Oxygen sensor tune up

[CamaroRScrazy]

Hey,

Do you guys change the Oxygen sensor after a certain miles or years?

[poormillionaire2]

I won't worry about it until your car throws a code for it. It should last for the life of the car if maintained properly.

[Mr. Luos]

They normally start getting lazy at around 100K miles.
Some go longer, some go shorter.

[silicon212]

What he said.

[Morley]

50k miles is the mfg reccommended time for 02 sensor changes. Since this is the only real feedback the ecm gets on weather the fuel/air mixture is correct...why wait until it says it is bad? You can have an 02 sensor that is completely out of tolerance and never throw a code.

[wrightz28]

With Morley

Your fuel mileage will suffer long before the MIL ever comes on, i fit does at all. OBD II is more more adapt to pick it up, but OBD I the CTS and o2 are your main players to set fuel trim. I usually replace mine at most every other year.

[Blue Bowtie]

The older, one-wire (non-heated) zirconia sensors were usually good for a maximum of around 50,000 miles of NORMAL operation and driving. If the engine has been running excessively rich for some reason, has had coolant leakage into the combustion chambers, has used fuel additives and/or "octane boosters" regularly, or has been assembled with non-sensor safe sealant compounds, you may get as little as 20-30,000 miles of accurate signal from a sensor. Bosch (formerly the OEM supplier to GM) used to recommend changing at 30,000 miles for peak performance.

The later style three- and four-wire heated sensors seem to produce good signal longer, but at about 50,000 miles they should at least be scrutinized closely. The factory still recommends testing of O²s and catalytic converter efficiency at 50,000 mile intervals, and replacement of sensors at 100,000 miles for most GM vehicles.

The typical failure mode of a sensor is a constantly weakening signal until the output level is below the minimum signal threshold. As wrightZ28 pointed out, this weakening signal is interpreted as a lean condition by the ECM/PCM interprets the input. This means the ECM/PCM will "adapt" to the weaker signal by adding enough fuel to satisfy the sensor. In reality, this usually means the engine is running unnecessarily rich just to satisfy a weak sensor. This can continue for years until the sensor finally fails to produce the minimum signal, wasting fuel and contributing to carbon buildup and plug fouling the entire time. On most older ECMs and newer PCMs, the lack of activity will eventually set an error code and light the MIL.

Unfortunately, it actually gets worse than that with some later PCMs, since the programming on some GM vehicles will check the main (front) sensors against the post-cat (rear) sensors routinely. If a front sensor falls below the minimum activity or signal level but the rear sensor is still within range, the PCM program will use a subroutine to "fool" the low O² input with a acceptable data, then use the rear sensor to trim fuel to some extent. All this while, the MIL is NOT turned on, and the driver has no indication of a dead sensor. What is even worse is that a tech who scans the PCM for data will see the dead sensor I/O data just as the PCM processor sees - with the "phoney" subroutine data instead of the real PCM input levels from the weak/dead sensor. Unless the sensor is checked directly with a meter or oscilloscope, there will be no indication of a weak or failed sensor. The only hint is the remote possibility of history or a pending error indicating a weak sensor, and that's a BIG maybe. My gut feeling is that this was done to prevent the MIL from turning on when the factory deemed it "unnecessary", even though a problem could exist. That's what happens when accountants are allowed to run a corporation instead of engineers, I suppose.

Regardless of the reasoning, you could be driving around on a weak or dead sensor without even knowing it, and scanning the PCM may not reveal it either. It pays to perform the recommended tests at the recommended intervals. Cheating will only cause problems, whether you are aware of it or not.