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How do I fix Odyssey with faulty O2 sensor heater circuit


Topic starter

Hi, Scotty and all.

I've a problem - code P135 - faulty heater circuit of primary (upstream) O2 sensor (new).

car: 2002 Honda Odyssey w 250000 miles. New catalytic converter w/ Y-pipe and both O2 sensors (ultra Power). Van has 1 upstream and 1 downstream sensors.

Simptoms: running rich (exhaust smells "unburnt"), at best 20MPG, and error code. Can't pass emission inspection.

Previous owner bypassed old O2 sensor with one black wire (for heater element) connected to a fuse below dash board (drivers side). I removed all these extra wires...

New upstream O2 sensor has 6 Ohm resistance between black wires (heater element). It must be bet. 10 and 40 Ohms.

Initial diagnosis (by automechanic)- bad CMU, replaced ECU (with VIN re-programming) - problem persists.

I spliced one of the black wires and installed a resistor (10 Ohms) - total resistance was now about 16 Ohms.

I traced white and green wires from O2 sensor harness to the CMU.

I check continuity  of harness black wire to the ground - good.

Still have code P0135.

I think that I need to purchase Denso upstream O2 sensor (part # 2344727). Maybe its resistance is within the specs.

What do you think? what would you suggest?

Thanks in advance...

I also need to add this: when engine running and cold, the scan tool shows voltage on primary O2 sensor (about 0.9 V, I think), then the volts start to drop every second or so to ZERO (takes about minute or two). Then voltage stay at zero.

5 Answers
Topic starter

Thanks, DayWalker.

I checked your link and eliminated a few possible causes. Still have to "retrace" wiring and, maybe, measure vacuum at airintake (thrre's a port - just before rubber boot meets throttle body).

It's hard to work on wiring, as I don't have wiring diagram. will post results in a week or so.


Did you use the OEM oxygen sensor? The symptoms say that the sensor is not functioning correctly. I would replace the oxygen sensor. 

the O2 sensor is aftermarket. I intend to buy Denso (part #2344727). Strange - both brands (aftermarket one and Denso) have the same part #. honda's genuine part is 36531-P8F-A11, but a dealer asks over 250 for it.

@wwdwgs this is what can happen not using OEM for certain parts:


The pre-heater in the 02 sensor isn't working.  Get OEM part.

New Denso O2 sensor installed, codes cleared, code P0135 comes up again.
faulty MAP?
I also noticed that sometimes engine stalls when I quickly let go off gas pedal (2000+ rpms). Usually rpms drop below normal idle and recover quickly.


Read through here and perform the troubleshooting they suggest:

Topic starter

New primary (upstream) O2 sensor (Denso 234-4727) is installed to no avail. code P0135 still comes up.
I used a troubleshooting procedure, posted in 2013 OdyClub site and got to the point where there's a short between primary O2 heater wire #4 and fuse #6 (ECU/Cruise Control internal fusebox by driver side).
Troubleshooting guide can be found here: file:///home/silver-dell/Downloads/P0135(1).pdf
by the way, new O2 sensor has 6.xx Ohm resistance, not 10-40 Ohm per specs.
Also, I found how to diagnose MAP sensor (on top of Throttle body between rubber boot and intake).
Per specs, center pin must read 1.4-1.9V at 2500 rpms. Mine did not, it read 0.78V or so. Although the voltage at idle was fine (1.11V) (normal is 0.9-1.1V at idle).
Primary O2 wire at #4 (BLK to BLK/YEL) terminal goes to a junction called J/C C107 where another 3 BLK/YEL wires joint. This junction is located somewhere "below center of dash". I could not find this junction/connection, not wires anywhere in that area.
Could the be in wire bundle plugged into the ECU? or is it a separate junction?
any thoughts? experience?