It seems there are 3 O2 sensors in the California Emissions equipped Camry -
1. The Air-Fuel Ratio sensor (Bank 1 sensor 1) - close to the engine
2. O2 (Bank 1 sensor 2) - before cat, (upstream), underneath car
3. O2 (Bank 1 sensor 3) after cat, (downstream), underneath car

Is this understanding above correct ?

My mechanic replaced (2) above, but the P0136 is back again within driving < 40 miles of replacing (2). Now he is suggesting to replace (1) which I doubt is the cause. Any suggestions ?

 

This code is bank 1, sensor 2. The link below has the locations. The engine model is 2AZ-FE.

http://alflash.com.ua/Learn/banksengine10.pdf

What brand of sensor was used? Some after market sensors (not Toyota brand) don't work as well as others. Suggest Denso or Toyota brand.

Other possibilities are a dirty MAF sensor and damaged wiring or connections to the O2 (A/F) sensor.

In doing the prior repair the mechanic cleared the trouble codes. The engine computer is always conducting tests on various components such as the 02 sensor. The sensor needs to fail a test twice over preset driving conditions which can take a few miles to accomplish. After two failures the check engine light comes on again.

If the mechanic has a good enough code reader they can check the voltages from the sensor to determine if it is within range. The computer checks these voltages and if not in range turns the check light on.

 

He used Denso part 234-4624.

Where is this MAF sensor ?

Here is the longer version of my story -
First time I got this P0136 was in February this year (2013) and my mechanic (repair shop) replaced that sensor with Denso 234-4048. Amazon/Autozone websites showed that the Denso part does not fit my vehicle, but my mechanic insisted it is the part I needed.

Sure enough, now in September, the engine light is back on with the same P0136 code. The same repair shop now says they incorrectly used a part for Japan built Camry (I doubt that is the case) - so they replaced the Denso 234-4048 part they had used in February with Denso 234-4624. However the engine light came back on with the same P0136 code after driving for < 40 miles after that Denso 234-4624 replacement!

The repair shop ran Diags and confirmed/concluded that the O2 circuit is good - connections are good. According to them everything looks as per spec for emissions. On the A-F ratio sensor they found that the voltage is 1.8V instead of instead of 2.8V - which they say is within spec, but lower than usual 2.8V. They suspect that low voltage might be causing the ECU to think O2B1S2 is bad. Hence the suggestion to replace the A-F ratio sensor.

 

Bank 1, sensor 2 (and 3) are listed as an 02 sensor, not AF sensor. Sensor 1 is an AF sensor. These sensors types are not interchangeable.

Denso 4048 is not listed for this car. Denso 4624 is listed as the intermediate sensor for car.

For more details go to the Denso website and look under you car in the emissions parts section. These sensors are emissions parts.

Issues that can cause this code are bad 02 sensor heater or wiring. The sensor has an heating element to warm it up.
-Bad O2 sensor or wiring.
-Bad AF sensor (Sensor 1, Bank 1). This is what the mechanic is thinking. AF sensors have different voltages.
-EFI relay.
-Leak in exhaust system.

Also check for dirty MAF sensor. Location is before throttle body and after cleaner. Do an internet search on cleaning 2AZ-FE MAF sensor.

AF sensor voltage should be between 2.8 and 3.8v at idle. Trans in P or N, all electrical items off engine warm. If voltage low also make sure the engine is not running rich for some other reason and the sensor is just detecting this.

Both AF and 02 sensor should have voltage swings within a certain range.

Bank 1, Sensor 2 should output 0.1-0.9V when driving at 44 mph. This voltage will swing up and down.

 

Update from today: Took the car to a different shop and they wanted me to try cleaning the MAF first since they found it was very dirty and sensors seemed working ok. They cleaned the MAF @ noon, and I could drive around 90 miles during the day until ECL came back with same P0136 in the evening! Shall call them tomorrow and take it back.

 

@Joey: They found that the EFI relay is good.
From your check list -
-Bad O2 sensor or wiring - checked OK
-Bad AF sensor (Sensor 1, Bank 1). - TBD
-EFI relay. - checked OK
-Leak in exhaust system. -TBD
-MAF clean - Cleaned

I also found the following thread which talks about checking wiring between the ECU and the O2. I'll ask the guy to check this as well tomorrow.

 

Post back with what you find out.

 

2nd shop did several tests. Only thing they could find different is that both the rear O2 sensors would not go beyond 0.7v whereas another Camry the guy owned would go to 0.9v under same tests. They suspected that there may be some short somewhere in the wiring.
Updated list of checks -
-Bad O2 sensor or wiring - checked OK
-Bad AF sensor (Sensor 1, Bank 1). - checked OK
-EFI relay. - checked OK
-Leak in exhaust system. - checked OK
-MAF clean - Cleaned

I was scheduled to take it to another shop today to check for short and further diagnosis. However a strange thing happened that is beyond me. Yesterday on my way back from work in the evening the "empty fuel" light came on and I filled 10.38 gallons of gas (kinda stopped in between while filling up). 2 minutes after I got out of the gas station the CEL went away on its own!!! That drove me crazy since it did not make sense to me why it would go away like that and right when I was supposed to take it for diagnosis next morning! I drove the car last night for 110 miles (mostly freeway) hoping that it may come back.. even after 150 miles now, it has not come back.. Spoke to the 3rd shop and they suggested to keep driving and take it back if it comes back again.....