CEL For P0138 Code - Definitive thread

There's been a few threads on this issue on the forum. Unfortunately none of the posters have been come back and posted what the fix was. They complain about the problem, get some advice, go away and never post back to the thread.

Did they fix the issue? Did they learn to live with it? Did they sell their truck in frustration? Who knows?

2012 2.7 Taco with 12X,XXX miles. I've had it for a month. The CEL was on when I bought it. The previous owner had it for 2 years and he said it was on for the whole time he owned it too.

Where I'm at right now on this is I've read everything I can find on it and watched a few videos. Most of the videos show you how to replace the sensor and (I ASSUME) that fixes the issue.

So...did that...didn't fix it.

The posts I've found on it in this forum indicate that often the problem is not with the rear sensor but is in fact with the front sensor. The front (wide band) sensor perhaps in error, thinks there is a lean condition, tells the ECU to increase fuel, injector duty cycle increases satisfying the front sensor, but the downstream sensor cries foul.

I have an upstream sensor sitting on my work bench and will install it tonight and see if that fixes the CEL.

Either way I'll report back for the next guy searching for this error code.

Hopefully I won't need "That didn't fix it so now what do I do advice"...but if you have run into this yourself when it was not the sensors...please let me know!

Phil

 

before you throw parts at you could have it diagnosed. Any high end OBD2 scan tool should be able to monitor the sensors and see how they are working in conjunction with each other. This will tell you if its a sensor problem or a wiring problem.

 

If I can't fix it, I'll take it somewhere that has this tool.

In my research on this, it looks like people were paying several hundred dollars to take it to a dealer and have them replace both sensors. Buying Denso sensors...the back one is about $70. The front one is about $120.
The wiring is all pristine and my VOM indicates it is good.

When I replaced the rear one, I went ahead and took the front one out and cleaned it. Looks to be original to the vehicle. Now that I've had it out and had the wiring connector apart, it'll take me longer to put it up on my lift then it will to replace the sensor.

Normally I'd agree that doing more diagnosis is better, but if the possible cure is $200 and the diagnosis is $200 too...I'll try the potential cure. Dollars to doughnuts if I take it to the Toyota dealer (closest one is an hour away) they'll replace the sensors as the first step in diagnosis anyway.

Phil

 

Get someone with a good scanner that you can actually look at the front and rear oxygen sensor voltages while running. Your front oxygen sensor should go above and below .45 V up to 10 times per second. The rear sensor behind the catalytic converter voltage should fluctuate very slowly. A graphing scanner is best to see this voltage reading. The front oxygen sensor will be bouncing up and down up and down up and down while the rear sensor will be a nice smooth long sine wave. If the rear sensor voltage is bouncing up and down as fast or almost as fast as the front sensor, your catalytic converter is not working properly to reduce emissions.

 

Exactly!

 

Sounds like one of us misunderstands something. On a 2012 anyway....

Doesn't the front AF sensor produce a 3.3 volt signal and adjust the amperage plus or minus based on the oxygen content of the mix? Then your high dollar scanner converts this to a reading of roughly 3.2 to 3.4 volts during normal closed loop operation?

Meanwhile the back sensor does as you suggest, fluctuating between 0 and about .9 volts?

Phil

 

Yes you are correct. I was explaining a front oxygen sensor and not a air-fuel sensor. You can still use a graphing scanner and see the front and rear sensor graphs and comparisons. The front sensor will have many small humps while the rear sensor would be a long shallow sine wave. Sorry for the misinformation.

 

Lefthook...I appreciate you taking the time to post...I'm trying to learn.

So....up on the lift and replaced the front sensor. Not ignoring your advice....just I had the brand new one delivered already. I cleared codes and took it on a test drive. It went about 12 miles before the CEL reset.

The permanent P0138 code cleared. The CEL is now on for p0420 Catalyst system below threshold efficiency.

I take this as a positive step. Now I have both oxygen sensors behaving acceptably, so the ECU made it past proving them out and was able to check the cat. That's a positive right?

The fact it failed is less than optimal.

I think I'm going to follow you guys advice at this point and set up an appointment with the man. I don't mind throwing a couple of hondo at a problem replacing a couple of 9-year old sensors in the process....but I want to get an expert to tell me it needs a new cat assembly. Looks like the non-oem version of that including both cats and (what I guess you'd call) the mid pipe back to the connection to the exhaust manifold is about $370. That's a Walker version that is not approved for all 50 states....but I think is in mine.

If I knew it would fix it....I'd just do it....but I know it might not.

Phil

 

Good job doing your own work and technical diagnostics. I agree that if you had the parts- to just “replace and pray”. They were not too expensive and some positive came out of it. It does look like you will need to replace the catalytic converters (just an educated guess without seeing your engine data on a scanner). You can be definite with your diagnosis if you want to pay someone who “knows” how to read a graphing scanner and diagnose “your” diagnosis. What most people do not understand is that a scanner 99% of the time does not tell you what is wrong, but points you in the direction of what could be wrong. The technician has to know what he is looking at in the data provided to him by the scanner and then continue to diagnose and test parts to find the problem. There are plenty of parts changers out there but you are proceeding correctly by using the data. Good luck

 

My old code reader got left in my kid's WRX when he moved away for the summer...so I bought a BlueDriver Pro OBD2 Scan Tool (bluetooth to tablet connection) that I'm using for the first time ever on this project. It does a heck of a lot more than scan and reset codes and has some rudimentary abilities to graph. Before I take it to the man, I'll try to play around with the scanner and see if I can get a graph of Sensor 1 voltage, sensor 2 voltage, throttle position, and maybe MAF? Maybe it will have useful info.

In the "throw parts at it" category....I looked the engine over last night and the PCV looks like it's probably never been changed...so I grabbed one of those at lunch. $8.

The guy I bought the truck from did a great job of keeping it clean. Paint, frame, interior, all that stuff looked great...but other than that....he just drove it. Still had the original spark plugs in it until I bought it for instance.

Phil

 

OP, before you spend any money on a new cat you may find this thread most intriguing:

 

Man....that's some serious MacGyver work there.

Phil

 

...and its relatively easy and seems to be working a very long time. One guy is now at >500,000 miles.

And the chemicals are dirt cheap. I got mine at Walmart (citric acid) and Home Depot (oxalic acid used for washing decks) for just a few bucks.

Unless the ceramic substrate is actually broken, I can't think of any reason why someone with even just moderate wrenching skills would not try this before shelling out big money on a new cats.

 

The walker non-oem replacement with both cats, bolts right in, is less than $325 at my local autoparts store....but something to think about! If my only choice was $2K OEM Toyota....Heck Yeah...I'd be all over it!

My next step is replace the PCV, just on general principal and see if I can figure out how to get any meaningful data with this new OBD2 tool I have. I need to figure out what it'll do anyway. So...this is a good test for me and it.

Phil

 

Just to close down this thread guys....in my case the P0138 code was apparently being caused by a faulty #1 sensor ahead of it. Replacing both sensors with fresh Denso units (about $200 for the pair) cleared the code BUT once the sensors were working properly the ECU was able to test Catalytic Converter Efficiency and the CEL lit for P0420.

Luckily I found that there was still an outstanding recall on my truck (certain 2010 to 2014 Tacomas with 2.7) that will cover installing replacement cats. So...fingers crossed, I'll be CEL free some day soon.

 

Someone just liked my original post....and I clicked on it and realized I hadn't totally locked down this thread on cause and fix...so in case you are here searching for how to fix the P0420 code....my Toyota dealer replaced my cat assembly as part of an outstanding recall (for free). I picked the truck up and it was clear of CEL's for the first time since buying the truck. That was over a year ago and the CEL has not been on since. So problem solved.

If Toyota hadn't stepped up in a huge way to replace it with OEM parts I would have tried the Walker route. This is not an option in all 50 states.