I need to know, on a 2001 T4R, is there a functional difference between clearing a code or codes with a scanner, and disconnecting the battery.

Thanks,
Michael

The obvious difference is you never get to learn what the codes were if you don't use a scanner.

I am not sure on this but I believe disconnecting the battery also erases more than just the codes. It also forces the ECU to re-learn everything it had learned about fuel trim and timing--so mpg and performance may be affected in the short term. Clearing the codes simply makes the ECU re-run and complete the various tests before you get a ready flag, but doesn't erase the other stuff.

I welcome correction from anyone with a deeper understanding.

Thanks to TheDurk:

The obvious part was truly obvious - it was the 2nd part of the answer I was really looking for.

I had a P0420 code that I thought I had corrected with the install of a new O2 sensor (rear sensor downstream from the cat). I have put on probably 500 miles, and gone through 2-3 tanks of gas since installing the sensor and clearing the codes then. Also, had an opportunity to disconnect the battery for several minutes last week when I had a battery related issue due to the cold weather we had.

So - today, out of nowhere, I get the P0420 code again. Wondering what could have caused it. Hoping its not the cats, not smelling anything, and the vehicle itself seems to be running fine.

Any ideas....?

Update: When I got this code today, I had to get a new scanner - the other one is very basic, and not available to me today. So, I run the scan when the CEL comes on, I see the P0420. Some of the indicator lights on the scanner are flashing - indicating monitors not ready. Two of them relate to the Oxygen Sensor, which I have to assume at this point is the one I just replaced a couple weeks ago. That replacement was a denso part, could it have been a bum sensor?

1st of these flashing indicators was for "Oxygen Sensor Monitor"

2nd of those two flashing lights were for "Oxygen Sensor Heater Monitor".

Two others were also flashing, related to the comprehensive component module, and the other was for the catalytic converter monitor. I would expect those given the P0420 code overall.

Any thoughts on this?

Weird. I don't see how it could set a P0420 if the sensor monitors are not complete. Those normally complete almost instantly--especially the heater monitor--and if something is wrong with one of the o2 sensors, a P0420 is suspect, as it is generated by comparing the output of the two sensors. I'd clear with the scanner, and let the ECU run its tests again.

I guess I should clarify - those monitor codes were flashing when the p0420 presented, and after I cleared it.
I Have done some more driving since, gonna go out shortly and run the scan again to see if any of them have gone active.

Most scanners to clear codes do such with key on, engine off (KOEO).

As Durk mentioned, the ECU has no relearning to do with a scanner, for it just has its code trip cache so to speak cleared. I can't speak on other merits or drawbacks for doing one or the others, other than maybe convenience.

Disconnecting the Battery is a thing of the past. I can't speak directly to our obsolete 3rd gens, but the newer vehicles keep the code regardless of the battery. And they set markers so even if the code is cleared it is still in memory for a number of start cycles or miles, or both. Scan tools have become a necessary evil, preferably one that has live data to help diagnose problems just like this one.

Thanks BallEngineer - Not to start an argument for sure, but, I literally just got through reading that when you clear the codes, everything, including the freezeframe in the ECU at the time of the code, is cleared. Nothing is stored in regards to the actual code. I believe there is cycle data stored if applicable, but not actual fault data, based on what I just read. (I was reading the online manual for the reader actually).

So, a quick update, I just scanned again and the O2 sensor monitors are now active, no longer flashing/pending. So now I am waiting to see if the Catalytic Converter monitors kick in as well here soon. Let the waiting begin.....sigh.

That is correct, the marker remains for emissions testing purposes. So you can't just clear codes and expect it to pass a test. They want the problem gone, meaning you have to drive it for the marker to clear. Sorry to not be clear about that. I don't fight, I just watch from the couch!

On GM and some others there is a history module that stores the data. I do not think its cleared with a battery disconnect, but you can clear the history.

You are absolutely correct, sir. Thanks very much for your input on this!


Michael

Just an FYI OP , there is no need for a fancy scanner like Scanguage to get codes.

I like my Scanguage but a $30 Bluetooth OBDII scanner from Amazon combined with $5 Torque app on your phone will be just as effective.

Actually, I got this Innova model 3020 on sale today for $29.95 at Autozone. Nothing fancy at all, it gets the job done. The other one I have that I did not have access to is even more basic than this one. So...now all I need is the app you mentioned....Thanks!

This isn't precisely correct, or at least isn't clear. Battery disconnect or with scanner: the DTC codes are cleared completely. The readiness markers are all set to Not Ready, and all the tests have to be rerun and completed, pass or fail, before they get set to Ready. But there is no memory retained of the previous Fail code whatsoever. I am talking specifically about the 3G 4Runners.

Some states allow one or more 'Not Ready' markers to pass emissions. NJ, for one, does not.

Most of the tests complete instantly or very quickly. Evap and Cat efficiency can each take quite a while. If you drive to fit the required drive cycle, you can get it done faster.