Help! CEL P0420

[kwkshft]

2009 Yaris S 23,0000 miles. No mods on engine, just a TRD exhaust which I've had for 3 years. Car is used for a lot of short in-town trips. I went on a longer trip last weekend. After the first 90 miles, I filled it since it was almost empty. Drove another 40 miles to Ann Arbor, MI. When leaving Ann Arbor, the CEL comes on. I immediately think it's the gas cap and keep going since the car is driving fine. I topped it up at the gas station that I filled it earlier in the day made sure the gas cap was on right. After a couple of days, CEL is still on so I take the negative terminal off to clear it. I made sure again that the gas cap was on right. CEL came back on after another 2 days. I took it to Advance Auto and their scanner said P0420 Bank 1. The guy there said its the O2 sensor. So did I get water in the tank? Thanks in advance.

[CTScott]

P0420 is: Catalyst System Efficiency Below Threshold (Bank 1)

That triggers when the two sensors are reading too similar of values (the one after the cat should not read the same as the one before the cat if the cat is working).

It can be caused by a bad CAT or by a bad O2 sensor.

[kwkshft]

The guy at Advance Auto said it just takes a little bit of moisture on the O2 sensor for it to go bad. So I thought water in the fuel system.

After watching this vid:

http://www.youtube.com/watch?v=9VZ5K8n5jj0

I bought an infrared thermometer to rule out the catalytic converter. I will update this post with what I find.

[CTScott]

The guy at Advance Auto said it just takes a little bit of moisture on the O2 sensor for it to go bad. So I thought water in the fuel system.

After watching this vid:

http://www.youtube.com/watch?v=9VZ5K8n5jj0

I bought an infrared thermometer to rule out the catalytic converter. I will update this post with what I find.


That's a good approach. I have heard many stories of people replacing the cat and then later finding that one of the sensors was the actual issue.

[esse10]

I don't think it's the CAT either. I think you got into some bad gas, or perhaps a rapid change in atmespheric temperature and pressure will also trigger that. Before changing parts I would trace back your steps; fill the tank with high octane gas at the station you used to fill all the time, reset the car's computer. Drive it for a while and see if that nasty P0420 stays gone. Then I would try the O2 sensors next............... and I don't want to ask how much does a CAT cost for these cars. :laugh:

[kwkshft]

Thanks! I will do that. Yes, I got into some heavy rain that weekend on the way to Ann Arbor and it was also damp when it came back on yesterday. Luckily, the cat is under warranty for 8 years/80K miles.

[marcus]

dreaded P0420 on the subbie.... hey scott any way to pass emission on forester thats using catless 3 inch down pipe..i can clear code but emission shows not ready...:) and sensor was not being used...

[kwkshft]

Well, did the temperature test and the temp before the cat was approx 200F, after was about 350F so the cat must be ok. Going to find a scanner next to see what the voltages are from the O2 sensors. Meanwhile I'm going to run it till quarter tank then drain it to see if I find any water.

[kwkshft]

A week after I reset the CEL the first time, I reset it again. Did not do anything to the O2 sensor. It has been 4 days and it hasn't come back on again. Hope it stays that way.

[kwkshft]

Update on this. I never did anything but kept re-setting the CEL. I kept getting it back after varying intervals. Finally took it to the dealer today to diagnose because they gave me a 20% off coupon. It turned out to be the cat which is still under warranty so I get a free cat next week.

[Golddeenoh]

Update on this. I never did anything but kept re-setting the CEL. I kept getting it back after varying intervals. Finally took it to the dealer today to diagnose because they gave me a 20% off coupon. It turned out to be the cat which is still under warranty so I get a free cat next week.

nice. nothing like a free cat, still have mine, she is fluffy and still running around the house.. :biggrin: