Changed both o2 sensors but still getting codes. Possible bad catalytic converter?

I've got a 2005 Tacoma with the 2.7L automatic. It has 137K miles on it. We bought the truck used in 2012, have put 60K on it since then, and it's been a good truck until lately.

The CEL came on and I checked the code. It said P0137 which indicated to me the O2 sensor had failed. It didn't seem unreasonable with the mileage as it is so I changed the sensor. The light went out and all seemed good for about 3 weeks. The light came back on and it said the same sensor had failed.

I did some research and learned that the Bosch, which the sales guy at AutoZone insisted was best for the truck, isn't such a great idea. They took the sensor back and gave me the Denso it's supposed to have.

After changing the sensor again the light was back on within a few miles. This time it was P0031 which would indicate the front sensor failed. That didn't seem unreasonable either. Some sources I've read say if one fails the other probably isn't far behind. I changed that sensor, with the appropriate Denso, last night. Within a few miles the light was back on again and the code is P0137 AGAIN!

I'm stumped at this point. I can't believe the sensor has failed again.

I'm not getting any other codes that would indicate a bad ECM/PCM or a bad catalytic converter. Could it be a bad Cat?

I've been fooling with the thing for a couple of weeks now and running out of time. I have to have it inspected by the end of the month. The state is kind enough to send me emails reminding me the inspection is due so I know they are aware of it. I'm concerned that with the computer systems as they are, just driving by a cop with a plate scanner could result in being stopped and having the truck towed.

There have been a series of issues with the Tacoma that have me discouraged. If I could afford to dump the thing I probably would. I've had to replace rear wheel bearings, U-joints (and still have a vibration in the drive shaft) and the A/C compressor imploded. The cost of ownership is getting heavy.

 

The following Toyota dealer parts site shows 8 different OEM numbers for your vehicle. I suspect half are for the front Air-Fuel sensor(before CAT) and the other half for the O2 sensor that follows the CAT.

http://www.villagetoyotaparts.com/s...=2007&ukey_category=29066&ukey_driveLine=6089

It's interesting that there are sensors for the "Mexico spec".

Tacoma's made in Mexico have the number 3 as the 1st number in their VIN. Some of these trucks ended up in the U. S. I suspect that's what is meant by "Mexico spec".

You might want to read thru this document as there are other components that can cause the P0137.

http://repairpal.com/OBD-II-Code-P0131-and-P0137


Yes - the Bosch sensors would not be my first choice for a Toyota. They are fine in European cars, but only the ones made in Germany. Many are now made in India.

I would also be suspicious of Denso sensors sold by Auto Zone. Denso is a good brand in general, but like a lot of parts mfgs., they might have a high end and a low end of parts.

You may have to dig this out.

If you end up replacing again with OEM, there are a lot of Toyota parts suppliers on ebay who sell dealer parts.

 

Firstly, codes don't tell you what part failed. It tells you what group that has the problem.

It can say an O2 sensor failure but that doesn't at all mean an O2 sensor has failed.

Test things before you replace them, it is cheaper than throwing money to replace parts that are not the issue.

 

Something else I'll pitch and it's not likely related to your issue, although it could be.

Clean your MAF sensor - mass air flow sensor. You can buy it in just about any parts store

http://www.pepboys.com/product/deta...m7Wc7sKwc-LKUi0KpqR_lkTXOZGF_Xdy30aAuyJ8P8HAQ

MAF is part number 22204 in this diagram:

http://www.villagetoyotaparts.com/s...=2007&ukey_category=19385&ukey_driveLine=6089

This part gets dirty over time and needs to be cleaned. Do not touch the platinum wires in the MAF. Just spray with the cleaner.

 

Thanks for the replies and advice thus far. I'll give an update on developments.

I used the code reader to see what was going on with the system in real time. I'm still getting a P0137 code but the real time data shows the voltage from the rear sensor is all over the place and changes very rapidly. The front sensor is a flat line at 0 volts. Just to see what would happen, I cleared the codes and the CEL stayed off for a few miles. When it came back on it was still giving me the P0137 code. I looked again after driving home and it had added P0606 which looks like the ECM/PCM is at fault.

I stopped by the local repair shop and talked with the owner to see what he had to say. His advice was similar to what I hear from you guys. He said that the Toyota products he has worked on have given him so many problems, he won't buy the sensors from any of the parts stores or aftermarket type. He doesn't like paying the price at a dealership for them but has far less trouble with them.

As for the P0606 code he said it could be any number of things besides an ECM issue. A broken wire or blown fuse have been the culprit with some vehicles that he has worked on. He wasn't certain but said that some vehicles have the "always hot" power supply to the ECM/s in addition to a fused wire that is only hot during cranking. The always hot supply will keep a vehicle running but the blown fuse will throw all kinds of crazy codes around.

Does anyone know if there is a separate fuse for the ECM? I haven't had time to research since I stopped at the garage and won't have time until I get out of work later.

One other thing I'm curious about but haven't had time to look into is what the exhaust gas temperatures should be. When I use the reader to look it appears that the exhaust exiting the catalytic converter is almost 200 degrees cooler than going in. It seems like it should be the other way around but as I say, I'm on my break and haven't had time to do any research just yet.

As for the MAF sensor, I can also see the possible relationship to the problem. I hadn't thought of that before. I did pull the sensor and clean it with an electrical contact cleaner about 5,000 miles ago. It's easy enough to do so I may see how it looks.

The mechanic at the local garage is willing to take a look and see if he can figure things out. If that fails I have a few days left on the inspection to get it to a stealership. I'm certainly not looking forward to that experience.

I'm remembering very quickly why I stopped working on my own vehicles a few years ago. I just don't have the tools or the technical know how to sort out this kind of stuff. I used to rebuild carbs for people, almost as a hobby, but those are as much dinosaur as this truck's tech makes me feel.

 

Just in case anyone happens to stumble on this thread while searching for information about sensors I'll give a simple update.

I couldn't figure out the issue or why the error codes kept changing. I finally took the truck to a local garage where someone with more experience and a lift could get under it and look around. It took him about 2 hours to find a tiny break in one of the wires right where the harness from the downstream O2 sensor takes an ugly turn into the cross member. He said he was probing for voltage and noticed the CEL had gone off. When he touched the wire again the CEL came back on. A nearly microscopic hole in the insulating plastic had a tiny bit of telltale green corrosion showing. I'm on the third day without a CEL on.

Had I done enough research to know ho to test the sensors, I wouldn't have jumped to the conclusion they needed changed. It would have still been painful trying to figure out what the problem was but I may have saved $$$ on sensors to begin with.