Seems like you know more than Toyota and the service manual they print. The connector for the air fuel sensors are a 4 pin and on the sensor between pins 1 & 2 you should have 1.8 to 3.4 ohms and between pins 1 & 4you should 10kohms or higher. If the results are not as specified replace . For the O2 sensors between pins 1 & 2 you should have 11 to 16 ohms and between 1 and 4 you should have 10kohms or higher.
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You need to read a little further into the Toyota service manual to find the DETAILED procedure for actually testing the output levels of the air/fuel and O2 sensors (Section ES, Engine Control System, SFI System, pages ES-23 and ES-24 in my manual).
If you measure the resistance of the heating element with an ohmmeter, and find it to be 1.8 - 3.4 ohms (air/fuel sensor) or 11-16 ohms (O2 sensor)
you are pretty well assured that the heating element is good and will function as designed.
If you measure the resistance of the zirconia sensing element and find it to be 10K ohms or greater, all you are verifying is that it is not grossly shorted.
The ohmmeter test will tell you NOTHING about the functionality of the sensor: output voltage levels, output voltage linearity, and response time.
These characteristics must be measured with the sensor at operating temperature, immersed in the exhaust gas stream, and with the circuit properly "loaded".
The sensing element is a high-impedance voltage source, and you are looking at a total range of output voltage that only swings from about 0.1V to 0.9V, which is why you need a measuring instrument that can accurately resolve millivolts (thousandths of a volt).
If you have a high quality digital multimeter with an input impedance of 10 megohms or higher, you can monitor the voltage output of the sensor while the engine is running, looking for a continuous variation in output as the fuel-feedback system adjusts the mixture to keep it as close as possible to 14.7:1.
Denso, the manufacturer of Toyota's sensors, recommends the following procedure to help assess the sensors functionality:
Check the voltage signal.
1. Using a 10-megaohm digital voltmeter, connect the voltmeter's red probe to the sensor’s signal wire and the black probe to engine ground. (use back probe method, DO NOT pierce the wire)
2. Start the engine and let it idle for about two minutes while watching the digital reading on the voltmeter. It should be fixed for a short period at around 0.1 or 0.2 volts.
3. After two or three minutes, the voltage reading will begin to fluctuate between 0.1 and 0.9 volts. If the sensor takes four minutes or more to begin fluctuating, replace it.
4. Record the lowest and highest voltage reading from the voltmeter in any one-minute period. The voltage should fluctuate constantly between 0.1 and 0.9 volts.
5.
Replace the oxygen sensor if the voltage:
a)
Goes above this range,
b)
Remains below 0.5 volts
c)
Stays fixed at a particular voltage
6. Open and close the engine throttle with a quick motion. The sensor’s output voltage should go up and down accordingly."
