Hi. I hope there is someone that can help me.

My wife's 1996 civic cx is throwing 2 error codes: P0135 and P0141. It's an automatic with a D16Y7 engine. The cat is attached to the headers with the primary and secondary HO2 sensors before and after the cat, respectively. The heating elements inside both sensors don't appear to be working. I've done every troubleshooting thing I can think of. Here's what I've tried so far on just the Primary sensor:

1. Checked the resistance on the heater element (Meter: 20 Ohms, Spec: 10-30 Ohms)
2. Checked to see if there was power on the fused side with the ignition ON (Meter: 14.4 volts)
3. With the iginition ON or OFF there was no voltage between the heater wires.
4. Checked for voltage in the line from the ECU to the engine ground (Meter: 0.0 V)
5. Checked for continuity in the line from the ECU to the sensor - there was.
6. Borrowed a working ECU from the a local garage - same problem.
7. Bought 1 used OEM honda sensor and tried it in the primary port - same problem.
8. Bought a new "cheap" sensor off ebay and tried it in the primary port - same problem.


I'm loosing my mind here. Everything was fine before I replace the transmission. I keep leaning toward the wiring, that somehow I may have broken something when I was doing the tranny swap, but I don't see how given the tests results I got. Any ideas?

 

If you reset the ECU, do both codes return?

With the key in ON(II), do you measure voltage to body ground at both top test tabs of installed 7.5A fuse 15?

 

Yep. I've reset the ECU several times. Usually by using my ODBII code scanner, but I've also done it by disconnecting the battery. I turn the ignition ON (II) and in a few seconds the engine light is on again with the same 2 codes.

I haven't tested the voltage directly from the #15 fuse to the body groud, but I did replace the fuse with a 7.5A fuse as I noticed someone had put a 10A in there by mistake. The voltage I measured in Step2 of the list I gave was from the wire coming from the junction box behind the intake, which I presume is going back to the 7.5A #15 fuse.

 

You might have a short. There was a TSB on a wire from one of the connectors near the intake manifold wearing out and then shorting out. Check the #15 fuse to make sure it's not blown.

 

Do this test:

 

I verified continuity in the #15 fuse so I'm good there. I did the test with the ignition ON (II) and measured both sides of the fuse and got 11.92 Volts.

 

Check whether the heater power wire at the O2 sensor clips has battery voltage to body ground with key in ON(II).

 

Yeah, it did the last time I tested it. I'll have to try again in the next day or so when I return. The last I remember, there was 14.4 volts from the heater wire to the body ground when the ignition was ON (II).

 

Voltage from the heater elements grounds at ECU A5 and A6. Check those two ground wires for a short (continuity with body ground).

 

ok, lets see. I tested the voltage from both O2 sensors to the body ground with the ignition ON (II) and got 0.782 Volts on both. I did the same for the ECU wires while the ignition was ON (II) and got 0.0 Volts on both. With the ignition OFF, I tested the continuity between ECU wires (A5 and A6) and the body ground - nothing. Tested the continuity between A5 and A6 and there was nothing...

 

What wire are you testing? Was the O2 sensor connector unplugged for this test? It should be unplugged.

 

I was testing the heater wire on the side of the #15 fuse. I tested it with the sensor unplugged from the harness to the body ground.

 

I will assume the test was done with the key in ON(II). You have 12V on both sides of fuse 15 but <1V at the Blk/Yel wire terminals of the two O2 sensor plugs. This tells you that the Blk/Yel wire has an open (is broken) at a point before it branches to each of the O2 sensor plugs. Start by checking whether this same wire in the VSS plug and the EVAP purge control solenoid valve plug has either 12V or <1V.

 

Yes, the ignition was ON (II) for that test.

If that is the case, the open in the wire would have to be before the junction box behind the intake manifold as, after I did the last test you recommended, I turned the ignition off and did a continuity test between the Blk/Yel wire on the Primary and the Blk/Yel wire on the seconary O2 sensor connectors and it was ok. Which, come to think of it, seems strange since nothing else in the car is tripping error codes.

You wouldn't happen to know what the wire colour is from the fuse box to the junction box?

 

Maybe the C115 junction connector is bad or not fully seated. A few continuity tests should sort that out.

 

That was it!! Problem fixed! There was an open in the line between the #15 fuse and the C115 junction.

I've been beating my head against the wall for 3 weeks trying to figure this one out. Thanks a million!!!

 

Nice job. Was the wire break related to rubbing on the intake manifold bracket, similar to what is reported in the Fuse 15 TSB?

 

Actually, the wire had corroded just before the C115 junction and had fallen off when I pulled on it. It was borken just as the wire meets the junction...

 

So you think it was unrelated to the TSB where the wires rub on the intake manifold bracket?

 

Yeah, there was no visible damage on the wires from the bracket. I'm not sure how the wire could have got corroded like that given that the connection still looked sealed to me.