D/C battery...CEL went off and never came back on

[sigley0527]

As I was slowly creeping in traffic a few weeks ago the dreaded "Check Engine" light turned on. That same day I went to Autozone to check the code with the scanner. I got one code, P0420. This was the first time my Runner ever threw this code. I understand this code could really be one or more of three things; downstream o2, catalytic converter, or an exhaust leak. I know that I do not have an exhaust leak and I've heard most of the time the problem is the downstream o2 sensor. So I bought the sensor from Toyota and its sitting in my room. Just for the record, I have a relatively new A/F ratio sensor that I put in last year in an attempt to fix a different code (ended up being a TPS sensor.)

ANYWAY, after I got home from Autozone I disconnected my battery cable in an attempt to reset the ECU. Long story short, I drove 650 miles since then and the light never came back on. I didn't even reset the computer with the scanner. Usually when I simply disconnect the battery, the light comes back on in like 5-10 miles. I have not installed my new downstream o2 sensor yet because the light has not come back on. I am thinking of bringing it back to Autozone and run the scanner again to see if the code is still in the computer. I am pretty sure at one point or another the light will come back on, but is it possible it was just a fluke that it came on in the first place?

 

[shootmymime]

Because you were in traffic it just built up some extra fuel (or lack) and the value was not in the range so i set the code. traffic is hard on vehicles.
since you have driven so much since then and no code i would not worry about it.
return the part and forget about it.

 

[TheDurk]

I'd hold onto the sensor. When mine went bad, it would go a few months between resets at first, then a few weeks. Eventually, it would come back within a tank of gas, which is as fast as the Cat Efficiency 2-trip detection can complete its cycle. It is by far the slowest test to complete. If your sensor was original, it is due. You'll need it eventually. Probably sooner than later.

There is no difference between a scanner reset and pulling the battery. The system has to rerun all the tests. Most are immediate. Some are two-trip but relatively quick. Cat efficiency and evap are more involved.

 

[BlackCat914]

Because you were in traffic it just built up some extra fuel (or lack) and the value was not in the range so i set the code. traffic is hard on vehicles.
since you have driven so much since then and no code i would not worry about it.
return the part and forget about it.

You do know that a P0420 takes more than one key cycle to set, right? It has to fail once, then fail again during the next key cycle.

 

[shootmymime]

what about sea foaming it to maybe clean the o2, if its blocked of clogged could be having problems.

 

[sigley0527]

Thanks for all of your replies.

Since I disconnected the battery, I have driven 1000 miles and the check engine light never came back on. Within the 1000 miles I have went through many many key cycles as well. I even took it back to Autozone today to have it scanned again and there were no codes that showed up. I was surprised by this because even though the CEL never came back on, I figured the code would have still been stored in the ECU. Considering the CEL did not come back on, and there are no codes showing up on the scanner right now, I am going to just get my inspection over with on Monday while I'm in the clear. I've decided to hold onto the O2 sensor that I bought from Toyota just in case. After my inspection, I might even change it just for the heck of it considering its the original sensor.

 

[sigley0527]

Because you were in traffic it just built up some extra fuel (or lack) and the value was not in the range so i set the code. traffic is hard on vehicles.
since you have driven so much since then and no code i would not worry about it.
return the part and forget about it.

I suspected something similar to this but it seems BlackCat has a point if indeed what he says about P0420 needing more than one key cycle to set the code is true. But even if BlackCat is right, I have driven 1000 miles after disconnecting the battery and the code isn't stored in my ECU anymore. So I really have no clue why it came on in the first place. All of the CEL's I've dealt with in the past have always came back on after d/c the battery or resetting the computer with the scanner. Usually it comes back on much faster when I only d/c the battery opposed to clearing the code the right way with the scanner. I guess I will just have to wait and find out what happens. In the mean time, I'll get this inspection while I can! :rapture: