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7thGenCelica
10-23-2010, 06:21 AM
Hello Folks,

Need to change my O2 sensor...got a P0402 bank 1 error on my scanner...is it the front or rear sensor? Thanks!

-DC

2001 Celica GT

kungFUBAR
10-23-2010, 06:33 AM
p0402 is for excessive egr flow, not an o2 sensor. Take the valve off and clean the soot out of it and out of the egr pipe.

If you meant p0420, it's a catalyst efficiency code. I'd start with the rear if that's the case.

7thGenCelica
10-23-2010, 09:03 PM
hello kungFUBAR,

you're right, it's a p0420 error...thanks for the advice, i will grab the rear and see how that goes...thanks!

donteatbugs
10-25-2010, 06:38 PM
check your spark plugs too. they may be worn and not getting full combustion and letting the car read rich. only do this if the o2 doesnt fix the problem or you havent checked them in 50k miles

7thGenCelica
10-26-2010, 05:11 AM
donteatbugs,

thanks for the reply...i just ordered the rear 02 sensor (denso), and a set of iridium denso spark plugs...i'm hoping this resolves my problem...thanks guys!

alltracman78
11-21-2010, 06:00 AM
You only have one bank on your engine. V engines have 2 banks, so they have a bank 1 and a bank 2. You will only have bank 1 [there's an exception for certain inline engines with 2 catalytic converters in the manifold, but it doesn't apply to the ZZ engines]. Sensor 1 is the sensor before the cat, sensor 2 is the one after the cat.


p0402 is for excessive egr flow, not an o2 sensor. Take the valve off and clean the soot out of it and out of the egr pipe.

If you meant p0420, it's a catalyst efficiency code. I'd start with the rear if that's the case.

No EGR on the ZZ engines, FYI.
P0420 is usually a bad cat, has nothing to do with the O2 sensors being bad. The ECU is looking for very low oxygen content after the cat, if it sees to much oxygen it sets P0420. A bad O2 isn't going to send a high oxygen signal to the ECU. If you had a bad sensor after the cat you would be seeing a code for B1S2 [bank 1 sensor 2].


check your spark plugs too. they may be worn and not getting full combustion and letting the car read rich. only do this if the o2 doesnt fix the problem or you havent checked them in 50k miles
Incomplete combustion [or misfire] doesn't cause the O2 sensor to read rich, it causes it to read lean. Remember, the O2 [or A:F] sensor ONLY reads oxygen, it doesn't read HC [Hydrocarbon]. Incomplete combustion leaves extra unburned oxygen. This will cause P0171, engine running lean.

kungFUBAR
11-21-2010, 06:32 AM
P0420 is usually a bad cat, has nothing to do with the O2 sensors being bad. The ECU is looking for very low oxygen content after the cat, if it sees to much oxygen it sets P0420. A bad O2 isn't going to send a high oxygen signal to the ECU. If you had a bad sensor after the cat you would be seeing a code for B1S2 [bank 1 sensor 2].

Not trying to call you out, but this is incorrect. Modern catalytic converters do 3 things: reduce NOx into oxygen and nitrogen, turn carbon monoxide into carbon dioxide and convert unburnt hydrocarbons into CO2 and water.

The ECU is going to be looking for O2 levels after the cat to be higher that before it. Consistent O2 reading across both sensors triggers a P0420 or P0430 code.

That being said, a misreading rear O2 sensor can cause catalyst efficiency codes to come up. If the problem was with the sensor itself you would have a code between P0136 and P0141.

Again, not trying to call you out or make a scene, just trying to prevent the spread of misinformation.


No EGR on the ZZ engines, FYI.
I did not know this. Interesting.

alltracman78
11-21-2010, 04:36 PM
Not trying to call you out, but this is incorrect. Modern catalytic converters do 3 things: reduce NOx into oxygen and nitrogen, turn carbon monoxide into carbon dioxide and convert unburnt hydrocarbons into CO2 and water.

The ECU is going to be looking for O2 levels after the cat to be higher that before it. Consistent O2 reading across both sensors triggers a P0420 or P0430 code.

That being said, a misreading rear O2 sensor can cause catalyst efficiency codes to come up. If the problem was with the sensor itself you would have a code between P0136 and P0141.

Again, not trying to call you out or make a scene, just trying to prevent the spread of misinformation.

This is one of the reasons I stopped posting, I got so sick of putting up information and then having to go back and retype again because someone that didn't know what they were talking about tried to tell me I was wrong.

The cat takes the excess oxygen in the exhaust and the extra oxygen from the NOx, adds it to the CO to make CO2; adds it to H and C to make H2O and CO2. There is LESS oxygen after the cat.

A correctly functioning cat will have the O2 sensor reading STEADY ~ .8v [very very rich, eg lack of oxygen]. The pre cat sensor readings fluctuate, post cat readings are supposed to be much more stable.

The ECU looks for a constantly changing signal [like the signal from the pre cat sensor] before it will set a P0420.
A rich signal means the cat is functioning correctly,
A lean signal is impossible unless either the O2 sensor isn't working correctly [will set an O2 sensor code] or the engine is running lean [will be backed up by the pre cat signal also reading lean],
A slow response from the O2 sensor will set an O2 sensor code,
A problem with the heater circuit for the O2 sensor will set an O2 sensor code


Anything else?

7thGenCelica
11-23-2010, 12:55 AM
alltrac,

so you're saying i should change the cat? i did change my plugs (which i had to do anyhow), cleaned my MAF (for the first time), but haven't made my O2 sensor swap yet since the CEL went out after i did those 2 things...but it has returned...haven't had the time to recheck the error code since i've been away quite a bit since then. i was about to return my o2 sensor, which is the one after the cat, but then the CEL came back. anyhow, will post update once i get some free time to check the code again. thanks for all the useful info :)

D

alltracman78
11-25-2010, 04:54 AM
Your cat is what's causing that code.
Depending how worn out the cat is you might get away with clearing the code instead of replacing the cat, especially if the light is going out and coming back on. If you replace it with an aftermarket cat that could cause the code to come back. They normally don't, but they don't clean up the exhaust as well as a Toyota cat [Not cleaning the exhaust is what causes the code in the first place]. The Toyota one will be much more expensive, but it also is a much better quality cat, and the exhaust piping is stainless steel instead of cheap steel, and it's double walled [2 pipes, 1 inside the other] instead of single wall. Also, Toyota replacement exhausts are lifetime warranty [But I think you'll have to have it installed at the dealer to get the warranty].

The reason your CEL is going on and off is because of how OBDII works.
OBDII works like this; your ECU is always looking for problems in the system. It runs what are called monitors [think of a monitor kind of like a self check of the system], to check different parts of the system. So there's a monitor to check the O2 sensor heaters, there's a monitor to check the EVAP system, a monitor to check the O2 sensor signals, and a monitor to check the catalytic converter [there are others, but I'm not getting into them now]. During the check if the monitor sees a problem it stores a code in the ECUs memory. Normally it won't turn the CEL on the first time it sees the problem. But, the second CONSECUTIVE time it sees the problem it will turn the CEL on. Some codes will cause the light to come on the first time, but most take two consecutive trips [2 trip code] to turn on the light [Think of a trip as starting every time you start the engine and stopping every time you turn the engine off. It doesn't matter how long, could be 2 min could be 2 hours]. If it doesn't see the problem during consecutive trips it won't turn on the light. So, for example if you drive one day and it sees a problem with an O2 sensor it will store a code in memory but won't turn on the light. Now, the next time you drive it doesn't see a problem with the O2 sensor, it won't turn on the light. Now lets say the next time you drive after that it sees the problem again. It will only store the code in memory again, it won't turn on the CEL, because it didn't see the same problem two consecutive trips. The point of this is to try and eliminate false positives [positive being a failed monitor].

Now let's pretend the ECU has seen a problem twice in a row and turned on the CEL. In order to turn the light off it has to see NO problem in that system for THREE consecutive trips. If it sees no problem twice in a row, but the third time it sees the problem again it will leave the light on and the three consecutive starts over again. Sometimes a part is only partially bad, so it will turn the light on and off again as it doesn't work for a while, then starts to work, then stops working, ect. This is why your CEL can go on and off instead of staying on.

There's one more thing with this. These monitors; some of them run all the time, like the one that checks your knock sensor, and MAF and VVTi. Every second your engine is running it is constantly checking and rechecking those systems. Some of the other monitors only run at certain times. The monitor that checks your cat is one of those. It only runs once the engine is warmed up, the O2 sensors are hot and you're cruising at part throttle at a relatively even rate. Also, it only runs once per trip [remember, engine start - engine stop]. If it starts to run and gets interrupted [like if you all of a sudden stomp on the gas], it won't try to run again until the next trip that everything is set up right [warm engine, hot O2s, steady throttle, ect].
This is important because if the monitor doesn't run it won't set or clear codes. So that 2 consecutive trips is actually 2 consecutive times the cat monitor runs. For the monitors that don't run all the time a trip that the monitor doesn't run, doesn't count. So if you drive once and the cat monitor runs and fails, and the next 5 times you drive the cat monitor doesn't run, but then the next time after that it does run and fails again, that's 2 consecutive trips to the cat monitor and it will set the code. It just took you 7 total "drives" for it to come on.

I'm going to leave it at that, I really hope that all made sense, I tried to keep in relatively short while still explaining it.

KooK
11-25-2010, 06:50 PM
Your cat is what's causing that code.


I like how confident you are in this simply because you still could be wrong, however you could be right. The only thing we know is it's narrowed down to two possible issues. Unless you actually know what that oxygen sensor is reading, you can't tell if it's tripped or bad.

Luni
11-25-2010, 07:48 PM
You kill me dude. Jeremy is for the most part, always right. I wouldnt question him on this at all. If he says its a cat, Ill bet money its a cat.

Hes a Toyota tech and hes been doing it for a LONG time. Hes probably the one person on the board that truly is qualified to be able to pull a diagnosis out of his ass and be right.

alltracman78
11-26-2010, 04:08 AM
Thanks for the vote of confidence Rob. :)


I like how confident you are in this simply because you still could be wrong, however you could be right. The only thing we know is it's narrowed down to two possible issues. Unless you actually know what that oxygen sensor is reading, you can't tell if it's tripped or bad.

I can tell you what the O2 sensor is reading. That's why I know the cat is bad. It's reading rich/lean/rich/lean/rich/lean.
When the ECU is running in closed loop mode it's constantly switching the fueling from rich to lean, back and forth. This causes the exhaust to switch back and forth from rich to lean. When the exhaust reaches the cat it's chemically changed, and most of the spare oxygen is combined with other molecules. This leaves very little free oxygen in the exhaust. So, the O2 sensor, which reads only oxygen, sees this as a rich condition [very low oxygen]. So, a correctly functioning cat causes a rich signal from the rear O2 sensor. Keep in mind this is only when the ECU is in closed loop and a constant throttle angle. Once you start changing the engine speed and fueling the exhaust temporarily changes as well.
If the cat isn't functioning correctly [not chemically changing the exhaust] then the exhaust gas after the cat will still be fluctuating back and forth from rich to lean. And, the rear O2 sensor will read this, and send the same back and forth signal back to the ECU. The ONLY time the ECU will set a P0420 is if it sees this back and forth signal from the rear O2 sensor.
A bad O2 sensor can't send this signal, nor can faulty wiring. So the only thing that can be wrong is the cat isn't chemically changing the exhaust the way it's supposed to.

The 2 exceptions that I can think of are if the ECU itself is bad [highly unlikely] and is setting the code out of no where, or if you had some type of electrical interferance with the wiring, causing the back and forth signal. Even more unlikely.

So, P0420 [or P0430] means you have a bad cat. Not a bad rear O2 sensor, a bad cat.

Luni
11-26-2010, 06:16 AM
Well, I look at it like this. I know my shit. I know I know my shit. Lots of people difer to me cause I know my shit. Theres people I difer to becuase they know their shit more than I do. Jeremy is one of them.