[DIY] Testing an O2 Sensor

[drexchan]

[DIY] Testing an O2 Sensor - and what it tells

Testing O2 sensors using simple tools

Those who are having high-FC problem might want to check the O2 sensor whether it's functioning properly in regulating the A/F, as well as emission control.

photos in http://drexchan.fotopic.net/c597354.html

 

[drexchan]

Let's start with some basic on how Oxygen Sensor works in the EFi system.

An O2 sensor generates a voltage output within the 0V to 1V range when fully warmed up to the operating teperature, 315°C and above. The voltage generated depends on the amount of oxygen in the exhaust stream and accurate reading can only be achieved when it's warmed up to the operating temperature.

The input from the O2 sensor is used by the ECU to keep the air/fuel ratio as stoiciometric (14.7) as possible. That means that for every gram of petrol the engine burns, it will need 14.7 grams of air, or 3.1g of oxygen.

This is commonly known as the O2 feedback system and when this system is operating it is said to be in "closed loop". When it is not operating, meaning the computer is not reading and responding to the oxygen sensor, it is said to be in "open loop".

All O2 sensors are vented to the atmosphere which contains approximately 21% oxygen. The exhaust of a ideally-tuned petrol-powered engine typically contains 2% oxygen, max.

Lean: If the exhaust has > 2% oxygen, it is lean (A/F ratio > 14.7). This produces a low voltage of < 0.3V.

Rich: If the exhaust has near 0% oxygen, it is rich (A/F ratio < 14.7). This produces a high voltage of > 0.6V.

The voltage signals are sent to the ECU and it reacts by adjusting the air/fuel ratio. The signal output is therefore fluctuating around 0.45V in a sinus pattern. Each cycle take less than 0.3 sec for a healthy, sensitive sensor.

In order to read the O2 sensor, most ECU send out a certain voltage to the output terminal of the sensor. This is typically around 0.45V. Since we know that the sensor sends low voltage (< 0.3V) when a lean condition is present and a high voltage (> 0.6V) when a rich condition is present, the computer can count the number of times the sensor crosses the 0.45V mark. Cross-counts are the number of times an O2 sensor crosses 0.45V. A scanner such as PDT2000 can read this.

The ECU can only use the sensor's output information under certain conditions.

A] the sensor must be hot to produce a normal signal (> 315°C).
This is why most sensors today have built-in heaters to counteract the cooling effects of prolonged idling and to achieve closed loop mode sooner during warm-up. Heating the sensor also keeps it cleaner and extends its life considerably. The heater usually gets voltage from a constant "key-on" source like the fuel pump relay or a fuse. For the Proton models (such as Wira with S-VDO ECU) are now using a 4-wire O2 sensor, from NTK, Japan. The four wires are:

1. O2 sensor output (black)
2. O2 sensor ground (gray)
3. 12V heater feed (white)
4. 12V heater ground (white).

B] The ECU is programmed not to go into closed loop operation until the coolant temperature sensor tells the computer the engine is warmed up. If the system tries to go into closed loop too early in the warm-up period, the leaning effect of the system would cause driveability problems and pollutants.

C] The ECU is also programmed to ignore the O2 sensor at near wide open throttle conditions. Maximum power requires maximum enrichment.

We can then conclude that O2 sensor feedback is used when the engine is warmed up, at Idle, and at part throttle (cruise) conditions.

 

[drexchan]

There are various testing methods including testing it during its operation (on the car, with the engine runing). Rich or lean conditions are induced and the voltage signal from the sensor is monitored using a High-Impedance Digital Voltmeter. This method is more accurate but require alot of work under the hood.

However, what will be discuss here is an easier way to test the sensor off-the-engine.

You need:
1. 22mm spaner
2. Digital Voltmeter
3. Cooking Gas Stove

Procedure:

1. Remove the sensor using the 22mm spaner (open end) when the extractor is cooled (if you have runned the engine before this). The sensor is located at the end of the extractor for Wira and Waja. For Gen.2, it can be easily reachable on the extractore head. TAke all precaution step if you want to jack-up the car, use jack-stands with parking brake engaged. Put a piece of brick or wood at the rear tire if neccessary.

2. Check the sensor, clean it if neccessary if eccessive carbon deposit is found on the sensor. Make sure that the 4 small holes around the casing, and the perforated tip of the sensor are free of any deposit. Do not use any solvent, petrol, water.. any liquid. Just rub with news paper. If you see eccessive carbon deposite, this is a good indication of having an extra rich burning in the engine.

3. Check the color of the wire. There are 4 wires in 3 colors: 2 white, 1 gray and 1 black. Identify the pin hole for each wire on the socket.

4. Jab the red (+ve) probe into the pin hole of the black wire, black (-ve) probe into the pin hole of the gray wire. Set the multimeter to DC20V. DO NOT measure the resistance as this will send a voltage to the sensor and might damage the electrode. See this.

5. Ignite the stove. Make sure the flame is in blue color for clean burning.

6. Put the tip of the sensor into the flame center and make sure that the flame fully covers the tip (see this). This is to heat up the sensor to the operating temperature at the same time preventing the sensor from contacting with the atmospheric oxygen. This is the reason of using a stove instead of connecting the heating element to a constant 12V (such as a small battery). It took me around 1 minute to start having a reading.

7. Let the sensor heats up until you see a max voltage reading above 0.9V. Now, the sensor has reached its operating temperature. I have got 0.95V, indicating that there's almost no oxygen in contact with the sensor. This is similar to a RICH condition in the engine where the A/F ratio is lower that 14.7, and all oxygen are used in the combustion process. If the reading doesn't reach at least 0.6V (> 0.9V if fully covered in the flame) after 1 minute, most likely cause is open circuit internally or lead fouling.

8. Continue to heat the sensor for 2 minutes. Observe whether there's any constant drop in the voltage. Sometimes, the internal connections will open up under heat. This is the same a loose wire and is a failure.

9. If it doesn't show any significant and constant drop in voltage. Remove the flame. This will expose the heated sensor to the atmosphere, similar to a very lean condition in the engine where high concentration of oxygen is being detected in the exhaust stream. The voltage should drop to below 0.1V within 4 secs upon the removal of the flame. (See this). If it doesn't or takes longer time to read below 0.1V, the sensor is most likely silicone fouled. Repeat step 7 and 9, skip 8.

10. If the voltage repeatedly reads > 0.9V when the sensor is fully covered in the flame, and drops within 4 secs to < 0.1V upon the removal of the flame, and pass the 2 minutes test. The sensor is good regardless of age.

11. However, good is subjective. The sensor must be responsive. Vary the size of the flame and see if the reading reacts as fast. One way of doing this is to try to get a flame size that give a reading around 0.45V. Gently fan the flame and observe the voltahe changes.

12. Cool down the sensor naturally in a save place, such as on the floor near a corner. DO NOT try to dip it into water to cool it down.

 

[drexchan]

What spoilt the sensor?

1. If it's carbon fouled, it's most likely that the engine is having a problem such as stuck open injector(s), faulty fuel pressure regulator (FPR), leaking exhaust manifold, etc.

2. If it's lead fouled, you must have added some kind of additive containing lead (Pd) into either the engine oil or the petrol.

3. If it's silicone fouled, you must have recently did something to the exhaust system, and while reassembling the parts, a silicone-containing sealant was used. Always look for the "O2-sensor-friendly" lable while purchasing such sealant. Check with your mechanic whether he's using such incompatible sealant. Insist not to have it used in your car and give him a lesson on "why".

 

[drexchan]

Please discuss

 

[jaywat]

Haven't read through the lost post...

Petrol price goin up again ...sigh.....

 

[GT20v]

nice write up mr drexchan...a really good job....i feel obliged to offer u a teh o ais for your effort.

we need more of this instead of talk kok in zth....
ive tried the same way like you....except cant get which wire to use..mine toyota with 4 wire .but 2 black and 2 grey..dunno which is for heating,O2 ecu input,ground etc....stupid toyota......sigh...

 

[delphinium]

What spoilt the sensor?

1. If it's carbon fouled, it's most likely that the engine is having a problem such as stuck open injector(s), faulty fuel pressure regulator (FPR), leaking exhaust manifold, etc.

2. If it's lead fouled, you must have added some kind of additive containing lead (Pd) into either the engine oil or the petrol.

3. If it's silicone fouled, you must have recently did something to the exhaust system, and while reassembling the parts, a silicone-containing sealant was used. Always look for the "O2-sensor-friendly" lable while purchasing sus sealant. Check with your mechanic whether he's using such incompatible sealant. Insist not to have it used in your car and give him a lesson on "why".

great write up!! Thumbs up. We need some tok kok but we do need this kinda stuff oso:)

My 02 sensor is 2 wire so I guess it's not that self-heating type one. Is 02 sensor different that much from a car make to another? Ie: can i use Honda's one on a let's say Peugot or Hyundai?

How do I detect wether the sensor is carbon/silicon/lead fouled?

I'm currently suffering high FC .. about 9km/liter. My ride is 1.8 Mivec.

 

[drexchan]

Yes, sensors are different from car to car.. some models from a certain manufacturer may be the same. But using a 2-wire (non-self-heating) in a 3-wire/4-wire (self-heating) design is impossible due to the socket design and vise versa. For 3-wire, the third wire is for the heating element's +ve, it grounds using the case through the extractor-engine-battery. For 4-wire sensors, the third and forth wires (white, in the case of Proton's NTK) are for the +ve or -ve (doesn't matter, non-polar) of the heating element.

"If the reading doesn't reach at least 0.6V (> 0.9V if fully covered in the flame) after 1 minute, most likely cause is open circuit internally or lead fouling."

"The voltage should drop to below 0.1V within 4 secs upon the removal of the flame. (See this). If it doesn't or takes longer time to read below 0.1V, the sensor is most likely silicone fouled."

Carbon fouling is easily observable with naked eyes, just like a bad spark plug, covered with black soot..

 

[drexchan]

nice write up mr drexchan...a really good job....i feel obliged to offer u a teh o ais for your effort.

we need more of this instead of talk kok in zth....
ive tried the same way like you....except cant get which wire to use..mine toyota with 4 wire .but 2 black and 2 grey..dunno which is for heating,O2 ecu input,ground etc....stupid toyota......sigh...

try the two black. polar doesn't matter. if jabbed wrongly, the worse you get is a negative reading. Then you should know what to do.. haha!!

 

[LilKeV84]

great article! :p

1 more way of finding out why my fuel consumption is so freaking high(delphinium: if u think urs high...mine is 7.6km/liter...sigh...)

but...does anyone know how much does a new oxygen sensor cost? mine is a Honda B16A, heard that there's 2 sensors on it...is that true?

 

[drexchan]

"is it true?"

Go find the truth! Enjoy your time looking for it. If it has two, one should be located before the cat-con, another one after.

Price.. no idea.. could be easily touching RM200. An universal NTK is being sold at USD60 - 80 per piece.

 

[drexchan]

IF THE SENSOR HAS BEEN IDENTIFIED AS CARBON-FOULED

Cleaning and Testing the Fuel Pressure Regulator

The FPR is located at the end of the fuel rail in an EFi engine. Some EFi cars don't have a FPR. The FPR reads the manifold pressure to regulate the opening of a valve in the FPR. IF the vacuum is high (such as idling, engine braking), the valve opens more and more fuel is returned to the tank, thus lower fuel pressure. If the vacuum is low (such as WOT or underload), the FPR opens less, thus less fuel is returned to the tank, and higher fuel pressure. This mechanism is not controlled by the ECU. However, if O2 sensors feedback shows rich burning at close-loop operation at any fuel pressure, the ECU will increase the pulse width of the injectors, thus injecting less fuel, at the same given pressure if rich burning is not present.

To clean and test the FPR. Take it out, fill it with petrol from the inlet port. Shake it hard, pour it out. You will be surprised to see thoudsands of tiny black particles of different sizes. Those are neoprene debris from the disintergrating fuel hose (the portion from the charcoal canister to the fuel rail). These debris, accumulating in the FPR valve, blocking the return path, making your car running on extra high fuel pressure and burning rich at open-loop operation. Producing eccessive unburnt HC in the exhaust stream, fouling the O2 sensor, reducing it sensitivity... and you know what happen next -- worsen FC and lost of drivability!

Clean it several times until you can`t observe any debris from the pour-out. Use a white tissue paper so that you can see the tiny particles clearer. I bet you need to do it at leas 10 times. It`s better if you have an ultrasonic bath, just give the FPR a bath for 20 minutes with the inlet port facing bottom, in petrol or ethanol, NOT WATER!.

After cleaning and completely drying the FPR (I use an electrical oven at 50 deg C, you may try a hair dryer), run a vacuum test on the air chamber. Connect a 60-cc shringe to the vacuum port. Fill the fuel chamber with some petrol. Pull the shringe all the way back and hold it for a minute. If there`s any liquid coming out of the air chamber, the diaphragm is cracked or leaking. Get a new one. IF it`s good at this stage, do the pressure test on the inlet port.

Similarly, using the same 60-cc shringe, connected to the inlet port with a proper fitting, pump the piston down. A 60-cc shringe when fully pump down, should generate about 40-50 psi of pressure. That`s about the WOT fuel pressure of many models. You may want to calibrate the shringe using a pressure gauge before this. Just connect the pressure gauge to the shringe and mark the shringe with the corresponding pressure for each 5cc compression. During the test, slowly pump it down and record the pressure required to open the valve. The compressed air will be vented out from the return port when the valve is opened. As a point of reference, WOT fuel pressure for Wira 1.5 EFi is about 42psi. If it requires > or < 5psi (considering the error in the pressure reading) to open the valve, it`s out. Get a new one.

Cleaning and Testing the Fuel Injectors

The above-mentioned debris will get into the injectors and accumulate in the valve too. You will either get (1 or more of) them stuck-open of stuck-close. This doesn`t mean that a stuck injector will not function, but, a stuck-open injector will result in rich burning, and vise versa for a stuck-close one. Sent the injectors for cleaning and testing. You can`t afford to have the instrument to do it, unlike those used in cleaning and testing the FPR.

By doing these two services, you will almost cartainly gain back the lost power and improve the FC. Hope these help.

note: I have personally done the FPR cleaning and testing (photos later). But I need to send my car for injectors cleaning in Autochild.

 

[drexchan]

I can sense that you guys are having digesting problem everytime I post something technical... :p

 

[CressTT]

just to add, you can test the o2 sensor while still on the car. Hook up a digi voltmeter, set it at 10 or 20 vdc and tap the positive (red) probe to the o2 sensor signal wire to your ecu. You need to find the pinout for your ECU. Did it on a evo2 ecu, one wire taps thru the signal from the 02 to ecu and the other just ground to vehicle body. Also did this same thing on a toyota 1ggte, the latter is easier since signal and gound can be tap at the diagnostic port.

If you're car in closed loop operation, voltage readings will fluctuate (1~4 times a second depending on refresh rate of your voltmeter) from 0.1 vdc to 1.2 vdc. Sometimes on extreme decceleration, it will jumps to a negative value for just a fraction of time. If boost is present, your ecu (well at least mine) will go out of close loop and starts to enrich the mixture and this will give a reading of about .7 to 1.2 vdc.