Celica Forums General Discussion General CEL... mine likes to go OFF and ON... and then back OFF again

I have a CEL PO420 that keeps coming up for the past 5 months... it would bother me for the first two months, then i found out what it was - it had something to do with my car's CAT system ( )

well, the light usta come on just about every time i hit half tank of gas - then I'd reset it again ands the light wouldn't come back on again until i hit half tank again.

for the last two months, i just ignored the light and it stayed on for those several weeks. I've grown to just pretend that it wasn't there.

now for the past 2 weeks, i keep forgetting about it being on. but during my drive, it seems to magically attract my attention... hmmmmmmmm

I decided to reset it again for the first time in 2 months. This time i reset it right before making the 300 mile trek out to Las Vegas last weekend - and sure enough, the light came on again within the first 150 miles (about 1/2 tank)

i ignored it... but when coming back from Vegas, i noticed that the light was not ON all the time... it only turned on at half tank, but was off all before that point (i wasn't really paying attention to it, up until the point when it turned ON again)

Now tonight, I actually witnessed the light turn ON again... then drove 10 miles to my parents with the light ON and during that time, i watched the light turn OFF... its been OFF ever since - but i assume it will be back again.

how weird is it that a CEL will just "fix it-self"?
...is this totally unheard-of

That's straange, don't know what to tell you but good luck figuring it out.

You are the MASTER baby!!!!
j/k

haha same here isaac, sometimes my CEL pops up and then after a day or so it just turns off.

must be something wrong or else the light wouldnt turn on and off for no reason. inspect the part that the code says. take it to dealer or something. all these 2 years im working by a shop, there NEVER a time where a problem is unsolved. i know im not helping you guys in any way, but i would definetely check it and fix it, cz when the CEL really needs to come on for something major, you wouldnt care or you wouldnt know that something else is wrong cz the light is already on

The only time I have had that problem is when I have a had a CEL for evaporative emissions (cant remember the number of the code). Well basically it was because the gas cap wasnt making a good seal and the light would come on with about half a tank and then sometimes go off on its own while driving and then it would also go off sometimes when I re-filled the tank.

i've noticed mine goes on when it's just about time for an oil change...and i get the same code. Something with the emissions system. i don't have the on-off problem tho.

My CEL has been coming off and on lately. I dont know what to do cause some days it will be on then somedays it will be off. Im sure if its something major the CEL will just start blinking out of control then i will take it in lol

CEL Diagnostic codes...
https://celicahobby.com/ubbthreads/thread.f_1870112628_0_collapsed_5__1.html

if your cat is fucked up, it might give different readings...


what i'd do:

1: replace the cat with a "performance" cat, ytou might make 1 hp in the process...

2: remove the cat, and get one of those switches that fool the sensors into thinking everything is good...

... thats if the cat is the problem...

the fact the the gas level has something to do with it is weird...

Isaac, evap. control codes do not reset themselves.

Chances are, if it's coming on/off by itself, you have a more serious electrical/ECU related problem.

EVAP/Emissions codes are programmed to remain in the "On" position until removed, and they are even stuck in memory after you flash the ECU for up to FOURTY times. (I.E. you leave the gas cap off, you will get a CEL from that until you reset/start/run the car for 40 cycles)

Have them do an electronic's dianostic check on your cars system...

i have done that already - thats how i knew it was PO420, CAT SYSTEM
https://celicahobby.com/ubbthreads/thread.f_1870112628_0_collapsed_2__1.html

unless you were taling about something else?
thanks, it is good advice though

Might wanna check/replace your heated O2 sensor.

i'm kindda at a loss here , I don't think I can help any.

Thats what I was gonna say. I had an O2 go bad, and got similar CEL activity. It also ran poorly tho..like bad gas!!

try running some good fuel system cleaner through the fuel system as that can clean dirty 02 sensors and may save you the trouble of getting new one(s) especially if your warranty is up. Redline is SI-1 is strong stuff.

O2 sensors are in the exhaust. Fuel cleaner won't help them