E46 Fanatics Forum banner
  • Hey everyone! Enter your ride HERE to be a part of this month's Ride of the Month Challenge!

1 - 7 of 7 Posts

·
Registered
Joined
·
255 Posts
Discussion Starter · #1 ·
My 2001 325CiC tripped the P0128 code for the thermostat. This happened once before and changing the thermostat fixed it. I know this is minor and I probably won't rush to fix it. Everything seems fine with the car.

Anyway, the light was on for about a week, and then it went off for about two weeks and now it came back on. I thought the light was supposed to stay on until you reset it. Any ideas why mine went out and came back on?
 

·
Registered
Joined
·
204 Posts
You turned the car on 3 times without tripping the sensor, so it went off. There is still a thermostat issue that I would fix sooner than later, its not something you wanna let slide
 

·
Registered
Joined
·
255 Posts
Discussion Starter · #3 ·
Kwaz..thanks for the reply. But I didn't fully understand. Are you saying that each time I turn on the car it rechecks the thermostat condition...and after 3 successes turns off the light? I thought once on, it stayed on until a reset was performed.
 

·
Registered
Joined
·
204 Posts
Exactly, it checks it every time you turn the car on. After succesfully checking it 3 times, it'll turn the light off however It will still be in your computer. The code itself won't go away until its cleared with a tool
 

·
Registered
Joined
·
10,428 Posts
WITHOUT THE CODES TODAY, YOU DO NOT KNOW WHY THE LIGHT IS ON.

The light constantly checks the system, and turns on for any number of faults. The fault today can easily be different than the fault yesterday. It could be the same, but you CANNOT assume so. If you are working on the idea that the problem yesterday was XYZ, and the light is on today so it must still be XYZ, that is a false assumption. It may be true, but you have to be sure it is true and not work on the assumption.

And, yes, the car will eventually turn the check engine light off after the fault condition goes away. The car runs 12 monitors, and if the monitors complete their run without any faults, then they set themselves to Ready. This can take various numbers of drive cycles for the different kinds of faults, and most people simply reset the check engine light to force it off. The monitors still run and have to reach the Ready state before the car can pass a smog test. If the check engine light comes back on, then the problem can be the same or a new one. You can spend hours working on the same problem when the issue is new. You have to pull codes...
 

·
Registered
Joined
·
10,428 Posts
MORE

The car stores Active and Pending Codes. Yesterday's pending code can be today's active code. You can have an active code yesterday that is corrected while the pending code is ignored. The active code can be corrected -- a repair action has been taken -- and that fault will clear, leaving the pending code to percolate a while longer before it bubbles to the top and becomes the active code.

BOTTOM LINE
You have to pull codes to know why the light is on.
 
1 - 7 of 7 Posts
Top