o2 sensors

when's a good time to replace o2 sensors? we have 140 000 kms, 0r 86 000 miles. whats the symptoms for replacement, would after market sensors be ok say
from bosch etc since its a bosch management system in the accent any way..? I think were slowly loosing mileage, we get about 500 km per 45 l tank in summer driving stop/go in town... im anal when it comes to maintenance, so id like to know whens a good time to look at replacing them..
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O2 sensors are no longer a maintenance item. They should be replaced when they stop working properly. Trust your check engine lamp to turn on when that happens.
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never had the light come on for any long period of time. but it does come on when we get real cold snaps in the winter, then goes off same day. doesnt always come on when super cold, just once in a while. seems to be when ever weve started it with remote starter.. by cold snaps i mean like - 45 f or more.

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I'm not sure about where you live, but by U.S. standards, -45 F sets a coolant temp code. If your car runs the same computer logic and sensor, that'd set the lamp. (The computer thinks the coolant temp sensor is open).
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I live in northern Canada, I also mentioned before that the rad fan runs at cold start when verrry cold out. the fan works normally in the summer. i have a new switch actually for that that goes on the thermostat housing (it has a square snap on end with two prongs inside) not the other one on the housing.. also, do you think theres a way i could get some more heat in the winter by replacing the origional thermostat? im thinking of replacing since were closer to 100 000 miles now...

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The switch you describe is the coolant temperature sensor.
It's possible your fan is coming on because the coolant temp sensor is reading so cold, the computer thinks there's a problem and turns on the fan to safeguard against overheating (because it thinks it doesn't know the actual coolant temperature).
You could try replacing the factory thermostat with a warmer one, but you could of course cause overheating.
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On my accent I had the check engine light go on. I unplugged the upstream O2 sensor there was no change when the downstream one was unplugged the light went off and is staying off. Should I replace the downstream sensor or did it correct itself ?
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No, you shouldn't replace anything. In fact, your actions have possibly set erroneous fault codes. You should clear whatever codes are in the ECM, wait until the check engine lamp reilluminates, and check to see what codes are present at that time.
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i havent replaced the sensor yet , ill wait t ill winter.... how long will it hold the codes?

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I don't recall off the top of my head. It's either a predetermined number of "drive cycles" or vehicle starts. In any event, if the lamp is off, I doubt it will be around come wintertime.
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thx tech , gonna put the new sensor in manana.. =)

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I either hadn't realized how cold it was or hadn't thought about the significance of the cold when you posted about this topic earlier. It's entirely possible that the temps are just too cold for the engine management system to function correctly regarding interpreting the coolant temp sensor and that replacing it will have no effect.
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