1997 Nissan D21 - Inescapable P0141 code

[Electrok-d]

I've got a really strange situation. Across a year of fixing issues associated with various trouble codes, I finally have one left: P0141. It is associated with the down stream oxygen sensor's heating element not reaching 600 degrees fast enough.

Since the original had over 100,000 miles on it, I figured just swapping it out with a new one would fix it. Nope. First I tried a Bosch universal, then a Bosch Platinum original fit, then a Denso, and then finally another Denso.

All the while, I checked everything along the list in the factory manual. The circuit actually works. The element gets hot. The sensor produces voltage and sends it to the ecu.

The factory manual states that the measured resistance of the downstream oxygen sensor's heating element should be 11.4 to 17.4 ohms at 77 F. Every sensor that I measured, even the original one I took out of the pipe, measured between 4 and 5 ohms. I even took the last Denso to the bench, applied heat, and took a measurement. I couldn't get it over 8 ohms resistance. That is three different manufacturers all measuring something different than what was stated in the manual.

I'm using a Denso 234-4703. This cross references to the OEM part number, which is 226A0-1S715.

Has any encountered anything like this before?

[Desert Rat]

interesting. had that code myself and cleared it with a Bosch original fit.

Could be you have a dead or dying cat converter. Given your mileage and age, it's not out of the realm of possibility. Other causes I believe are a major vacuum leak, which would seem pretty obvious in how the truck runs.

[Electrok-d]

From what I've gathered from reading forums and other people's problems with oxygen sensors is that a vacuum leak typically affects the upstream sensor, with regard to producing erratic voltage. In this case, the trouble code is specifically designed to warn against and incomplete circuit at the downstream heated oxygen sensor. The ecu thinks that the downstream sensor isn't heating up fast enough.

This makes sense if the sensor's internal resistance is 5 ohms when in fact it should be 11+ ohms. The more resistance the faster it should heat up. What I'm having trouble wrapping my head around is why the factory manual states one set of values for resistance and the all of the sensors I've tested read something completely different.

As for the cat, I'm pretty sure if it were shot I'd be getting an improper dataset in the scantool with regard to the downstream sensor's voltage produced. It appears to be reading the exhaust properly and communicating with the ecu as it should. The ECU just wants it to heat faster than its current resistance will allow.

[nalz]

Sometimes manufacturers realize that certain check engine lights and/or codes are set erroneously, and they issue a technical service bulletin to alert their technicians of that possibility. They have you check that the hardware in the system indicated by the code is working properly, and then recommend a reprogram of the ECM/PCM. If there is a way for you to check for the existence of such a bulletin, then that might save you a lot of time and money.

[Electrok-d]

I just contacted Denso directly. Apparently, Nissan has told them the resistance for both the upstream and downstream sensors are 5.1 ohms. This doesn't jive with the manual, so it is likely that something has changed and there is indeed a service bulletin about it released at some point.

I need to do some more digging...

I'll let everyone know here. Hopefully the info can help someone in the future that experiences the same issue.

[Desert Rat]

thanks. I know how frustrating this stuff can be.

[Electrok-d]

i ended up calling a local Nissan dealership and talked to their parts counter. They couldn't offer an explanation on the resistance difference between manual and after market parts. I was told there are five dealerships in the country that has OEM sensors in stock, so I called one of them. Apparently the part was in some far off warehouse and was told someone would get back to me with a measurement. That was 4 days ago.

An interesting note though, Denso gave the the model number of a universal fit sensor that is manufactured at 15.7 ohms. Technically, this hack should work. So should installing a separate resistor within range, completely bypassing the sensor's heater element. I hope to try that sooner than later. My trip permit runs out in 6 days!

[Electrok-d]

More info from the battlefront...

I bought a 15 ohm, 10 watt resistor and used it to bypass the sensor's heating element. I jumped the sensor's two other wires to the plug and confirmed they were producing voltage both at the scan tool and with a multi meter. I confirmed the resistor was indeed 15 ohm and that the ECU provided ground as it should with the key on. The circuit was complete and fully functioning.

I started the truck completely cold. Once the coolant temp reached about 170 F, which is its max, I let it run awhile. No trouble code. On the surface, it appeared that the ECU liked this little stunt I pulled with the resistor. I knew not to get too excited though. I've been down this road before about a million times before.

As is tradition with this truck, immediately after I turned the key off and let it sit for a few minutes, the check engine light appeared. After clearing it with the engine running and immediately rechecking, the light will instantly appear. The only time there is a lull in its appearance is when the truck is super cold.

As far as I'm concerned, every single check in the manual's troubleshooting guide has been performed. The disparity in resistance between the manual and the aftermarket parts doesn't seem to matter, as shown in my test above. The wiring from the computer is obviously functioning because the ECU is clearly providing ground as it should. This is a very simple circuit.

An ECU has been tracked down at a price of $240. This is not a part I just want to throw at the truck. Can anyone think of any more tests before I drop coin on this thing?

[Rmv0809]

Having the same problem ,how did you correct it?

[Electrok-d]

I replaced the computer. It was the last thing I ever expected I'd have to do.