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Discussion Starter #1 (Edited)
The engine management light had come on and been reset twice with around a month before the light came on. I can't be absolutely sure what the code was but it was sensor related both times. So I asked the Toyota Dealer to have a look when it went in for the MOT. At that point the light was off. Dealer reported:
"Engine Management Light comes on intermittently - carry out diagnostic 0.40 check - P0420 Catalyst stystem below threshold (Bank 1) carry out checks to O2 sensor - Bank 1 sensor out of spec. Delete fault code and road test."
Advised that removing old sensor could be difficult if lots of corrosion and could be more than car is worth in worst case. Suggested delay repair until I was experiencing problems with fuel economy or performance.

The very next day the light came on again. It has been on ever since (1 month) until..
I removed filler cap - heard a hissing as if there was pressure in the tank and refilled tank. When I got back in car light was off and has remained off for the past week.

Could a vacuum/excess pressure in the fuel tank be causing the light to come on with that code?
What next?
I am thinking that maybe I should buy an ELM327 v2.1 bluetooth OBD dongle and software and see what I can discover. I guess a cheap one from ebay is as good as any for my needs?
That said my wife's Chevrolet tacuma has an ABS light coming on intermitently and local garage could not drill down to discover exact fault. So I would pay a bit more for a more flexible scanner.

RAV4 XT3 5dr 2.0 VVT-i Auto April 2004
 

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P0420 has nothing to do with the EVAP system. The dealer said the O2 sensor was out of spec. You loosened the fuel cap and heard a whoosh. That is not uncommon.

My educated guess is that the O2 sensor has been working fine for the past few days. You could have added 3psi to the left rear tire and the light would have gone off. The light WILL come back on. We are assuming the dealer told you the correct code, and not a P045x series code. If you have an AutoZone, O'Reilly's, NAPA or CARQUEST near you, they will read the code for free. Even though the light is off, the code will still be stored.

The scanner I use at home is the ALLScanner VCX. It comes with Techstream, the same software Toyota uses. The VCX will not update calibration files (reflash) so f you need that functionality, then you need the VCX Plus.

At the shop, I use a SnapOn Solus Pro, which works with many brands, but does not have the functionality of OEM tools.

This eBay seller is the only seller I have found on eBay or Amazon that sells this ALLscanner.

Toyota Allscaner VCX Nano | eBay

I have no experience with the Nano. I am referring to the blue device on the right.
 

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Discussion Starter #3
Thanks FixIt, just a co-incidence then, that's a shame.
If the P0420 code switches on the engine light doesn't it stay on permanently even if the fault goes away?

I will sort out a scanner (I am in uk). Don't need to update calibration files (as far as I know) and that would probably be beyond me. Initially I want to read and reset the codes but thought that the ability to see/save/chart what is going on would be interesting and might help pinpoint the faults.
 

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Hi all,
i have this same fault code.
before considering changing my catalytic converter, I would first like to reassure me that the problem does not come from the O2 sensor.I have an OBD II scanner and I use it with Torque application.I can see 4 O2 sensor on torque and i want to know their signification (O2 1x1; 1x2 O2, O2 1x3; 1x4 O2).I wonder because the graph of 1x2 O2 often shows a curve that rises and slopes down, but all other graphs show nothing.
 

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I have never replaced an O2/Air Fuel sensor to correct a catalytic converter code. I have replaced an upstream sensor to cure a downstream sensor code.

The PCM checks the amplitude of the waveform of the downstream sensor and compares it to the amplitude of the waveform for the upstream sensor. When the amplitudes are similiar (way simplified explanation) the PCM turns on the MIL and sets a code for the catalytic converter.
 

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On a 2005 RAV4 I had a persistent P0420 code that would randomly appear a few days or weeks after resetting. I replaced the downstream and upstream sensors, and the MAF sensor, to no avail. All other params were normal.

In the end after having my mechanic review things, he recommended just trying an "extension" tube on the downstream sensor to pull it slightly out of the exhaust path (basically back the sensor off by about an inch.) This doesn't fake out the sensor signal but can help make a marginal/borderline conditions go through. In my case it worked and CEL hasn't been on for 6 months.

It was a cost/risk thing. Replacing the cat would have been a much more expensive "test fix" that had no guarantee of fixing anythings. So we tried the spacer tube and it ended up working for now.
 

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On a 2005 RAV4 In the end after having my mechanic review things, he recommended just trying an "extension" tube
That is circumventing the emissions system, and is illegal for a person employed as a mechanic to do. You could do it yourself, but someone gainfully employed as an auto repair service provider cannot circumvent the emissions control system.


From Document Display | NSCEP | US EPA

"As of August 8, 1977, all persons engaged in these businesses are prohibited by Federal law from removing or rendering inoperative any emission control device or element of design that is installed on a motor vehicle or motor vehicle engine."

I am aware of these devices. They are actually called non-foulers and were originally used as a spacer between the spark plug and the cylinder head. If a cylinder burned oil, the electrodes of the spark plug would accumulate a conductive coating of oil, causing it to misfire. The non-fouler spaced the spark plug back out of the combustion chamber to shield it from the oil, and yet still be able to fire the air/fuel mixture.

Essentially, the PCM looks at the waveform for the front sensor and compares it to the waveform for the rear sensor. When the vehicle is new, the waveforms are vastly different in amplitude. When the converter degrades, the burnt mixture passes through the converter almost untouched. The oxygen level is nearly the same going in as coming out. so the rear waveform will be very similar to the front waveform. The PCM logs this into memory, and if on a subsequent trip, if the same fault is flagged, the PCM turns on the MIL and logs a catalytic converter code. Using a non-fouler results in the amplitude of the rear sensor decreasing to mimic a catalytic converter that is still efficient, yet still provide a signal to the PCM that the sensor is working.

I have never replaced an upstream or downstream sensor for a catalytic converter code. An oxygen sensor does not generate voltage by itself. The only way the PCM would see high oxygen sensor voltage is if the heater element shorted to the signal wire. I have seen this, only on Jeep products. Watching the waveform jump from .45 volts to 14.3 volts stumped me the first time I saw it happen. The engine usually jerked badly because the PCM didn't know what to do with that much voltage.


I would advise your "mechanic" to not offer this "service". The fine is $2500 per vehicle.
 

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I had the same codes P0420 and sometimes P0430. Put up with it for over a year by resetting the codes myself. Finally got tired of it and replaced the cat. No more codes.
 

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Hi all,

Is there a diffrence between A/F sensor and Upstream sensor?

Where are located Oxygen sensors (upstream and downstream) on a RAV4 2004 which has just one Bank?

I wonder because the graph of 1x2 O2 often shows a curve that rises and slopes down, but all other graphs show nothing.
 
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