Ok so my runner has really got me concerned about mileage...last winter I skewered a rod of rebar into the underside of my runner and it created a rattling noise.

Turns out I knocked a O2 sensor loose, no big deal, got it on a hoist and saw that they look to be original sensors (11 years old) This runne ris my next project so I've been hesitant working on it till I can do everything at once (We all know the pita it is to build a project and keep it driveable)

So I'm wondering, since a pair of o2 sensors 500 bones, If you can just replace to front one (ahead of the cat) and not worry about the rear...I've heard the rear is not important :confused: but as long as the front one is maintained everything is good...any insight? The motor is pushing 290,000 km so I'm hoping at the 300,000 mark I'll be freshing her up/ maybe rebuilding it...though the motor is not giving me troubles, I'm just seeing little maintenance issues arise...

The second 02 is there to check that the cat is working properly. Some people on customtacos.com have been removing the 02 and replacing it with a resistor.

BigMike was using a reader from http://obddiagnostics.com/index.html for $80. It will show you if they are working or not. In addition to read and clear codes.

Sorry but a simple resistor in not going to emulate the second O2 sensor.
It's a timed type of signal. The ECU is looking for the output to change in output and in such timed manner based on how a healthy CAT works and the what the input from the primary O2 sensor is.

You can check them out with any decent DMM. You can do an even better test by removing it and using a propane torch and the DMM. Now that is for the common 1 volt output types not for the newer current output type O2 sensors.

If you knocked an O2 sensor loose with a hunk of rebar, you should also be concerned with a leak in the exhaust. Especially around the O2 sensor as it has a gasket.

An air leak will create problems with what O2 sensors outputs ending up with sending shit for info to the ECU. If the ECU isn't pitching a code, it could very well be that the sensor element itself is OK but is telling the ECU garbage because of the air leak and it's none the wiser. Same thing as crap in, crap out. If the ECU sees more O2 than it should be seeing from O2 #1, the ECU says oh OK the engine can take some more fuel to burn with that extra O2 so it richins things up. You then wind up with raw fuel dripping out of the exhaust. And the CAT will probably burn its guts out with all that extra fuel in the process. Then you get to buy yourself a new one of those too.

If you knocked an O2 sensor loose with a hunk of rebar, you should also be concerned with a leak in the exhaust. Especially around the O2 sensor as it has a gasket.

An air leak will create problems with what O2 sensors outputs ending up with sending shit for info to the ECU. If the ECU isn't pitching a code, it could very well be that the sensor element itself is OK but is telling the ECU garbage because of the air leak and it's none the wiser. Same thing as crap in, crap out. If the ECU sees more O2 than it should be seeing from O2 #1, the ECU says oh OK the engine can take some more fuel to burn with that extra O2 so it richins things up. You then wind up with raw fuel dripping out of the exhaust. And the CAT will probably burn its guts out with all that extra fuel in the process. Then you get to buy yourself a new one of those too.


Wow, thanks for the info, I'm kinda sitting on the fence here, as I'm collecting parts I don't want to fix it till I have all the pieces, I'm happy with the 3rz, but the rig seems to have been a little neglected by the previous owners.

I'm planning to do a complete exhaust with cat and header and o2's, along with the rest of the build...I just wanna try and hold out for a little longer (may be leaving for police training soon) I never thought about the fuel problem from damagaing the sensor/exhaust...