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Discussion Starter · #1 ·
This is concerning my 2001 Camry 4cyl auto with 215k miles and has been maintained very well.

I have the dreaded p0420 and replaced the exhaust manifold/front cat with toyota oem. Code went away for 300 miles then came back.

Original toyota O2 sensors on the car. I have a scanner and have the following freeze frame info.

Trouble Code ....P0420
Engine Speed ...1492 rpm
Calc Load ........47.8%
MAP ...............48 kpa
Coolant ...........185 degrees
IAT ................118 degrees
STFIRM1..........-1.6%
LTFIRM1..........-1.6%
Veh Speed .......37 mph
Fuelsys1 ..........Closed

This is my daughters car and I have been longterm unemployed. I want to pinpoint this as best as possible.

Thanks in advance

John
 

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2013 Camry XLE
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2,183 Posts
I have a 1999, but here is what I did for a 420 code. Take the car out for at least 1.5 hours at 65mph+ with O/D off. After that I reset the comp and haven't seen a code on 3 months. Other than that most I have talked to say first try replacing the down stream O2 sensor as usually that is the one that gets "lazy"
 

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1MZ powered MR2
1991 MR2
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try replacing the down stream O2 sensor as usually that is the one that gets "lazy"
+1000

The ECU basically compares the front O2 reading to that of the rear O2 reading. Too much variation and you throw a code, on the other hand, too little variation will throw a code too. Actually, in your case, it'd be a front Air-Fuel Ratio sensor.

The ECU wants to see the downstream O2 voltage change in accordance to injection on/off time.
 

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Camry 1996 V6 LE
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1,128 Posts
uhm... well, at least measure the resistance between black wires of the oxygen sensor, should be around 16 Ohm, if I remember correctly.... (check FSM)
 

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1MZ powered MR2
1991 MR2
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1,065 Posts
Any other thoughts???
Your fuel system is working beautifully based on your fuel trims. It's probably slightly negative from the high intake air temperature.

I'd change out the rear O2 sensor first. The 2001s have two catalysts - one at the manifold, and one in the exhaust pipe. Doubt both of them went bad.

You can try Seafoam to clean up the catalysts too.
 

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V8'sRGone
95 Cam
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1,956 Posts
The temp on that caught my eye too!

Just a thought, if the intent is to run fuel system cleaner, would it not be preferred to do that on the old sensors and when done install the new ones? The point being all the carbon, soot, lead, contaminants are hard on the sensor so why subject it to it?
 

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Discussion Starter · #8 ·
The temp on that caught my eye too!

Just a thought, if the intent is to run fuel system cleaner, would it not be preferred to do that on the old sensors and when done install the new ones? The point being all the carbon, soot, lead, contaminants are hard on the sensor so why subject it to it?
Thanks for the info!

I reset the error and it came back quickly with basically the same freeze frame data except the following:

STFIRM1 - 1.6%
LTFIRM1 - 4.6 %

I actually removed the front o2 sensor and cleaned it with Seafoam. I know the rear sensor is much harder to deal with.

Does the rear convertor have anything to do with this error considering the o2 sensor are measuring BEFORE the rear convertor?

I see no other issues. The car runs great. It accelerates much better since I replaced the Exhaust manifold/front CAT.

Thanks again.
 

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1MZ powered MR2
1991 MR2
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1,065 Posts
The rear O2 sensor should be mounted after the 2nd catalyst, so it measures oxygen content from both catalysts. The front catalyst (on the manifold) is just a pre-catalyst or warm-up catalyst to help reduce cold start emissions. The rear catalyst is the main one that provides most of the emissions reduction.

You said you're still on the original Toyota O2 sensors? I think it's time to replace BOTH of them.

The front O2 sensor is not an O2 sensor, it's an air-fuel ratio sensor or a wideband sensor, which is completely different.
Here's the front one for your 2001 Camry 4 cylinder: [ame]http://www.amazon.com/Denso-234-9010-Oxygen-Sensor-Ratio/dp/B000C5WD08/ref=sr_1_1?ie=UTF8&s=automotive&qid=1285065352&sr=8-1[/ame]


The rear O2 sensor is a regular lambda sensor.
Here's the rear one: [ame]http://www.amazon.com/Denso-234-4137-Oxygen-Sensor-Ratio/dp/B000C5UFQ2/ref=sr_1_1?s=automotive&ie=UTF8&qid=1285065421&sr=1-1[/ame]
 

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Discussion Starter · #10 ·
The rear O2 sensor should be mounted after the 2nd catalyst, so it measures oxygen content from both catalysts. The front catalyst (on the manifold) is just a pre-catalyst or warm-up catalyst to help reduce cold start emissions. The rear catalyst is the main one that provides most of the emissions reduction.

You said you're still on the original Toyota O2 sensors? I think it's time to replace BOTH of them.

The front O2 sensor is not an O2 sensor, it's an air-fuel ratio sensor or a wideband sensor, which is completely different.
Here's the front one for your 2001 Camry 4 cylinder: http://www.amazon.com/Denso-234-901..._1?ie=UTF8&s=automotive&qid=1285065352&sr=8-1


The rear O2 sensor is a regular lambda sensor.
Here's the rear one: http://www.amazon.com/Denso-234-413..._1?s=automotive&ie=UTF8&qid=1285065421&sr=1-1
Thanks for the great response. I am a little confused. The Camry is a 2001 with Non_California emissions. I was told by two different Toyota shops (on the phone) that the error code was the result of the front CAT being bad.

From what you say it is the rear CAT that is most likely throwing the code. I see where the rear O2 sensor is after it.

Would you suggest replacing the rear CAT?

Thanks !!!
 

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1MZ powered MR2
1991 MR2
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1,065 Posts
Thanks for the great response. I am a little confused. The Camry is a 2001 with Non_California emissions. I was told by two different Toyota shops (on the phone) that the error code was the result of the front CAT being bad.

From what you say it is the rear CAT that is most likely throwing the code. I see where the rear O2 sensor is after it.

Would you suggest replacing the rear CAT?

Thanks !!!
No, just the sensors for now. If replacing the sensors doesn't do the trick, then your rear cat is likely shot.

2001s are all 50 state emissions, which means you have California emissions (all 2001+ Camrys are like this). Only CA emissions 5SFEs (4 cylinder engine) have the front catalyst. It won't be listed as CA emissions because Toyota now classifies its engines as 50 state compliant from 2001 forward.

Think of the front cat as a supplement catalyst.
The rear cat is the main one.

The ECU compares the voltage readings from the front AFR sensor and rear O2 sensor and determines if catalyst efficiency has dropped. If the sensors aren't reporting the correct voltages ranges because they've aged and are now reporting slower, you end up with codes (sometimes a catalyst code, sometimes an air-fuel ratio sensor code or rear O2 sensor code).
 

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i have the same problem with my 2001 Camry 2.2L. I have replaced the front air/fuel sensor and that did not fix the problem, I was going to try the trick of extending the rear O2 out a little to trick it to think the cat. is working well, if that doesn't fix the problem, then replace the rear O2. any thoughts, the car has 170,000 miles on it.

Thank you
 

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2002 Solara SLE V6
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you need to replace the post-cat oxygen sensor, that is the one ECU uses for monitoring catalytic converter's efficiency. maybe as easy as that.

upstream o2 sensors (or AFR sensors) are mostly used for adjusting the fuel injector pulse width (fuel trims).
 
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