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Secondary O2 Sensor Replace

Old 03-17-2010, 12:22 AM
Thread Starter
Join Date: Mar 2010
Posts: 1
Default Secondary O2 Sensor Replace

My original post is gone, so I am a bit confused.
I wrote this before I knew that.
Anyway, this may give someone courage to perform the task I just completed.

Replacing the Secondary O2 Sensor, 2004 Honda Accord, 4 door, 4 cylinder.


The Secondary O2 Sensor is screwed into the right side middle of the catalytic converter. The electrical connector is under the front passenger seat.

Local stores and online sales showed that Bosch 13704 and Bosch 15409 both fit this car, but the electrical connectors are slightly different. The 13704 would have required me to splice the old original connector onto the new sensor wires. I did not want to splice the wires, so I chose the
Bosch 15409.

The car's computer monitors the return voltage from the sensor and its heater, and if out of narrow limits, turns the Trouble Light on. Universal types of sensors require splicing the wires so you can keep the original connector plug. The splices underneath the car will be exposed to grime and moisture, possibly causing additional resistance, which the computer will detect as a problem. It is best if you do not have to do any splicing.

The new sensor will include a tiny packet of anti seize compound. Make sure it is in the box. You should not install the new sensor without it.

The new sensor I bought did not have identical wire anchors, so I cut three of the rubber barrels off the old wiring and put them over the new wires when I anchored them in place, using the original anchors.

Steps to Replace the Secondary O2 Sensor:

1. Gain access to the electrical connector by loosening the front passenger seat:

Note. The seat is not going to be completely removed, just tilted back.

There are three tabs on each of the rear attaching bolt covers. The tabs are not visible. You will have to feel for them. Use a hook to release them. One tab is at the center rear of the cover and a tab in on each side of the cover about one inch forward from the rear. The tabs only have to be pulled about 1/8th of an inch away from the main body of the bolt cover to release them. Pulling too far will distort the plastic cover and it will not properly reattach later.

Lift the covers off the seat attachment frame.

Slide the seat fully forward, and remove both of the rear seat attaching bolts.

Slide the seat back and remove both of the front seat attaching bolts.

Carefully tilt the seat back, being aware the air bag wiring is still attached.

Prop the seat in the tilted back position so that no strain is put on the wiring, but you can work underneath the seat.

The floormat is split just behind the front position of the seat. The four wire O2 Sensor connector is under the part rearward of the split. Remove one push fastener and lift the floormat to see the connector.

2. Unfasten the in cabin parts of the sensor wiring harness.

The connector has a spring lock to keep it from coming apart. The body (male) side of the connector has a tab about 1/10th of an inch long and 1/16th of an inch wide that fits into a corresponding hole in the Sensor side of the connector. If you turn the connector so that you are looking at its side, you can see the locking tab.

Use a very small screwdriver and pry the tab on the body side of the connector so that it releases the tab. Pull the connector until it separates.

About 6 inches back from the connector, the sensor wires go through the floorboard with a rubber boot that is permanently attached to the sensor wires. Push the boot down at the edges and allow the sensor wires and connector to fall to the underside of the car.

3. Disconnect the sensor wires from the hangers underneath the car.

The wires are supported by 3 hangers. Two of the hangers can be removed by pulling them off the plastic channel they are attached to. Roll the wire out of the third hanger so that the wiring harness is dangling from the sensor.

4. Remove the 4 bolts attaching the lower catalytic converter baffle.

Drop the lower half of the baffle down and put it out of the way so it does not get bent.

5. Remove the sensor from the catalytic converter.

Use an appropriate tool to unscrew the sensor. The wrench size on the sensor is 7/8 inch, 22 MM. The connector on the wire will not go through the hole in a 7/8 box end wrench. Cutting the connector off the wiring harness is not the answer, because the new connector will not go through the hole in the wrench either. The special tool for removing the sensor has a slot in it that the wires can pass through. The tool comes in offset and deep socket. Your choices are; use the loner tool that the parts store provides, buy the special tool, or CAREFULLY use an open end wrench. In any case, you should soak the area where the sensor meets the catalytic converter with a good penetrant to loosen any corrosion. If the sensor doesn't budge with reasonable force, soak it with penetrant some more. Work carefully, so as to not round off the flats on the sensor. If a wrench doesn't unscrew the sensor, Vice Grips are not going to either.

6. Install the new sensor in the catalytic converter.

Screw the new sensor hand tight only into the catalytic converter. Make sure the crush washer on the sensor touches the catalytic converter. If it does not, remove the sensor and clean the threads in the catalytic converter. When it will fully screw in with only hand pressure, take it out again.

Apply anti seize compound to only the threads of the new sensor. Many sources say that getting contaminants on the bulb portion of the sensor will cause early failure, so be careful with it.

Screw the sensor into the catalytic converter and tighten to 33 Ft. Lbs.

7. Reinstall the lower half of the catalytic Converter baffle.

The threads on the 4 hex head screws will be corroded. Spray them with a good penetrant and wipe off any excess. Tighten the bolts to 4 Ft. Lbs.

8. Secure the sensor wiring harness under the car.

Route the undercar portion of the wiring harness through the three clamps. Push the connector through the hole in the floorboard and push the rubber boot into place.

9. Fasten the sensor connector.

Route the wire harness under the floor mat the same as it was originally. Push the two halves of the connector together until the lock engages. Look at the tab in the hole to make sure that it is properly locked.

10. Secure the floormat.

Fold the rear portion of the floormat down and secure it with the push pin you removed earlier.

11. Reattach the seat.

With the seat slide in the forward position, screw the two rear attaching bolts in hand tight. Slide the seat to the rear position. Install the two front seat attaching bolts and tighten to 25 Ft. Lbs.

Slide the seat to the forward position again. Tighten the two rear seat attaching bolts to 25 Ft. Lbs.

12. Install the rear seat attaching bolt covers.

Properly align the plastic cover and press firmly at all three clip positions.

13. Clear the trouble code.
Old 03-17-2010, 06:58 AM
JimBlake's Avatar
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Join Date: May 2006
Location: Wisconsin
Posts: 17,518

Excellent post!
I don't know what happened before, 'cause this seems to be your first post.

I think I'll move this into the DIY section, because it's a good detailed procedure.

The general procedure should be good for 1996-onwards. But the details of where the plug is located (under the seat) and how the wire is fastened will differ over the years. 2003-2006 all should be pretty close.

I'll also add some diagnostic information...

Check-Engine-Light error-codes P0136, 37, 38, or 39 indicate a problem with the oxygen measurement. That's difficult to verify by measuring. P0139 (slow response) is one where you should probably trust that it's a bad sensor.

CEL error-code P0141 indicates a problem in the heater circuit of this sensor. You can measure it, by unplugging the sensor. In the wires going to the sensor itself, look for 2 same-color wires (black?). Measure resistance between them, it should be:
1996-2002 = 10-40 ohms
2003-2006 = 5.0-6.4 ohms
Now 5 or 6 ohms is pretty low vs. the measurement uncertainty of most household meters, so it might not be perfect. A bad heater normally would be open-circuit. A good heater resistance means you should look for problems in the car's circuits. That's for another thread.

Last edited by JimBlake; 03-17-2010 at 07:10 AM. Reason: more information
Old 04-18-2010, 03:02 PM
Join Date: Apr 2010
Posts: 5
Default Error code reset?

I had a free diagnosis done at Autozone and replaced my secondary O2 sensor (thanks for the excellent DIY post ST Bubba!). Now I need to know how to clear the code to turn off the engine light. How is this done?

Thanks in advance for any help ....
Old 04-18-2010, 03:53 PM
JimBlake's Avatar
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Join Date: May 2006
Location: Wisconsin
Posts: 17,518

Different ways to clear the CEL error over the years. What car are you talking about?
Old 04-19-2010, 02:14 AM
Join Date: Apr 2010
Posts: 5
Default Sensor code reset

Sorry, it is a 2004 Accord EX 4 cyl.
Old 04-19-2010, 07:26 AM
JimBlake's Avatar
Super Moderator
Join Date: May 2006
Location: Wisconsin
Posts: 17,518

Check this thread about "readyness codes".
Look at the different ones about cat & A/F sensor. If you satisfy all the requirements for those, it should clear itself. Unless the original problem was with the wiring in the car.
Old 04-23-2010, 10:11 AM
Join Date: Mar 2010
Location: Denmark
Posts: 6
Default Secondary O2 Sensor Replace

does any one know how to wire up the secondary o2 to the ecu or can you tell me if what i am doing is right i wire it up this way but dosnt work


e4-sensor ground

please let me know or a way to bypass the secondary o2 to get the check engine light off so i can pass inspection thanks

update: this is how i have it wired now
sensor ground-e4
a/f sensor-a10

it is getting power but no ground i can put a ground to it and it will heat up but check engine light still on also i know the primary needs a relay does the secondary need a relay too ?????

oh by the way it is a pnf ecu
Old 04-23-2010, 11:50 AM
JimBlake's Avatar
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Join Date: May 2006
Location: Wisconsin
Posts: 17,518

The missing ground is back to the ECU - that's how the ECU "knows" when the heater fails.

Sounds like an open-circuit back to the ECU. Possibly check the plugs on the ECU itself to see if one's loose?
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