warthog

Can an O2 sensor go bad and cause you to run rich without popping the CE light. And explain how if you can,
Thanks

djb383

I would think highly unlikely with no CEL, especially OBD-II. Even in "open loop", presuming all is functioning correctly, the system functions only slightly rich (cold engine running on factory programing/mapping). EFI atomizes fuel very well even when cold, far better than any carburetor so the A/F ratio just doesn't need to be very rich, again, presuming everything in the system is functioning correctly.

4.3L XJ

Yes it can. The CEL goes on when the heater fails. They can wear out. They should be replaced regularly, about every 50,000 or less

djb383

Yes it can - what......go bad and not trigger a CEL? Not all O2s are heated, so there's no heater to fail.

4.3L XJ

The late model O2 sensors are heated. If it has more than one wire, it is heated. Here is something from Wikipedia

The sensor element is a ceramic cylinder plated inside and out with porous platinum electrodes; the whole assembly is protected by a metal gauze. It operates by measuring the difference in oxygen between the exhaust gas and the external air, and generates a voltage or changes its resistance depending on the difference between the two.
The sensors only work effectively when heated to approximately 316 °C (600 °F), so most newer lambda probes have heating elements encased in the ceramic that bring the ceramic tip up to temperature quickly. Older probes, without heating elements, would eventually be heated by the exhaust, but there is a time lag between when the engine is started and when the components in the exhaust system come to a thermal equilibrium. The length of time required for the exhaust gases to bring the probe to temperature depend on the temperature of the ambient air and the geometry of the exhaust system. Without a heater, the process may take several minutes. There are pollution problems that are attributed to this slow start-up process, including a similar problem with the working temperature of a catalytic converter.
The probe typically has four wires attached to it: two for the lambda output, and two for the heater power, although some automakers use a common ground for the sensor element and heaters, resulting in three wires. Earlier non-electrically-heated sensors had one or two wires.

O2 sensors generate their own electricity, and just like solar panels, give diminished output with use. Check the Wiki website and you will find that with diminished output, the mixture will become richer. You just have to change them. If you put 300K on them they are not going to work, but may not generate a CEL

warthog

Thanks for all the Info. I have 160K on mine and not sure if the O2 sensor has ever been changed. I have been told there are two and i am sure they are both important but seeing as cost so much. Should they both be changed at the same time to get the best results?

Thanks again for all the responses

storm101

I think if you have an issue with an o2, it would be the upstream, IF and i mean IF i am correct with what i think i know on this lol, the upstream one is what sends the signal back to correct the fuel ratio from what its reading (which would obviously be wrong if its not working properly).....and the downstream is pretty much a supervisor for the cat...not totally sure how they tie into eachother or how replacement goes as I havnt really had to fiddle with them too much, but if youre going to do one, I would say the upstream would be the one to start on.

I have on the other hand seen some go bad on some other vehicles (not mine) that didnt toss a code....

4.3L XJ

The upstream, or the first one is the most important. It measures the exhaust before the cat and is the one that determines the air fuel ratio. If you are going to change one, that is the one. If you change it and get a CEL, it is because the second one, being worn out is, "detecting" a discrepancy in what the mixture should be after the cat.

ca441970

I changed my upstream sensor, and the NEXT DAY, I got the code for the downstream. I will just replace both at the same time when they are due. They are so easy to get to on XJ's anyways.

warthog

Yeah i guess i will just save up the extra bit of money and get two new ones at the same time.
Thanks

wildbill1

What brand O2's are recommended? I'm thinking of replacing the O2's in my '99 XJ (120K miles) to improve fuel economy.

tiwvr1

bosch for the brand, as far as getting a code for the downstream after the upstream has been changed, clear the code and it should go away. It is reading that the long term fuel has changed. As soon as compu. relearns it self it should go away.