ohms (both within spec according to shop manual). But… I cannot get any resistance across Pins 1 & 4 (the ‘heater’ ?) on either the ‘new’ or ‘old’ O2 sensor (cold): should read 10k ohms or higher according to shop manual: two different DVDMs read both as an open circuit. Unusual for both ‘new’ & ‘old’ O2 sensors to be bad… given ‘old’ sensor never triggered O2 codes before the cat was changed.

Does anyone have any insight as to what’s happening here, or what I’m not seeing or doing wrong?

Thanks!

 

Aftermarket. When the original Bank 1 Sensor 2 went south (approx. 35,000 kms ago) I bought a pair thinking I'd replace both O2s in the bank. But unlike other cars I've had, I discovered that Bank 1 Sensor 1 is not an O2 sensor, but an A/F sensor. So when I changed the cat, I put-in the new sensor I'd never used. What puzzles me is the 'old' O2 sensor (that never triggered a DTC code) I pulled when I changed the cat has no resistance across pin 1 & 4... just like the 'new' sensor. I have to assume both can't be 'bad' and I'm doing something wrong.
Q: Does the 02 sensor have to be at operating temperature to measure the resistance of its heater? Nothing about this in the manual, except that the sensor circuit (pin 1 & 2) has to be measured at 20 degrees F. Also, a PO136 is supposed to be a voltage issue (reads OK at 0.8V), and PO139 is supposed to be a problem with the sensor's response time. I'm flummoxed ;-)
Thanks!

 

Thanks for the pin info. Scanner says I'm getting 0.8V (hot) from the sensor, and voltmeter reads 9.0 ohms (cold) across Pins 1&2 (heater) which is also within spec. according to the shop manual. But I'm not getting any resistance across Pins 1&4 (on either sensor) - which the shop manual says should be 10K ohms.
You mentioned Pins 3&4 is the sensing element, but I can't find info on any mention or measurement for those two pins in the manual.
Q1: Do you know what voltage I should be getting across Pins 3&4?
Q2: Does Toyota make their own 02 sensors, or do they use Denso?
I think I got the pair of 02 sensors I have at RockAuto... just checked... RockAuto doesn't have Toyota or Denso O2 or A/F sensors, so the two I have aren't OEM.
Q3: What brand(s) would be a qualified OEM replacement?
Otherwise, I'll have to bite-the-bullet and go to (gasp!) the dealer ;-)
Thanks again for the info - much appreciated!

 

OK, I'll look for a Denso or NTK O2 sensor... may have to go to the dealer. Thanks for the advice!
BTW: I get 800 mv on my scan tool (warm, running), and found the resistance measurement info for the O2 sensor on page EC-51 in the Repair Manual for Gen 2 Prius