I have 2007 corolla and I installed racing heading. Since then it gives P0420 code. I have tried to install an oxygen simulator but simulator gives me P0136 and P0137. I also heard add a spacer between exhaust and O2 sensor could fix this problem. What should I do? My tag has expired and I really need to pass emission test asap. Help plz!

ps my simulator is from http://www.ubertechnics.com/o2_simulator_2.5.aspx

The simulator has to output a specific waveform that takes ques from the upstream one. The one that you have is too simplified for the ECM - hence the P0136/P0137 codes. If this was on a pre-2005 Corolla, might work. Starting with the 2005 model year - they switched to a DBW throttlebody system and went with a wideband AFR sensor vs the conventional O2 sensor.

 

Assuming that the precat was likely removed to make room for the header? Still have the existing downstream cat attached or test pipe? Keep in mind that, depending on where you live at, even if the P0420 code disappears, the car will still fail a visual inspection, if any of those converters are gone - if the header doesn't have a CARB or EO exempt number, might be grounds for automatic failure as well.

The spacer you are inferring to sounds like the sparkplug defouler trick. Just happens that the rear O2 sensor and a sparkplug defouler have the same threads. You will have to drill out the center of the defouler to allow the O2 sensor's tip to be exposed to enough exhaust gases to take a reading. Will be an iterative approach, as too small an opening or too large - and you'll see a P0420 code thrown out. Just grab sparkplug defouler and the O2 sensor, see if they will thread into each other. Some had to stack two defoulers to get far enough away from the exhaust stream to fool the ECM.

I also wouldn't rule out a possible exhaust manifold leak - as those can also throw a P0420 code. Who makes this specific header? Hopefully you got the PPE header - as that is one of the few that is actually engineered to work well with the Corolla family of engines with 3-5 WHP gain with just a header swap were seen. The others are mainly for exhaust note and visual appeal - power gains from those are spotty, some actually make less power than the Toyota OEM tubular exhaust manifold.

I did install a PPE header. I also have a midpipe installed. My question is there is no simulator can fool ECM any more right? What should I do if sparkplug defouler trick doesn't work?

As far as I know, there is no guaranteed way to fool the later Toyota ECMs (now looks at multiple sensor inputs, if something doesn't fall inside of tolerance, it pops the code). Also keep in mind that Toyota ECMs are not completely "flashable" like some other manufacturers, hence, no easy software fix at the moment.

 

There are a number of other simulators out there that might do a better job (ie, much more expensive) - but the amount of time and effort needed to get the car to the point of passing. Not only will it have to have some intelligent feedback to read the existing waveform, but it also have to match the impedance of the original sensor. Gone are the days of just hooking up a couple of load resisters to act as a voltage divider - now lots of the more modern ECMs will purposely spike the A/F mix to illicit a particular waveform at the downstream sensor. At that point, be almost faster/cheaper to just thrown on the OEM exhaust manifold and cat - take the test, then swap them back out.

The sparkplug defouler trick needs a lot of trial and error to see if the P0420 stays away long enough to pass emissions. But since it allows alters the flow of exhaust gas around the probe, might be the best shot at tricking the ECM.

I tried to put on a sparkplug defouler, cleared the code and after 30 miles driving it came back on again. Should I try a different or longer defouler?