I'm on my day off today soI put this together, to help people understand O2 sensors.

 

Thanks for the detailed post

 

Also...
Remember that they measure oxygen. They don't really measure mixture. So if you're engine is doing some misfiring, it will read the 'unused' oxygen & the ECU will interpret it as being lean. The O2 sensor can't measure or understand that there's unburned fuel in there too...

 

And a little history.
Back in the late '70s when O2 sensors were first used, they were heated by the hotexhaust gas. See, the sensor only operates when the ceramic sensingelementis really hot.

Then theEPA got a bright idea that it really wanted the O2 sensor to start working immediately when you start the engine. So now there's a little electric heating element built into the sensor. This heater can fail, and Honda has different error codes for that. If you use the flashing CEL method,code 1 is the sensor measurement, and code 41 is the heater. If you use an OBD-2 reader, there's more detail, like this...
P0131 (1)= primary sensor low voltage (P0130)
P0132 (1)= primary sensor high voltage (P0130)
P0133 (1)= primary sensor slow response
P0141 (41) = primary sensor heater circuit

Then the EPA got even more ambitious (with your money...) & decided that you need another O2 sensor to measure whether your catalytic converter is working. That's the secondary sensor, it's located after the catalyst, usually mounted in the steel housing of the catalytic converter.
P0137 (63) =secondary sensor low voltage (P0136)
P0138 (63)= secondary sensor high voltage (P0136)
P0139 (63)= secondary sensor slow response (P0136)
P0141 (65)= secondary sensor heater circuit

If you have a code reader that has error code text saying something about 'bank 1' or 'bank 2' sensors, that would be for a V engine with a separate sensor on each cylinder bank. Not many cars have separate sensors on each bank.

 

An O2 sensor needs about 800F to start working, that is not really a problem with the one in the exhaust manifold, but It would take quite a while to achieve that temperature in the downstream sensor behind the catalytic converter, that's one of the reasons those are almost always heated and the more expensive of the two O2 sensores. O2 sensors tend to respond rather slowly to changes in the exhaust gas mixture, so their output lags behind somewhat. There is something called a wideband 02 sensor that responds much fasterto mixture changes, and they are rather expensive. Widebands are used a lot in dyno tuning applications where instant data is required to make mixture changes in the computer. I used to own a turbo charged Suzuki Hayabusa motorcyle that had a wideband sensor welded into the exhaust collector, which was connected to a data recorder(HAL meter)that collected the mixture info along with RPM and throttle position. This was a big help in making adjustments to the fuel injection mapping in real world conditions which the dyno can't always simulate. Unlike most modern cars this was a total open loop fuel injection system with a preset fuel map. She made 390hp with 14psi of boost, not bad for a 1300cc motorcycle engine on pump gas.

 

My 05 Accord V6 is an Ultra Low Emission Vehicle (ULEV).
Emissions in Reactive HC (unburned gasoline) in g/mile:
1975 1.5
1990 0.41
2000 0.25
2002 0.075 LEV
Current 0.04 ULEV
Current 0.01 SULEV (Super Ultra ...)
Gas cheap your area?
Or are you mildly interested in extracting MPG for the cost?
The Enviro's are hot on CO2 emissions. Before this is over you may be riding a bicycle.
My interest is efficiency and economy.
The 05 has two second generation wideband air/fuel ratio sensors Upstream.
Cost $108 each before S/H.
It has two Downstream O2 conventional sensors to measure system performance.
Cost $61 each.
The wideband sensors are designed to result in ideal O2 in the exhaust. Zeropercent or close. Measured Bank 1 and Bank 2 long term fuel trim. Car Chip.
The older sensors oscillate rapidly between rich and lean with an average voltage of 0.45. Less accurate.
CO2 will be about 15.2 percent.
Stoichiometric or 14.2 parts of air to 1 part of gasoline is the ideal.
Measured by Lambda which includes HC, CO, NO.
Model here:
SoftWare@JaysAutoService.Com
Lambda of 1 or very close is the emission design goal.
California has mandatory biannual testing to reduce emissions.
Tail pipe tests may be available elsewhere if dynamic testing isn't required.
Some areas use the OBDII dump as a system emission test.
That's why downstream O2 sensors. The vehicle is performg aself test.
Controversy on next OBD to auto report failures to the State for notification and correction.
Coming is a CO2 tax. Make sure you vote for it Nov 08.
"You are the solution" (and the problem) as one leading candidate spins.

 

Ok, great tech support from obvious experts. Bear with me on this since I'm old (enough to know how to set points on a Chevy without a feeler guage! lol)

Have my 98 Accord coupe with the P0138 code. I checked the sensor at the cat and it had about 18 ohms of resistance but NO voltage supply. So, maybe I'm going to get lucky and not need an o2 sensor. BUT, according to my very imperfect schematics this sensor connects to the ECM. I believe I read somewhere that it's located behind the pass. side kick panel. Since I'm somewhat proficient at electrical, and without first tearing it apart, is that where I can test for output voltage to the sensor?
BTW I did a visual of the wiring harness around the pass shock tower and there was no obvious wear or damage as I'd read previously here.

 

The ECU is located center, forward, & low. Look in the passenger's footwell, but the kickpanel at the center console, not the outboard kickpanel. Peel the carpeting down, the ECU is lower than you'd expect if you're accustomed to a transmission tunnel.

The signal for the 2nd O2 sensor is at terminal A-23, if you can read the tiny little labels of the terminals. Backprobe the connector soyou can measure voltage with it still connected.

Not sure where the signal ground is for that sensor. Maybe C-18? I didn't keep perfect notes from a repair manual which I don't have any more...

Your resistance measurement was probably bogus. If the sensor was cold it should read open-circuit. If the sensor was hot, the voltage it produces would fake out the resistance-measuring circuit of the multi-meter.

Maybe an 18-ohm resistance when cold is enough evidence that the sensor has gone bad? There's 2 same-color wires (black?) on the sensor. Those are the heater leads, and 18-ohms is good for those. A failed heater would throw a different code.

 

Hi Jim,
Thanks for the great tips. There's NO way I can get close enough to read pin numbers under there, that's for kids! lol I'm going to look for a code reader so I can try narrowing this down without running over the local AutoZone so they can read it...

My electronic manual says to reset the ECM and try again so that's where I'm at. I really want to do it myself rather than take it in. The money I save will pay for the reader. Any suggestions on brands?
L

 

Ihave a$40reader from Harbor Freight, but it doesn't work on the Hondas. Apparently there's like 4 different data protocols under OBD-2, and this little thing only knows how to read one.

My wish list is for a scan tool that can dodata-logging. Not just an error code reader. That way you can display O2 sensor voltage on the display of the reader. That's more expensive. Maybe a cable & some software to use a laptop computer to plug into the OBD-2 port? I'm kinda interested myselfin what other people like along those lines.