Testing an O2 Sensor on Vehicle

Testing an 02 sensor is an extremely easy bit for anyone vaguely familiar with EFI systems. This should (but sometimes doesn't) include most technicians. Especially at a dealership. And especially one asked to diagnose an emissions failure problem. Anyway, the car should be up to operating temperature. Exhaust gas temperature is not a real variable here. No dynomometer need be involved. All you need is a decent DVOM (digital volt-ohm meter). Simply probe the connector at the o2 sensor and read the voltage being produced. Rev the engine to around 2500 to get a good whoosh of gas through the pipes. It should occilate from .2v to .8v about 8 times every ten seconds. Thats it.

Although I know service advisors love dispensing technical guidance, no 'break in' period need be allowed with a new sensor. Once the new sensor is installed in the exhaust stream, it'll either produce the proper voltage or it won't.

Vince wrote:

Well I work on Toyotas sometimes at my part time job. Sometimes on some old Toyotas you will find a little black plastic box, when you open the cover you should see a diagram of the wire terminals in the black box. If you have a digital volt ohm meter you can set the meter to dc volts and measure Ox with respect to ground, usually E1 which is Earth 1 (British Earth is American Ground). Sometimes the voltage will fluctuate so fast you look at the little bar graph if your meter has one. Sometimes the box has a bad connection so you will have to probe the oxygen sensor connector directly. One wire oxygen sensors always have that wire carrying the o2 sensor signal to the computer and signal ground must be through sensor body. 2 wire sensors are usally o2 sensor signal and ground to computer. 3 wire is probably 02 sensor signal and ground through sensor body with 2 other wires heater supply volts (usually 12V) and heater ground. 4wire is probably 02 signal, 02 signal ground, heater supply volts, and heater ground. So you must probe the right wire with your voltmeter. The old 1 wire sensors without heaters needed a long time to heat up (5 minutes and up) and sometimes they would cool down and not work during engine idle even after they warmed up. But the heated oxygen sensors probably work within a minute or so after starting and they didn't cool down during idle. The oxygen sensor needs to hit around

600 degrees fahrenheit before they work. With the old unheated sensors the computer would let a timer run out and look at the coolant temperature sensor to determine whether to go into closed loop (listen to and believe oxygen sensor signal). Since the oxygen sensor signal is stuck at 0.68 which for a normal signal is rich. What causes the sensor signal to fluctuate is the back and forth dance of the computer and oxygen sensor. The sensor sends a lean signal (under about 0.45) and the computer richens mixture till the oxygen sensor sends a rich signal (over about 0.45) and the computer leans mixture, and does it over and over again. Sometimes a computer doesn't want to go into closed loop because of defective inputs from other sensors like the coolant temperature sensor or mass air flow sensor. Most time the Toyota manuals say to run the engine in park at about 2000 rpm for 5 minutes and the oxygen sensors should be hot enough to work. You shouldn't need a dynamometer, but even if you did just run on street for 5 minutes or so. The oxygen sensor shouldn't need a break in value but the computer to store long term block values to use the oxygen sensor most efficiently may need a few days break in.

I noticed yesterday that the vehicle's clock indicated approximately 2 hours and 15 minutes behind the actual local time. This indicates that the vehicle's electrical supply system (battery neg I would think) was disabled for that accumulated amount of time.

I then asked myself why the electrical supply system had been disabled? The only reason that I can think of myself is that resistance measurements to evaluate the O2 sensor circuit were taken.

Any other reason than that one ?

The service department probably disconnected the battery to clear the ECU's memory and turn off the MIL. I doubt if it was disconnected for that long. More likely, the clock reset at 12:00 when they reconnected the battery. If the shop did not write down your radio station presets, they are probably gone as well.

No, it indicates the battery was disconnected only. The time discrepancy is because the mechanic didn't set the time, and they automatically start at either 12:00 or 1:00. A little math working backwards should correspond to the time they were working on it.

There are many procedures where they tell you to disconnect the battery even though the odds of something bad happening are vanishingly slim - Sorry, but no big conspiracy here.

(And if a mechanic is working on a customer's car with the battery still connected when it shouldn't be, and something DOES go very wrong, he's going to be in a big steaming pile of trouble.)

Whenever you are working near the starter motor or other components with high current supplies or unshielded power terminals, or on computer system leads where you could accidentally fry something very expensive, the FSM calls for a battery disconnect.

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