My 2002 V6 ATQ (191K 29MPG Highway 23 City) was due for state inspection so I needed to come up with a fix for my P0421 P0431 CEL codes. I planned to replace both rear O2 sensors, but first wanted to be sure this wasn't due to poor combustion performance or an exhaust leak. So I put 1 bottle of Techron fuel injector cleaner into a 1/2 full tank of premium, then hooked up my shop vac to one of the two the exhaust tips and plugged the other tip to determine if there was a noticeable air leak in the exhaust. I ran the vac about 1 minute, but the air would only force its way out the other exhaust tip which suggests I don't have a leak. The next day I drove the car and noticed improved throttle response and idle. After 2 more cycles of warm up and cool down the CELs turned off. Yesterday I took it in for state inspection and it passed.
Since I've used Techron multiple times before and never really noticed an immediate performance improvement, I think running the shop vac in the reverse direction of the exhaust flow may have loosened up some crud in the cat which may have at least temporarily improved the efficiency of the cat. I'm 100% sure either the Techron or shop vac leak test caused my CEL to turn off, but I don't believe Techron would have such a big impact after having it in the tank for such a short period of time. I would be interested in knowing if anyone fixed a P0421 + P0431 after using a shop vac to test for exhaust leaks, or after using Techron in the grey bottle. I will update this post with any changes in a few months.
P0421 Warm Up Catalyst Efficiency Below Threshold Bank 1
P0421 code means that the vehicle's control module has detected that the catalytic converter is not working properly as in it is not as efficient as the factory is expecting. Replacing the oxygen (O2) sensors may sometimes fix the code, and in most cases the catalytic convert needs to be replaced to fix the problem. The P0421 code is set when the Engine Control Module (ECM) detects rear O2 value difference becomes closer to value of front O2. The ECM monitors when vehicle speed is 25-60 MPH, load is 22-45 percent and engine speed is 1000-3000 RPM.