Testing Oxygen Sensor

[Z2TT]

Hello,

I have taken my oxygen sensor out of my 1g-gte. It's a 3 wire type. I have a multimeter and don't know which wire to test. There is 2 black wires and 1 blue wire. What are all these 3 wires used for and which one is the one that sends the voltage out to the ECU? Aren't some wires used to heat up the sensor on start up?

How should I go about testing this while it is out of the car, some websites on the internet say to heat it up with some kind of blowtorch or strong lighter.

Thanks.

[thechuckster]

you'd only use a blow-torch if it wasn't heated (e.g. 1 wire)

The colour coding suggests that the blue wire is the sensor output, the case is the sensor earth and the two black wires a the heater and that part is not polarity specific.

[Z2TT]

Ok I tried heating it up on my Stove.

I measured the voltage from the blue wire but it was 0. I heated it up heaps and still it is 0.
I am going to try measuring the voltage when I put it back into the exhaust manifold.

Is there any way to clean oxygen sensors. Many websites say use Petrol, where as other websites say cleaning wont help or will ruin them.

[thechuckster]

you'll never a reading above 0 until you remove most of the oxygen. It's a gas analyser not heat monitor.

source of CO2: get candle, get deep tin large a milo tin, put candle in tin and light it, put lid loosely back on tin, when candle dies you have an O2 poor environment, dunk heated sensor into atmosphere of tin and check readings.

You could also try a CO2 cannister from a soda siphon - using it to force a large amount of CO2 into a space with the sensor in the space.

btw: power up the heater side (andlet it warm up) when you do this and make sure you are measuring voltage between the flange/case and the blue wire.

[petergoudie]

Jaycar has a kit for about $14 that you can test the sensor with. When operating the O2 sensor's voltage output cycles from 0 to 1 volt (as I recall). The jaycar kit has 10 leds each for 0.1 volt (the kit is basically a voltmeter). The testing above might show if it works but you have to ensure it cycles correctly as well. When they cycle slowly they are on their way out. Do a google search for testing O2 sensors.

[Z2TT]

Yeah I have, most sites recommended heating it up with a blowtorch and use a voltmeter to see if the voltage fluctuates. I have to use the blowtorch to heat it up because its currently not in the car. Once I put it in the car I can test it.

So as far as I know the readings should rapidly fluctuate between 0.2v and 0.9v, is that correct?

[petergoudie]

I'm not sure of the voltage but it sounds OK. It will only cycle with the computer connected. It is supposed to cycle like the lights on the Knightrider car.

P.S. There are some really cheap generic O2 sensors on ebay. If your O2 sensor has more than 100k km it might be just as eay to buy a new sensor and forget the blowtorch.

[Z2TT]

Today I tested while the oxygen sensor was connected inside the manifold. I disconnected the plug and measured the output wire with a Volt Meter. After keeping the engine at 1500RPM for quite a while to get it hot, it just shows 0 Volts.

[petergoudie]

Found this on the internet::

Testing O2 sensors that are installed
The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop operation. Attach the positive lead of a high impedence DC voltmeter to the Oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Check the specs for your meter to find out. Set your meter to look for 1 volt DC. Many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire. When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check your connections. Next, check your leads to make sure you won't wrap up any wires in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes the sensor as hot and active once there are several cross counts. You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through, your sensor is good. If not, is it steady high (> 0.45) near 0.45 or steady low (< 0.45). If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. If the reading is steady low, add richness by partially closing the choke or adding some propane through the air intake. Be very careful if you work with any extra gasoline, you can easily be burned or have an explosion. If the voltage now rises above 0.7 to 0.9, and you can change it at will by changing the extra fuel, the O2 sensor is usually good. If the voltage is steady high, create a vacuum leak. Try pulling the PCV valve out of it's hose and letting air enter. You can also use the power brake vacuum supply hose. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good. If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor. If you are not getting a voltage and the car has been running rich lately, the sensor may be carbon fouled. It is sometimes possible to clean a sensor in the car. Do this by unplugging the sensor harness, warming up the engine, and creating a lean condition at about 2000 rpm for 1 or 2 minutes. Create a big enough vacuum leak so that the engine begins to slow down. The extra heat will clean it off if possible. If not, it was dead anyway, no loss. In either case, fix the cause of the rich mixture and retest. If you don't, the new sensor will fail.

Testing O2 sensors on the workbench.
Use a high impedence DC voltmeter as above. Clamp the sensor in a vice, or use a plier or vice-grip to hold it. Clamp your negative voltmeter lead to the case, and the positive to the output wire. Use a propane torch set to high and the inner blue flame tip to heat the fluted or perforated area of the sensor. You should see a DC voltage of at least 0.6 within 20 seconds. If not, most likely cause is open circuit internally or lead fouling. If OK so far, remove from flame. You should see a drop to under 0.1 volt within 4 seconds. If not likely silicone fouled. If still OK, heat for two full minutes and watch for drops in voltage. Sometimes, the internal connections will open up under heat. This is the same a loose wire and is a failure. If the sensor is OK at this point, and will switch from high to low quickly as you move the flame, the sensor is good. Bear in mind that good or bad is relative, with port fuel injection needing faster information than carbureted systems. ANY O2 sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, AND pass the two minute heat test is good regardless of age. When replacing a sensor, don't miss the opportunity to use the test above on the replacement. This will calibrate your evaluation skills and save you money in the future. There is almost always *no* benefit in replacing an oxygen sensor that will pass the test in the first line of this paragraph.

[Z2TT]

Thanks, I have come across this article before.
So far I have tried the sensor outside the car on the workbench and inside the car, twice. 0 Volts reading.

Why must the o2 sensor remain connected to the ECU when testing?
It will still show a voltage, but its just the Voltmeter reading it, not the ECU.

[thechuckster]

it doesn't need to be connected to the ECU when being tested.

I think the 7MGE (ex MA70) has fewer wires in it - will go outside and check

edit: 1 wire - e.g. narrowband, unheated

[Z2TT]

Thanks,

The thing I don't understand, if my o2 sensor is not functioning, why hasn't my CHECK ENGINE light ever come on??

[thechuckster]

ECU would only throw an error if there was an electrical fault (e.g. a short crcuit from O2 pin to earth) or if it got a spurious reading (e.g.12V instead of 1V) .

The sensor data is only used during cruise and warm idle - and if it's stuck at 0V, then the ECU is only going see that as lean.

Remember, all a narrowband sensor does is tell you if it's lean or rich, not by how much or what the current AFR is.