Yes the heater circuit is separate from the signal circuit, if the ECU does not see a rise in amperage over a period of tome it will fail the monitor.

 

Older vehicles did not have oxygen sensor heater circuits and the engine speed had to keep the oxygen sensor hot (over 600 degrees F) or it would stop working.

So to keep emissions down while the engine was getting warm , they incorporated a heater circuit to take the place of requiring high RPM.
The PCM monitors this and if it doesn't see the heater circuit ,as mentioned, being active at certain times, it allows the sensor to cool down and increase emissions.
Are you going to do the checking yourself or have a shop check it out?

 

so this is what the connectors look like. It's hard to get my multimeter probes in there but I can manage. Not sure which are the two pins between which I should be measuring resistance

 

How about you start with checking for power coming to the oxygen sensor to power the heating element?

If you turn the key on one of those terminals should lite a test light or read 12 volts.
I suspect , you may find none.That could be why the code for heater circuit is coming up.
Let us know what you find and we can go from there to continue checking as you wanted to check the heating element.


Are the wire colors on your vehicle the same as the ones shown in the table.

 

Thanks, I just did that. Found 12v on both sensor connectors. I also measured the resistance between pins 1 and 5 on the sensor side. 92 and 101 ohms respectively. All seems reasonable, I'm ready to give up and take it to the shop for them to figure out. Never expected passing a smog check to be this hard!

 

Which 2 wires did you get 12 volts with the key on, colors also please?
Something does not seem right there.
I hope you were using a digital meter to measure because any meter with over certain power will destroy the sensor.

 

With a digital multimeter, of course. Between wires 1 and 2. But after I plugged them back in I'm getting codes for the two sensors even though they seem to be functioning fine from the OBD tool. Also other tests that were ready (e.g. EVAP) got reset to incomplete. The codes have not gone away despite 30 miles of driving.

 

What codes are you getting?

 

p0135 and p0155

 

P0135 Code: Heated O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1) - In The Garage with CarParts.com
P0155 Code: Oxygen Sensor Heater Circuit Malfunction (Bank 2, Sensor 1) - In The Garage with CarParts.com

You have heater codes, for both banks, upstream sensors (closest to engine)
With both banks reporting error you may have a fuse blown or a wiring issue..
Doubt both sensors have quit at the same time..