2001 Toyota Corolla CODE 0420

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Engine Performance problem
2001 Toyota Corolla 4 cyl Front Wheel Drive Automatic 150000 miles

My wife's Corolla has "check engine" light. OBD ll displays code 0420. Catalyitc converter efficiency below threshold bank 1.
We have first replaced down stream O2 sensor, still have same code #. Next replaced catalytic converter, still have same code #. Finally replaced upstream O2 sensor and still have same code #.
Upstream O2 sensor voltage not changing much after engine is hot, with downstream sensor voltage changing alot. What is next to get replaced the car or the Engine Coolant Temperature?

Do you
have the same problem?
Wednesday, October 15th, 2008 AT 11:59 AM

1 Reply

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O2 sensors and catalytic converter replaced with OEM-I'll check the coolant temperature sensor

The OBD II system can't actually measure the concentration of HC or CO in the exhaust, so it compares the upstream and downstream O2 sensor readings to estimate how well the catalyst is actually doing its job of removing pollutants from the exhaust.

The upstream O2 sensor will typically show a lot of switching activity because the engine computer is constantly adjusting the fuel mixture between rich (more fuel) and lean (less fuel). When the engine is first started, the catalyst is cold and doesn't do much. During this time, the switching activity of the upstream and downstream O2 sensors are essentially the same because nothing is happening inside the converter.

When the converter reaches about 600 degrees F, it is hot enough to start reacting with the gases in the exhaust. This is called the catalyst's "light off" temperature. The converter now starts to clean the exhaust and remove the pollutants. This causes a sudden drop in the switching activity of the downstream O2 sensor, and the sensor's output voltage levels off to an average reading of around 0.45 volts. This tells the OBD II system that the catalyst is doing its job and that everything is fine.

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Wednesday, October 15th, 2008 AT 2:29 PM

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