How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Jan Andersson Your Own Question
Jan Andersson
Jan Andersson, ASE Master Tech
Category: Toyota
Satisfied Customers: 834
Experience:  20 years of auto repair and racing both in Europe and USA. Have had Toyotas before and love them!
Type Your Toyota Question Here...
Jan Andersson is online now
A new question is answered every 9 seconds

2002 Toyota Prius: The check engine light came..stalling

This answer was rated:

The check engine light came on on my 2002 Toyota Prius. When I hooked up my diagnostic tool I got the code P0420 which is something about the Catalytic Converter system not performing at peak efficiency. (I know this is not the exact wording for the problem but you get the general idea I think) The car seems to be running just fine, starts fine, no stalling or any other mechanical issues that I can detect. Unless this is a super serious issue I am not planning on doing much with this at this time but I would like to know what effect if any this could have on my cars engine. I should tell you that the car currently has about 325,000 miles on it.



here's the description for the code



P0420 - Catalyst System Efficiency Below Threshold (Bank 1)



There are two O2 sensors in your exhaust system. One measures exhaust gases before the catalytic converter, and the data is used by the engine management system to adjust the fuel feed. The second, "downstream" O2 sensor is locates in the exhaust pipe after the catalytic converter, and it is used only to monitor the efficiency of the converter, by comparing the values between the two sensors. Because the converter is between the two, the later sensor should detect much cleaner exhaust gases if the converter is working right.


With time, two things inevitably happen: the converter efficiency goes down, and the sensor accuracy is reduced or it stops working.


So... your computer is telling you that the second sensor is reporting insufficient operation of the catalytic converter. The real cause is either just that, a failing converter, OR a failing sensor reporting wrong values.


I would first replace the O2 sensor, because they have a shorter lifespan than the converters. If the problem repeats, then you need a new converter. Usually when you replace a converter, you should replace the O2 sensor(s) too. But this way you already have a new one in there. With luck, the O2 sensor is all you need. With bad luck, you also need the converter. This is a logical way to approach it and it starts with the lowest cost repair first.


This code cannot be a result of a loose gas cap. Other codes can, and Double checking the gas cap was a good move though, a first step when the light comes on. Most people don't think about it or know about it.


The problem is not critical. The catalytic converter is just getting worn, and not cleaning the exhaust gases as good as it should. In somewhat rare cases they can start crumbling and clogging up internally, and then they can cause driveability problems due to restricted exhaust flow. You are not there yet. Your only problem now is increased emissions. The car won't pass emissions testing.



Jan Andersson and other Toyota Specialists are ready to help you