The problem is leak in evap (evaporative emissions system). Also this will be resulted in many complications in the system as you weren't able to detect the problem. Please check it again with your OBD-2 scanner. Possible faults can be in transmission ( Transmission Fault Codes ) .

In my amateur opinion, OBD scanner is a must. As cheap as can be found (paid $23 USD for mine FRM Amazon) all DYI'ers should have one. Ashamed to admit I should've bought one much sooner. Could've circumvented lots of time, $$, and guessing.

After 5100MI since fixing the EVAP problem, another CEL/MIL appears....this time P1135. Quick reset with the scanner and almost immediately, CEL/P1135 re-appears. Tried 3 more times, comes back everytime. Little research reveals (among other factors) strong possibility of A/F sensor just FWD of CAT.

I carefully disconnected the harness connectors and cleaned contacts with Deoxit. I didn't feel like removing it (although you really don't have to, to ohm out) and checking the heater or sensor resistances. Re-connected, cleared CEL and good after 1000MI. Seems too easy/lucky coincidence. Betting this will come back. Engine portion of the harness looked satisfactory & serviceable, but didn't feel like doing anymore than a good visual inspection of it's condition.

So.........This sensor has been replaced twice over the 20yr life of the vehicle (2002 SR5 4Runner). Although I didn't have a clue back in the day, so just took it to shop and let them fix it. Invoice identified A/F/O2 sensor back in 2005 & 2012. Even back then the OEM unit shop identified seemed pricey.

As cheap as the Denso's are (today and to me anyway, ((it's also my understanding Denso makes these OEM for Toyota)), I said screw it and replaced the FWD (p/n 234-9002-my cost $138) and the sensor just AFT of the CAT (p/n 234-4261-$51.68). With a little patience, research, and the correct tools R&R of these sensors fairly uneventful & easy task. Probably should have waited until the failure returned, but wanted to take advantage of my motivation for the moment as I hate working on vehicles.

Sensors replaced. Cleared MIL with scanner. Going on 3000MI since, I swear besides the P1135 not coming back, the vehicle even feels like it runs better, although I think that part of it is all in my head ;)

My old 2002 just turned 20 this past November @313,045MI! I've never kept any vehicle that long. This one is really worth very little for insurance purposes, but I think I'll just keep doctoring it along to see just how long it will last!