0420 questions

[bgambino]

now I thought I always read on this post that the first thing to attack (assuming there are no exhaust leaks) is the "front" o2 sensor

 

I was PMing another member on this board about the double spark plug anti-fouler he used to solve his problem...ends up he used it on the rear 02 sensor.

I assume it should work the same on the front

 

Someone just correct me if I have this wrong

1---if you are running with a bad front 02 sensor, it's a fact the performance and/or gas mileage will be affected..?(what I don't know is whether the 02 sensor actually tells the computer to trim air/fuel mixtures etc)

 

2--if I use the anti-fouler on the front, and the light goes away, the car should run exactly as if there was a brand new o2 sensor in place....?

(now, IF the 02 sensor DOES indeed tell the computer to make certain adjustments......it would seem obvious that using the anti-fouler will prevent normal function...to what detriment)

 

I want to avoid a $110.00 OEM sensor in lieu of a $5 fix

 

Also...I have been soaking that sucker with PB blaster for a few days...put a 22mm open end on it and nothing....any tricks here?...i really hate to drop the Y pipe...i'll probably have to end up re-tapping some of the threads....does not look like space for a socket.

[johnceggleston]

dis connect the wire and use a closed end wrench and tap on it. make sure you are going the right direction. some folks like to warm up the exhaust first.

[OB99W]

The ECU needs data from the front (upstream) O2 sensor in order to properly set the A/F ratio, and the sensor has to respond rapidly to changes in the exhaust. Lessening the sensitivity of that sensor will not only result in poorer performance/mileage -- it will also increase emissions (the catalytic converter will be less effective) and make the generation of code P0420 more likely.

 

The rear (downstream) O2 sensor data is used by the ECU to determine if the catalytic converter is working well. Under normal operation that sensor responds more slowly and to a smaller degree than the front one. Reducing its exposure to the exhaust stream tends to reduce triggering of P0420.

 

Although a ''lazy'' front O2 sensor is a common cause of P0420, it's hardly the only one. By using anti-foulers on the rear sensor, you might stop the P0420, but to the detriment of engine operation. If a bad front sensor is the problem, it's even possible that could reduce gas mileage enough so that eventually a new one would pay for itself in lower gas costs.

 

Here are a couple of good links to explanation of O2 sensor operation:

http://www.aa1car.com/library/o2sensor.htm

http://www.autoshop101.com/forms/h37.pdf (slightly Toyota-biased)

 

Some info on P0420 -- the second link gives several possible causes of the code:

http://www.motor.com/magazine/pdfs/102006_09.pdf

http://www.catalyticconverter.org/news/news_page.cfm?Key=catalytic_converter-&News=120

[bgambino]

The ECU needs data from the front (upstream) O2 sensor in order to properly set the A/F ratio, and the sensor has to respond rapidly to changes in the exhaust. Lessening the sensitivity of that sensor will not only result in poorer performance/mileage -- it will also increase emissions (the catalytic converter will be less effective) and make the generation of code P0420 more likely.

 

The rear (downstream) O2 sensor data is used by the ECU to determine if the catalytic converter is working well. Under normal operation that sensor responds more slowly and to a smaller degree than the front one. Reducing its exposure to the exhaust stream tends to reduce triggering of P0420.

 

Although a ''lazy'' front O2 sensor is a common cause of P0420, it's hardly the only one. By using anti-foulers on the rear sensor, you might stop the P0420, but to the detriment of engine operation. If a bad front sensor is the problem, it's even possible that could reduce gas mileage enough so that eventually a new one would pay for itself in lower gas costs.

 

Here are a couple of good links to explanation of O2 sensor operation:

http://www.aa1car.com/library/o2sensor.htm

http://www.autoshop101.com/forms/h37.pdf (slightly Toyota-biased)

 

Some info on P0420 -- the second link gives several possible causes of the code:

http://www.motor.com/magazine/pdfs/102006_09.pdf

http://www.catalyticconverter.org/news/news_page.cfm?Key=catalytic_converter-&News=120

 

 

OB99W...I was hoping you would weigh in..

altho I dislike your answer...LOL:banghead:

$100.36 for a front 02 sensor ...OEM mail ordered from 1st sub + shipping....yuck...and no guarantee that it is it...sheesh

[Log1call]

Strictly speaking the front sensor should post it's own code if it is faulty, but as has been said, it does influence the rear sensors operation to a degree.

 

Since the front sensor isn't posting a code, and because the rear sensor is checking that the O2 levels after the cat are more stable, I'd suspect the cat or the rear sensor.

 

If you could get live readings with a subaru scan tool or a laptop you could know exactly hat is happening.

 

If you can't get a scan, then take care of all the simple things first, check for exhaust leaks, consider the ambient temperatures, check both O2 sensors with an electrical gauge.

 

Definatly do not put the modification on the front sensor.

[bgambino]

Strictly speaking the front sensor should post it's own code if it is faulty, but as has been said, it does influence the rear sensors operation to a degree.

 

Since the front sensor isn't posting a code, and because the rear sensor is checking that the O2 levels after the cat are more stable, I'd suspect the cat or the rear sensor.

 

If you could get live readings with a subaru scan tool or a laptop you could know exactly hat is happening.

 

If you can't get a scan, then take care of all the simple things first, check for exhaust leaks, consider the ambient temperatures, check both O2 sensors with an electrical gauge.

 

Definatly do not put the modification on the front sensor.

 

 

Log1...what do you mean the front sensor should post its own code? WHere? How?

 

Is there a way to check an oxygen sensor (electrically?)

 

thanks

[Log1call]

If the O2 sensor can't keep the mixture within parameters, which is why most O2 codes are set, or if it gets an electrical fault like a short or open circuit, it will set a trouble code just like all the other codes.

 

You can test an O2 sensor with a multimeter. At idle if you leave them connected to the car and connect a sensitive multimeter to them, they should cycle backwards and forwards around the point four five of a volt range. As the motor goes a bit lean the readings will drop and that causes the ecu to richen the mixture which then send the voltage up which tells the ecu to make the motor leaner again. If everything is good the swing will be several times a minute and will not vary too far from four point five volts.

 

Some second O2 sensors use a five volt system. The procedure and operation will be the same though.

 

Get the manual for your car and have a read if you want to work on it yourself, otherwise take it in to be put on a scanner or have the voltages read manualy.

 

There is no need to throw parts at these cars on the basis of a guess.

[OB99W]

Is there a way to check an oxygen sensor (electrically?)

As in the first link I posted above, http://www.aa1car.com/library/o2sensor.htm -- see the section ''OXYGEN SENSOR DIAGNOSIS''.

Edited September 17, 2009 by OB99W
fixed link

[OB99W]

If the O2 sensor can't keep the mixture within parameters, which is why most O2 codes are set, or if it gets an electrical fault like a short or open circuit, it will set a trouble code just like all the other codes.

While bgambino's problem may or may not be O2 sensor related, it's common enough for a marginally faulty front O2 sensor to not trigger a code for itself while still causing trouble.

 

 

You can test an O2 sensor with a multimeter. At idle if you leave them connected to the car and connect a sensitive multimeter to them, they should cycle backwards and forwards around the point four five of a volt range. As the motor goes a bit lean the readings will drop and that causes the ecu to richen the mixture which then send the voltage up which tells the ecu to make the motor leaner again.

A multimeter can provide a rough idea what the O2 sensor is doing, and a sufficiently defective sensor might become obvious. However, a marginal one probably won't be easily detected that way. More-sophisticated equipment is usually needed for a reliable diagnosis.

 

 

If everything is good the swing will be several times a minute and will not vary too far from four point five volts.

Actually, the voltage swing at the front O2 sensor should be much more rapid than that, which is one reason why a multimeter isn't that effective a tool for testing it. Just to clarify things, the voltage around which the swing should occur is about 0.45 volts.

 

 

There is no need to throw parts at these cars on the basis of a guess.

Yes, that's an ideal I can second. Unfortunately, some people don't have the knowledge or equipment to do a proper diagnosis (and some are apparently unwilling/unable to obtain them). Also, sufficient experience with certain symptoms may lead to the conclusion that a particular part is likely causing the problem.

[Mikevan10]

Interesting discussion, fellows. Wish I could add something that might be helpful to Gambino, but I cannot. But I am curious about how an O2 sensor would "know" to trigger a code for itself (other than a short or open circuit). Wouldn't another sensor need to detect that the O2 sensor was not keeping things within appropriate parameters? And, if so, what other sensor would that be???

 

Mike V.

[98sub2500leg]

The ECU needs data from the front (upstream) O2 sensor in order to properly set the A/F ratio, and the sensor has to respond rapidly to changes in the exhaust. Lessening the sensitivity of that sensor will not only result in poorer performance/mileage -- it will also increase emissions (the catalytic converter will be less effective) and make the generation of code P0420 more likely.

 

The rear (downstream) O2 sensor data is used by the ECU to determine if the catalytic converter is working well. Under normal operation that sensor responds more slowly and to a smaller degree than the front one. Reducing its exposure to the exhaust stream tends to reduce triggering of P0420.

 

Although a ''lazy'' front O2 sensor is a common cause of P0420, it's hardly the only one. By using anti-foulers on the rear sensor, you might stop the P0420, but to the detriment of engine operation. If a bad front sensor is the problem, it's even possible that could reduce gas mileage enough so that eventually a new one would pay for itself in lower gas costs.

 

Here are a couple of good links to explanation of O2 sensor operation:

http://www.aa1car.com/library/o2sensor.htm

http://www.autoshop101.com/forms/h37.pdf (slightly Toyota-biased)

 

Some info on P0420 -- the second link gives several possible causes of the code:

http://www.motor.com/magazine/pdfs/102006_09.pdf

http://www.catalyticconverter.org/news/news_page.cfm?Key=catalytic_converter-&News=120

 

While bgambino's problem may or may not be O2 sensor related, it's common enough for a marginally faulty front O2 sensor to not trigger a code for itself while still causing trouble.

 

 

 

A multimeter can provide a rough idea what the O2 sensor is doing, and a sufficiently defective sensor might become obvious. However, a marginal one probably won't be easily detected that way. More-sophisticated equipment is usually needed for a reliable diagnosis.

 

 

 

Actually, the voltage swing at the front O2 sensor should be much more rapid than that, which is one reason why a multimeter isn't that effective a tool for testing it. Just to clarify things, the voltage around which the swing should occur is about 0.45 volts.

 

 

 

Yes, that's an ideal I can second. Unfortunately, some people don't have the knowledge or equipment to do a proper diagnosis (and some are apparently unwilling/unable to obtain them). Also, sufficient experience with certain symptoms may lead to the conclusion that a particular part is likely causing the problem.

 

I am also getting poor milage (about 20-21mpg). My machinist told me that even though the O2 sensor will work after you had a HG problem when you previously had a coolant leak into the engine. He said it may or may not throw a code, but will still work and be at the high end of the tolerance scale of the sensor. These sensors usually get plugged up with coolant in the exhaust after HG, or just high mileage. However, I believe if possible,the part should be tested properly before being replaced.

There must be a tolerance for testing the resistance rating with a multimeter, but I would think to test it properly you it you need to take a reading while the engine is running. However, if you disconnect the sensor to test it wouldn't the mixture will be thrown off to the ECU thus giving an erroneous reading? I would think the expensive diagnostic equip. would plug into the ECU side and give it the voltage it is looking for so the sensor could be tested while the engine is running open loop. I can't think of any other reliable way it could be tested.

OB, is this the case?

He also recommended heating up the pipe before removing to make it easier if replacing it.

[Log1call]

Ah yes, the voltage is indeed meant to hover around point four five, not four point five volts. That was a slip of the tounge.

 

The idea that it should swing several times per minute was the minimum to expect. They should indeed swing much faster and not very far if everythiing is working correctly. An analouge multimeter can detect the fast swing far better than a digital one, unless you happen to have a digital gauge with an oscilloscope function, unfortunatly not many people have either an analouge or the oscilloscope, so a digital will at least check that the change is occuring.

 

The electrical check is, in my opinion, the better test of the O2 sensor because the figures we get out of the ecu are calculations, which can be faulty. Then again, they are the figures the ecu is controlling the motor with, so, for testing the O2 sensor I think the electrical test is better, but to test the O2's interaction with the motor the ecu reading is probably the better and more relevant procedure.

 

The O2 does need to be attached to the wiring when we are testing it, whichever way we are doing it, and the motor must be warm, which puts it into closed loop, not open loop.

 

The O2 code can and often is set because the ecu can not keep the A/F ration within the desires range. Missfiring for instance can set it. As with all the trouble codes, it is the ecu that detects, calculates or deduces there is a problem with some component. All the trouble codes are an aid to diagnosis and need to be interperated and tested before replacing parts.

 

Hope that clarifies and confirms what OB99W has said. Basically he is correct.

Edited September 17, 2009 by Log1call

[OB99W]

[...] But I am curious about how an O2 sensor would ''know'' to trigger a code for itself (other than a short or open circuit). [...]

I think we're getting into semantics, Mike . What I said was ''it's common enough for a marginally faulty front O2 sensor to not trigger a code for itself while still causing trouble'' (emphasis added). Perhaps what I should have said was ''it's common enough for a marginally faulty front O2 sensor to not cause a code to be triggered for itself while still causing trouble''.

 

Of course, it's the ECU's programming that determines when parameters are far enough out of the ''normal'' range to trigger a code. Subaru refers to that as ''detecting criteria'' -- some of them are simple, and some complex, involving calculations using data from multiple sensors under varying conditions. For O2 sensors, a simple example could be that the voltage is too low or high (P0131, P0132), rather than swinging about a mid-point.

[OB99W]

I am also getting poor milage (about 20-21mpg). My machinist told me that even though the O2 sensor will work after you had a HG problem when you previously had a coolant leak into the engine. He said it may or may not throw a code, but will still work and be at the high end of the tolerance scale of the sensor. These sensors usually get plugged up with coolant in the exhaust after HG, or just high mileage. However, I believe if possible,the part should be tested properly before being replaced.

There must be a tolerance for testing the resistance rating with a multimeter, but I would think to test it properly you it you need to take a reading while the engine is running. However, if you disconnect the sensor to test it wouldn't the mixture will be thrown off to the ECU thus giving an erroneous reading? I would think the expensive diagnostic equip. would plug into the ECU side and give it the voltage it is looking for so the sensor could be tested while the engine is running open loop. I can't think of any other reliable way it could be tested.

OB, is this the case?

He also recommended heating up the pipe before removing to make it easier if replacing it.

Measuring the resistance of O2 sensors (other than when checking the heater in ones that have it) isn't the right way to test them. The sensors generate a voltage when they're hot enough -- the voltage is highest when there's the least oxygen, and lowest when the most oxygen is present. The openings in the sensor can become obstructed, or the sensor element itself can become contaminated. Either could cause the sensor to not respond sufficiently (not enough voltage swing) or rapidly enough. Sometimes a prolonged hard run on the highway can burn off contamination if it's not ongoing -- that goes for the catalytic converter as well, by the way. A ''good'' way to kill an O2 sensor is to use silicone sealant that isn't ''sensor safe'' on anything that the engine oil comes in contact with or otherwise connects to the crankcase. Besides the exhaust, the sensor also needs to reference the outside air, so contamination of or any sealant applied to the outside surface can also damage it.

 

As Log1call said, testing on the car is done with the sensor connected, and an oscilloscope or digital meter with such a function would be two ways of seeing what the sensor is putting out. With the system in closed loop, the voltage swings should easily be evident. You could ''play around'' with an O2 sensor when off the car by carefully clamping it in a vise (by the hex, not the body), attach a voltmeter, and heat the sensor tip with a propane torch. When left in the flame long enough to get it hot, the voltage should rise to about 0.9 volts (little oxygen). Remove the flame, and the voltage should drop to about 0.1 volts (detecting oxygen). Flick the flame past the tip and the voltage should switch between the two voltage extremes.

 

By the way, although an analog/analogue voltmeter might make seeing the sensor voltage transitions easier, they (especially inexpensive ones) can have relatively low input impedance, which will load the output of the sensor and give inaccurate readings. It's best to use a meter or scope with a high impedance input.

Edited September 17, 2009 by OB99W

[bgambino]

As in the first link I posted above, http://www.aa1car.com/library/o2sensor.htm -- see the section ''OXYGEN SENSOR DIAGNOSIS''.

 

 

oops:banghead: