Worth Replacing O2 Sensor on Crummy Exhaust?

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TLCA #19645
Dec 6, 2008
Tujunga, CA
Like the title says, is it worth replacing my o2 sensors if my exhaust is crummy? Or should I replace / repair the whole exhaust and then put new sensors in?

I've been getting an intermittent Code 25 CEL that I think I have tracked down to one of the o2 sensors, and it's causing some rough running / idling and higher fuel consumption than normal. But, my exhaust is in terrible shape and I just don't have the money to even think about fixing or replacing the exhaust yet. It's burned through and leaking in a couple places both before and after the o2 sensors and I worry that it will be a waste to put new sensors on a crappy exhaust. What does everyone think?
short answer is yes. as long as you have determined that there is not air being pulled into the exhaust ahead of or just aft of the sensor. there is also a diagnostic test for the O2 sensors you can run to see if it is failing/failed. it is in the FSM. I'd run that before spending any more time/money on it. back when I changed my sensors, I got steady CEL with 25+26 codes, then ran the diagnostics on the O2 sensors and had one that didn't pass, so swapped them both with new densos. been fine since(well, the O2 sensor circuit has been fine ever since...)if your exhaust is cracked up and not sealed, you might think about doing it all at once...HTH
Just wanted to update this thread in case anyone was wondering the same question. I ended up having both O2 sensors replaced, as well as a new muffler and and exhaust hangers underneath the truck. One of the O2 sensors was definitely the culprit; when tested for voltage according to the FSM, it was showing only 0.5 volts for one sensor when warmed up, which means that sensor was very intermittent, and causing a lean/rich imbalance. The difference was night and day, and my exhaust was confirmed not pulling air in before the sensors, so very minimal repairs and changes were needed. So for anyone else curious, even if your exhaust sucks (as long as its not before the sensors), replace them!! Totally worth it!
@LAMBCRUSHER I've been getting the 25+26 CEL for a few months now. I typically pull out the fuse to reset and it's good until I have highway trip more than 40 miles. I'm going to poke around underneath today, take a look at the 02 sensors. Is there anything I should be looking for? Thanks in advance
My 3Fe in my 1991 FJ80 had a rough idle. I changed both O2 Sensors. It wasn't a huge change. Worth it, sure. But I would not do it for the gain for fuel consumption.
My biggest change and I tell any one to do this as it's super easy is to take the two vacuum hoses that come off the top of the motor and insert a BB gun pellet into each one. It's not permanent. But it doesn't allow vacuum to go to the EGR valve, so it stays closed.
My idle was night and day better and the rough running went away. Also my MPG increased from single digits to 12-14.

Depending on where you live you can do an EGR delete or not. But this way it sort of does that by keeping the valve closed. That is if the valve still works.
the o2 sensors- look at the wiring harness...if the signal the sensor is supposed to send the ecu isn't getting there intact, then the ecu is just going to see jibberish, and return jibberish, and an open loop mode with a code; codeS.... look for damaged wires, loose connectors, corrosion or dirty connections...test the sensors at the diagnostic box via a volt-ohm meter ala FSM test, analog meter is easier to read from than a digital...
Just took a look and all seems intact. I don't know the names very well but yellow connector on both sensors looked to be in good shape and the part that connects to the pipe both seemed fine too, although hard to look too closely with the little guards blocking.

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