P0420

This 280k mile 3.4 L V6 beast is throwing P0420. It runs beautifully; no smoke, rich smells, or rotten egg smell. Gas mileage is in the 20s.

Had a reputable shop replace the upstream cat. 50 miles later, CEL is back on.

The upstream sensor shows 3.3 V, and remains steady through a RPM range of idle to 2500 RPM.

The downstream sensor shows 0.75-0.9 V at idle and drops to 0.09-0.15 V at 2200 RPM.

The downstream sensor heater passes a resistance test (13.4 ohms across B+ and HT).

Temp entering (new) upstream cat is 295 F, exits at 410 F. Temp entering downstream cat is 365 F, exit temp is 320 F.

Seems to me that the downstream cat is spent, but does the downstream sensor output seem normal, given a bad downstream cat? Thanks in advance.

 

Had the same issue a month ago. All it was an exhaust leak.

 

Thanks for the response. I had the muffler shop go through it when they replaced the front cat. I replaced the battery in the truck this morning as it wouldn't start, and I knew I needed to start the truck several times today. I reconnected the downstream O2 sensor after checking the heater resistance and took it out for a highway ride for 50 miles. There was no light, and I monitored the OBDII reader. The voltage from the sensor fluctuated between 0.17 and 0.75. I'm quite sure "El Chiflado Pequeño," as I've come to name this cantankerous little beast, will wait to throw the code as I drive up for inspection!

 

In the process of replacing the spark plugs. They all looked very nice, light brown, with no wet, black, or oil on any plug. In the process, I found a split in a manifold vacuum line, which I replaced. I also replaced the air filter. "El Chiflado" started right up, and I could tell there was an improvement in the idle and the road response. I did not reset the CEL. I've run it 25 miles, and the light is still glowing.

How many miles will it take for the light to turn off? It took almost 100 miles for the light to illuminate.

 

https://www.tacomaworld.com/threads/addressing-some-common-misnomers-of-the-p0420-code.396763/

 

I kept replacing the o2 sensor but the MIL kept coming back every few months. It ended up being one of the catalytic converters.

 

I’ve had this code for years and I finally replaced the downstream o2 sensor and the light has stayed off. I’ve driven about 600 miles since.

 

My P0420 code was from an exhaust leak.

 

I found a crack in my exhaust several feet after the second o2 sensor. I doubt this is causing my P0420. Where did you find your leak?

 

I believe the leak that mattered was between the second cat and the rear O2 sensor. I also had leaks close to the muffler and at the exhaust manifold.

 

^^^^might be worth investigating as stated
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