P0135 and P0155 at same time - 2002 Accord
 

My son's Accord, 2002, 4 cylinder, about 70k miles, has CEL on. Codes are heater circuits for both O2 sensors. Hard to believe both fail simultaneously. I see many many posts saying check the fuses, but I can't seem to find which fuses they are. The best I've seen is to look at certain fuses in the under dash fuse panel - but I'm not sure a 2002 has this. I checked the under hood and both of the boxes on the sides of the dash. I didn't find anything specific to the O2 heater circuits. I did check all the 40 amp fuses under the hood. All were good.

I do have power to the bank 1 sensor. I couldn't get a resistance reading across the heating element but I'm not sure I was getting good contact on the connector contacts. I also didn't get a voltage reading between the B+ and the harness side ground. I've seen the posts that discuss looking for shorts but there isn't too much wire that can easily be traced.

I was wondering if this could also be caused by a faulty ECT sensor since it is an issue with elapsed time between the coolant reaching some temp and sensors reaching some functional state. I assume the ECT is the one under the distributor, but I can't fit my hand in there to pull the connector and then check it.

OK - that's probably enough of my woes to get folks started. Guidance on figuring this out is much appreciated.

P0155 - There is no P0155 applicable to the 4 cylinders, as there is no bank 2.

P0135 is Primary O2 sensor heater circuit malfunction.

That was read at Autozone - could it imply the downstream (post cat sensor). Anyway, they got two codes and that was them.

It is likely referring to the same primary O2 sensor.

Thanks - I'll try a little harder to get a resistance check on the sensor heating element. One reference mentioned checking between pins 3 and 4, but there are no contact position numbers on the connector. Any guidance on which wires I should check between? Also, I would still like any help you could provide on which fuses to check, especially is there a 4th fuse box, one under the dash, on a 2002. The owners manual shows just the 3 I know of.

The heater element in the sensor has 2 wires of the same color (black?).

So IIRC the wires from the sensor itself are green - white - black - black. Measure resistance between the 2 black wires.

If that resistance is OK, then it still might be a break in the corresponding wires in the car's wiring harness. That'll be fun to find.

Thanks - I'll give it a check. Question, on checking the ground through the ECM, does the key need to be in run in order to cause the ECM to switch to ground or is it normally at ground?

Just a guess, but maybe so. It seems Honda's customary way of doing things is the (+) is usually hot & they switch the ground side of most circuits.

Nothing I do can find a circuit between black with yellow stripe and black with white stripe leads on the censor, well, unless I open the meter to 2 Mohms, then I get a 250k reading. So unless I have a very poor contact between the ohm meter probes and the harness contacts, I guess the heating element is toast.

But what is bothering me still is I can't get a good ground from the engine harness side through the ECM. With key on, engine off, volt probe on Bat + I get nothing. With engine on I get 3 Volts.

I do get 12 volts on the engine harness side lead into the heater element leads.

Any thoughts, other than replace the sensor and see what happens?

thanks

just to wrap things up
 

Bought a Denso direct fit for replacement $66 after 20% online Advance Auto discount. Checked resistance on it before installing and it was right in the proper range. I guess my meter was reading correctly when it showed the old one to be open circuit.

Installed the new one. Amazingly the old one turned out without a fight. Started up. CEL stayed off. Been good ever since - it's been about a week.

I have no idea why I couldn't get a 12-14V indication from the bat+ to the ground lead through the O2/ECM ground circuit as some trouble shooting guides indicated. Hopefully that isn't indicating a bigger issue somewhere else. Time will tell. Thanks all for your help