2006 Prius with Check Engine Light - P0420 Code

I've read on here about how over-filling with engine oil causes oil instead of crankcase fumes to get into the throttle body and then from there it can impact the catalytic converter. I'm sure if you do some searching on here you'll find lots more info...

 

*Always* check the oil level yourself before driving away from every oil change. This can prevent all kinds of grief.

 

I have a 2006 Prius which I bought brand new back in November 2006. I currently have a little over 290,000 miles on it, mostly highway miles. I noticed the "check engine light" come on after taking it to a Toyota service center. The light was not there before I had a routine oil change for my car. The light actually came about 5 miles after leaving the service center so i immediately turned around and returned to toyota. They told me my catalytic converter has to be replaced. I was furious not because of what they told me but rather the timing of the problem surfacing after an oil change. Could they have tweaked something in order to generate more income? I've taken good care of my prius that's why i'm upset i now have to deal with this. I asked my brother-in-law to check it with this new monitoring gadget he bought to find out what the code is and sure enough it's P0420. I've been driving my car and I noticed the check engine light goes on about 80 miles after an oil change but goes off after about 200 miles, then on again for a period of time, and that cycle has been going on now for about 2,000 miles. Any ideas why this is happening? Thanks.

 

Well, the obvious idea is that the catalytic converter has failed, which would not be a surprise considering the car has logged 290K miles. The light is turning on & off because the catalytic converter is operating marginally.

You might try replacing the air/fuel ratio sensor which is upstream of the catalytic converter to see if that would help. That sensor provides feedback to the engine ECU so it can manage the fuel injectors.

The engine ECU also compares output from that sensor to the oxygen sensor output which is downstream from the catalytic converter, to assess catalytic converter efficiency.

The air/fuel ratio sensor is less costly than the catalytic converter, and a new sensor may help the engine ECU to decide that the catalytic converter is "good enough" to keep DTC P0420 from being logged.

 

Did you check the oil level? Is it overly full? Using exactly 3 1/2 quarts of oil is important in these kinds of cars, but the dealers would rather not have to add more oil later so they add a bit extra, which can lower mpg, or if way too full can dirty up/damage throttle body and possibly Catalytic converter...

 

I agree, however, I would like to suggest the oxygen sensor also be replaced before the catalytic converter. I have seen this work in the past (not with Prii, however, I have not seen many Prii with P0420s) plus the operation of the catalytic converter depends on the operation of those sensors. I always recommend replacement of all air/fuel ratio and oxygen sensors before catalytic converter replacement.

Note: Many people mistakenly say that the rear oxygen sensor only monitors the operation of the catalytic converter. THIS IS INCORRECT! It is a known fact (not known by all) that Toyota uses both the front and rear oxygen sensor signals in their fuel strategy.

 

I agree that the rear oxygen sensor should also be replaced, to see whether that helps the engine ECU to decide that the catalytic converter is in "good enough" condition.

The Prius repair manual provides this description of Active Air-Fuel Ratio Control, which appears to be implemented only for a 15-20 second period when the engine is warm. If any of the three listed DTCs are logged by the engine ECU, that points to the rear oxygen sensor as a candidate for replacement:

"Active Air-Fuel Ratio Control
Usually the ECM performs the air-fuel ratio control so that the A/F sensor output indicates a near stoichiometric air-fuel ratio. This vehicle includes ”active air-fuel ratio control” besides the regular air-fuel ratio control.
The ECM performs the ”active air-fuel ratio control” to detect deterioration in a catalyst and the heated oxygen sensor malfunction. (Refer to the diagram below)
The ”Active air-fuel ratio control” is performed for approximately 15 to 20 seconds during a vehicle driving with a warm engine. Under the ”active air-fuel ratio control”, the air-fuel ratio is forcibly regulated to go LEAN or RICH by the ECM.
If the ECM detects malfunction, it is recorded in the following DTCs: DTC P0136 (Abnormal Voltage Output), DTC P0137 (Circuit Open) and P0138 (Circuit Short)."

That test basically gives the rear oxygen sensor a chance to demonstrate that it is working, as the lean or rich A/F mix should cause the sensor's output voltage to decrease or increase as appropriate. If the voltage doesn't change as much as it should, the engine ECU logs the appropriate DTC.

 

In my opinion, a worn (slow responding) oxygen sensor would, in theory, mask a inefficient converter. The reason I would replace it (both sensors) is to make sure the correct ratio of air and fuel is going into the converter as this is essential for proper operation. In other words, we want to make sure the poor catalytic performance is not due to a marginally performing sensor.

Lets not forget that vehicles often have problems without lights or codes showing up.

 

Sorry to hear, but yes changing out sensors first seems wise... Of course it'd be even smarter if their was a way to pull the existing sensor and figure out how to test it to make sure its working properly rather than just swapping them out... Anyone else on here know what has to happen to test these sensors?

 

I just got the same DTC for the second time, so I guess it is not a fluke. I have been swamped with other things right now (my home hot water heater starting leaking today!) so I will problably put off working on this for a while, the car runs fine, and my inspection is not due until July, so I have no pressure to get it done soon. I did check the O2 sensor with the AuteEnginuity, but it does not seem to have inputs for both sensors. When I have time, I will try and run the test outlined in the repair manual, but I might just replace the sensors. The first thing to do I suppose is to look under the car and make sure there are no obvious exhaust leaks. The downstream sensor is about $70 but the upstream is twice as much. I have 65K miles, not sure if this is a warranty repair, but if it is the CAT, it would be.

 

The dealer is correct. Replacement parts are warranted for the balance of the original vehicle warranty (which in your case has expired) or the one-year period, whichever is longer.

 

Sorry to hear that. My car's CEL has not come back on since I reset it, but I expect it will take 1 to 2 weeks.
We will see.

 

BAck on today. No time to look at it until next weekend.

 

I ran some tests yesterday. It looks like the CAT is not working. According to page ES187 of the manual, the heated oxegen sesnor after the catalyst should be stable, but is was jumping around. I could not locate the sensor before the CAT on my Autoenginuity. Anyone familiar with that? I think with 67000 miles, this should be under warranty, but I really don't trust the dealer.