Check engine lights on and off periodically
 

ES 2010, owned since 2010, check engine lights on and off very infrequently around 2013-14, then the lights were on more frequently, and then off automatically. Replaced the gas cap twice, but did nto solve the problem.

Last year, the issue became out of control. The check engine lights were on most of the time, every month, may be they were off for 5 days (with no human intervention).

Brought to Toyota dealer, paid 129 for diagnostics. Tested, found no issue. I insisted on checking the canister, then they found the canister was sticky. Canister was replaced, also the battery was also replaced, with a total of $1200.

This is the the only repair I did for this ES for my 9+ year ownership. Maybe it was the damage I did to the car when topping off gasoline a few times many years ago. Would never do that again.

It is my understanding, and I could be wrong, that if the engine light comes on, then goes off, the engine computer will still store the code that caused the engine light to come on, unless you clear the memory by disconnecting the battery or using a code reader to clear the codes before retrieving them. That could apply only to certain codes cause they say it does no good to clear an engine light with a code reader, then attempt to get the car smogged because the technicians will look for erased codes.

A couple of comments:

If the MIL/SES was on solid then the dealership should have provided you code(s) that was read and that could lead them to fix whatever. When the car runs, there will always be pending codes that don't turn on the MIL/SES light, and depending on what they are and what their trigger is will depend on if they will become permanent MIL/SES code or not. Dealership will almost never repair anything based on just the pending codes, not until they become permanent codes.

The fuel filler pipe has a breather tube attached to it, and topping up the fuel up to the cap can cause the fuel to get into this breather tube and mess up the charcoal canister. Because of this, there is a note in the OM when filling the tank to fully insert the nozzle and to stop after one click:

4.4 Refueling
Observe the following precautions to prevent fuel overflowing from the fuel tank:
●Securely insert the fuel nozzle into the fuel filler neck -> breather tube is near the cap and inserting nozzle all the way ensures that the fuel is not pumped into it
●Stop filling the tank after the fuel nozzle automatically clicks off -> this is so that one doesn't fill the filler neck to the cap