Check Engine Light (again)

[t4x4_offroad]

Hi Guys,

I wrote a few weeks back regarding the check engine light that pointed to a bad O2 Sensor. (the code was P0136). What happened next puzzles me...

I left the check engine light on for a few weeks, planning to fix the O2 sensor soon. A couple of days ago, the check engine light was gone! I thought maybe the bulb was blown but I checked (by turning the key to the "on" position to check all the bulbs in the dash) and the check engine light bulb was ok. So prior to a couple of days ago, the check engine light would stay on with the engine running. Now, the light does not come on when the engine is running.
I plugged in my OBDII code reader last night and was surprised to find that the code (P0136) was still stored in the engine's computer, even though the check engine is now gone.

So my big question is, does the engine computer automatically turn off the check engine light but still storing the code, or does this mean that O2 sensor "fixed" itself?

Any advice or suggestion appreciated. Thanks.

Dennis

[Axel Kain]

is the car running rich, or lean, if not then it was a system glitch, it happens sometimes, it just takes a little while for the computer to cycle and get back in sync with things.

[Mike Gerber]

The check engine light will return soon. It went on and off a couple of times in my 98 Camry 5SFE engine. I had the same code. I replaced the rear O2 sensor with a Toyota OEM and it never came back on.

In answer to your other question, the computer (ECU) keeps the code in memory until it is erased with the proper scan tool. Some have said they were able to erase it by removing the battery cables and then stepping on the brake pedal to dissipate any remaining electrical charge still in the system.

Mike

[lalo]

DIAGNOSTIC TROUBLE CODE CLEARANCE
1. After repair of the trouble areas, the diagnostic trouble
code retained in the ECM memory must be cleared out by
removing the E F I fuse (15A) from J/13 No. 2 for 10 seconds or
more, with the ignition switch OFF.
HINT:
• Cancellation can also be done by removing the battery
negative (–) terminal, but in this case, other memory
systems (clock, etc.) will also be cancelled out.

[t4x4_offroad]

Thanks for the reply guys. I also failed to mentioned my car is a 1997 LE 4cyl (5SFE) auto with about 90000KM or 54000 miles. The O2 sensor at fault is the one between the cat and the muffler.

From the replies, I gather that I have no choice but to change the O2 sensor. I will be doing that soon.

Thanks again!

[thisisntjoew]

From what I learned in auto shop class, the light turns off after so many "key cycles" so you probably need to address the problem because its still stored in the computer. (A key cycle is a key turning on the car)