I looked around the forum but didn't see anything that addressed my issue.

The check engine light came on my 1997 Accord 4 Cyl LX 2.2L (155,000 miles) a couple of days ago. I went to the shop and I was told it was a P0135 O2 Sensor 1, Bank 1. I wasn't surprised, since I've never changed the O2 sensors on this vehicle since new. However, I did some electrical tests and here's what I found:

(1) The heater circuit resistance was within spec (around 16 ohms)

(2) With the vehicle ignition on, and with the connector disconnected from the vehicle, pin 3 (Yellow/Black wire) on the vehicle side connector gave 12V but the ground pin (no.4 - Orange/Black wire) was open circuit - effectively zero volts between pins 3 and 4. The vehicle was stone cold.

Now, the diagnostic material I have gets a little hazy on where to go upstream to diagnose the O/C. It talks about PCM connectors and relays but I'm not exactly sure where I need to go (having trouble visualizing things) and what I need to do next.

Is there anyone here who has been in this position and has an idiots guide on what to do next?

Thanks in advance for any and all help.

 

Go to the ECM/harness connector w/ the wires for the O2 sensor heater. Check resistance at that point. It should be nearly same as measured at sensor harness.

If you measure open circuit (infinite resistance), then one of the wires is open. You can find out which by shorting the engine side harness of each wire to ground and rechecking. One will be open.

If wires and resistances check good at ECM, and code returns, the ECM is suspect.

We've been seeing more ECM faults lately in the 94-97's.

good luck

 

Thanks. I Googled for a pin-out diagram for the ECM and came across this entry in another forum 97 CL ecu pinout - Honda-Tech

When I read this I see only one pin in the "A" connector (A6) and one pin in the "D" connector (D7) but no pair of pins. I'm confused. Do you have some insight on what connects to what, here? Thanks again.

 

The Org/Blk is checked at A11 (connector A, Pin 11).

The Yel/Blk ties into various points and comes back to D20. Because of other circuits tied in, a test of this wire will be ambiguous. This appears to be the 12V supply side w/ A11, the ground side provided by ECM to monitor system.

good luck

 

Thanks. I will work on this tomorrow and report back.

 

Well, I went back to this today and, low and behold, I got an O/C on the sensor heater element. For a brief second the meter indicated something but then went O/C.

I also learned a little more about the grounding to the ECM. It appears that the ECM may not allow a ground under a couple of conditions:

(1) If there is a sensor fault present.
(2) If the engine is not running - this makes sense, otherwise the sensor would burn out.

I disconnected the battery to erase the code and started the engine and looked for the ground. Unfortunately, there wasn't one, which seemed to pour water on the above logic. However, I discovered that the CEL had come on immediately (due to the sensor being disconnected), which of course would prevent the ECM from completing the ground.

In any case, I have ordered a new Denso OEM sensor off one of the big parts suppliers on eBay and it should arrive by the end of the week. It is a full $30 cheaper then the same part in the local auto parts store. There must have been about 6 big suppliers who were selling the part within a dollar or two of each other.

Luckily, this sensor is easy to get at. I took the exhaust shield off and was able to loosen the sensor with no issues. This was a welcome relief because, when I did a replacement on my Jeep, I spent over half a day trying to get it out of the catalytic converter.

The car is back together so I'll drive it around until the new sensor arrives.

Thanks again for your help. I've learned a lot in the process.

 

O/C?

Thanks for posting your results. Look forward to hearing the new O2 sensor resolves your problem.

good luck

 

OC = open-circuit?

You mention getting it out of the catylitic converter... This is the forward sensor we're talking about, right (P0135)? It's screwed into the exhaust manifold right up front. (Maybe the converter comment was about the Jeep)

 

Yes, sorry, I meant open-circuit - OC and not O/C.

Also, I'm replacing the upstream sensor right on the exhaust manifold under the hood. I can see the confusion with the Jeep story. That was the downstream sensor in that case.

 

Problem resolved with new sensor. Have completed multiple trips totaling over 800 miles.

Strangely, when I had the old sensor on the bench, I found continuity in the heater circuit, about 14 ohms. I can't account for why I had continuity, then no continuity, then continuity again during all my testing.