OK from the top, the malfunction indicator light (MIL) is more commonly known as a check engine light, does not light up in response to a need for routine maintenance. Despite its rather misleading name it is a malfunction indicator light.
The ECM in the car monitors a whole bunch of sensors, and their operations. Among these sensors are the oxygen sensors. Older cars had one oxygen sensor, newer cars (OBDII) have two one in front of the converter, and one behind. If you have a V6 or V8 engine you might have 1 or 2 sensors per bank, ie a total of 2 sensors on an early car, and four on an OBDII car.
So what do they do? On a proper running engine, the front oxygen sensor(s) indicates if the engine is running slightly rich or slightly lean from what has been programmed into the ECM mapping for that running condition (There is no way to hold an exact fuel mixture, so the ECM hunts just slightly rich or lean of the where it wants to be). It should work like this:
Exhaust rich -> O2 indicates rich -> ECM leans mixture -> exhaust lean -> O2 indicates lean -> ECM enriches mixture -> lather rinse repeat.
The rear sensor on an ODBII car acts as a “traffic cop” to report if the converter is actually working or not. Again in a proper working car the front sensor should be cycling rich/lean/rich/lean. However on the back side of the converter the O2 sensor should be indicating a steady (or fairly steady) signal indicating that the converter has converted the HC, CO, and NOX in the exhaust. If there is a steady signal from the rear sensor, then all is OK, if the rear sensor is cycling rich/lean/rich/lean just like the front sensor, the ECM interprets this as a bad converter. Rear sensors may also have some slight fuel trim effects. In otherwords if the front sensor is always just a little rich, it cycles OK, but its switchover point is just a little richer than it should be, the rear sensor will report this. The ECM will then adjust the fuel mixture to compensate for a front sensor that is just not quite right. I don’t think this is on all OBDII systems, but I know some systems do this.
So what happens if I say fuck it and ignore the light? Well if they do emission testing where you are, you can kiss off passing. Pretty much every emission test I have ever seen has a step where the inspector turns on the key and verifies the light comes on, then starts the engine to verify that it goes out. if that does not happen you fail.
But will I do any damage? That as they say is the 64,000 question. The answer is you could. Depending on just how and which sensor fails, you could any of the following outcomes:[ul]
[li]Nothing obserable :) [/li][li]Increased fuel consumption :( [/li][li]Melt down the converter :eek: :( :eek: [/li][li]Wash down the rings from too much gas, and wear out the engine. :( :mad: :eek: :( :mad: :eek: :smack: [/li][/ul] (in the above list you may substitute signs for smiles)
Unfortunately without knowing a bunch more about the exact code being generated, and seeing the engine and the data stream out of the ECM I cannot give you an estimate of just what is wrong. If you would like to mail me your car I can fix it.*
So what causes the damn thing to go bad? Well, lots of things, and many of them are not obvious. In no particular order any of the following can either kill a sensor, or give a reading that makes the ECM think the sensor is toast[ol]
[li]Air leak in the intake[/li][li]Air leak in the exhaust (could be so small it makes no audible indication)[/li][li]bad injector(s)[/li][li]Bad Mass Airflow Sensor[/li][li]Bad ground[/li][li]Oil leak[/li][li]Silicone spray[/li][li]RTV sealants that contain silicone[/li][li]Blown head gaskets[/li][li]Oil leak[/li][li]Improper installation[/li][li]Leaded gas[/li][li]Octane boosters[/li][li]Bad thermostat[/li][li]Misfire[/li][li]Overheat[/li][li]Dielectric grease or any other type of grease on the connector to the O2 sensor[/li][li]Plus probably some other that I am not awake enough to recall[/li][/ol]
Notes on the above
!, 2. any extraneous air will cause the computer to think there is a problem. Air leaks can be either on the intake side or exhaust side of the engine. Yes an exhaust leak can draw in air and cause bad O2 sensor readings.
3. poor spray patterns, or leaky/drippy
4. If the MAF is bad but not far enough out of range to alert the ECM the failure may show up as a adaptivity code, which can lead to a O2 sensor replacement.
5. This is an electrical circuit. You have to have a good ground. Some sensors ground through the threads of the sensor, and then up to the engine block through the exhaust. If the threads got rusty (which they can) sensor readings went all to hell)
6. Sensor need a supply of outside air to reference. If oil were to leak into the reference chamber, then the sensor goes to shit. Some systems pull the reference air from inside the wires that feed the sensor. If oil gets into the harness (yes it can happen) then the sensor gets contaminated. A replacement sensor also gets contaminated.
7, 8, 9. Silicone kills oxygen sensors. Period. Silicone sprays, sealants containing silicone, and if there is any kind of head gasket leak, coolants that contain silicone will kill the sensors. Don’t use silicone spray anywhere in the engine room. If you use an RTV sealant it must say either low volatility or sensor safe of the package.
12, 13. lead also kills sensors. You probably can’t get leaded gas anymore, but some octane booster may contain lead. Lead fouls O2 sensors.
14. If the thermostat open just a bit too soon, and the engine never quite reaches operating temp, the ECM will continue to provide a little bit of enrichment. Meanwhile on the other side of the ECM the O2 is reporting a rich condition. If this goes on for too long it may be recorded as a mixture code that may interpreted as a bad O2.
15, 16. A bad misfire or overheat may damage the O2 which will then show up later as dead.
17. Can be the same as an oil leak into the harness. Contaminates the sensors from the inside.
OK, I will stop teaching now. Getting back to the OP. Damage the fuel line? Scuse me but WTF? A bad O2 reading can damage many things, but the hard plastic/steel line running from the gas tank to the engine is not one of them. :dubious:
A hoax? :rolleyes: Again WTF? Spoken like someone who technical automotive knowledge stops at “Fill er up” :rolleyes:
Front more important than the rear? Well maybe, but the systems is designed to operate with both. I cannot predict just what the limp mode is for you particular car if the rear fails.
Can you replace it yourself, is the aftermarket one as good as the factory one? Depending on the car, the aftermarket one may be fine. Some cars will not tolerate anything but the factory piece. Other don’t care. Can you do it yourself? Beats me, Can you? do you know exactly what is involved on your particular car to change the sensor? $200 is a lot of labor. It probably is not a 15 minute job. On some cars the exhaust has to be dropped to access the sensor. On one model Audi you have to remove the engine to change the O2 sensor. :eek:
*this is why I always love it in car repair threads where somebody says, just go to Auto Zone and have them read the code. The code alone without the data stream will not always give you the answer.