What I learned about replacing the O2 Sensor

[nan00k]

I recently replaced my oxygen sensor so that I could pass the California Smog Test. In case you need to replace your O2 sensor - and you don't know your ECM from your OBD - here are a few notes from my experience. Hope you find it helpful. (My car is a 98 Civic EX with 92k miles.)

 

1) If your check engine light (CEL) is on, try to find somebody to run a diagnostic scan for free. Some shops will do it for $40, others $80, others for free. A friendly mechanic at an oil change station hooked me up.

 

2) At the first hint of a problem, go straight to your library, or your library's web site and check out the Chilton manual, or perhaps Hayne's. My library even has an on-line auto-maintenance reference. These resources are very helpful, easy to read, with good diagrams, etc. Might save lots of time and money.

 

3) I shopped around for O2 sensors and found prices ranging from $40 to $300 with no obvious reason for the huge price discrepancy. I'd get the cheapest one.

 

4) The O2 sensor sticks out of the exhaust manifold. Look for 4 wires protruding from about the middle of the engine, b/t the engine and the radiator fan.

 

5) You can remove the sensor w/o first removing the manifold heat shield if you have the special slotted socket that is designed to slip over the entire sensor. But if you don't want to buy a special socket ($25 at Napa) just remove the shield, and use a good long crescent wrench.

 

6) Before you remove the sensor, drive around a bit, or just run the engine for a while to get it good and warm. This will help a lot. (And dont' forget to unplug it first! You might need a small slotted screwdriver to disconnect the pigtail. Pigtail = the 4 wires coming out of the sensor.)

 

7) Installation is reverse of removal.

 

Now, so far so good, but the check engine light will probably not turn off. If it did turn off skip this next bit. Mine didn't. In my case, the trouble code was still stored in the engine control module so regardless of the new sensor my chk engine light remained on. I tried to reset the ECM by disconnecting the negative battery terminal for a few minutes but that didn't work. Another technique mentioned on this forum (and in the Chilton manual) is to unplug the 7.5 Amp "Back Up" fuse in the fuse box located under the hood, passngr side near windshield). Unfortunately I misinterpreted this and unplugged the wrong fuse - so presumably this would do the trick.) I ended up going back to the same mechanic and he hooked me up again - erased the code through his computer.

 

9) Apparently it is not enough to have the light off, the ECM will not be ready for Smog Test until you drive the car a certain amount. You need to cycle the ignition several times and go for a drive, sustaining 65 mph for ten minutes. I followed this protocol and it worked for me.

 

10) Incidentally, in case you want to test the sensor or cable harness, the key to the wires is as follows: when looking at the pigtail connector electrodes, the top two wires are signal while the bottom two wires provide power to the "heater circuit" (at 12V). Top left is positive signal, top right is negative signal, Bottom left is positive power, bottom right negative power.

 

11) The Chilton manual says that the heater circuit should be b/t 10-40 Ohms, but my new sensor measured 4.6 ohms. I thought this might be why my light had remained on after installing the new sensor, but it doesn't seem to be a problem. That is, the check engine light has not returned since the ECM was reset.

 

 

Good luck!

[Ben.]

Not bad, bud.