I apologize in advance for making another thread on this well discussed topic.
I got a CEL on my 4Runner a few weeks ago. Using diagnostic software, I was able to determine that my ECU was throwing a P0136 code. Based on the info on YT, I got a Denso O2 sensor to replace the one after the cat. I ran the diagnotistic software again last night and it shows that Sensor 1 is alternating between 0.5V or higher and 0.4V and lower, whereas Sensor 2 constantly stays below 0.4V. Which confirms that Sensor 2 is faulty. However, my main issue is that the fuel economy has gone down considerably in the past few weeks. If I understand correctly, Sensor 2 doesn't have much to do with fuel economy. At first I thought it was because of bigger tires but my last tank gave me barely 15mpg. Even with the bigger tires I shouldn’t be getting 15mpg (I have the 2.7l motor) because I was averaing 18-20mpg in the city with the same tires before. This being said, I am tempted to change out sensor 1 instead, even though the computer says it's working fine, to restore my fuel economy. The connector for Sensor 2 appears to be fine and no visible leaks that I can see. Is it possible for a cracked exhaust manifold sick in air and cause the ECU to throw this code? I don’t think my manifold is cracked but I am certainly paranoid about it. What do you guys suggest that I do next? Thanks

 

So there is no CEL being thrown now? Is that right?

 

Same question.

Fink

 

If you still have a P0136 and want to get rid of it the FSM show only 2 possibilites: http://www.ncttora.com/fsm/2000/Repa...on/cip0136.pdf

If you want to work on MPG problems the front sensor or many other things could be tried. Yes an exhaust leak before the front sensor could be telling the ECU its running lean, which in turn could be adding too much fuel.

 

try cleaning out your MAF. Its It's quick and easy to do, and if it doesn't fix your problem (which it might), at least you have a clean MAF. It's what did it for me anyway.