Help with po138 code

My 06 4.7 is throwing a o2 code every 30 to 50 miles. My OBD Fusion reader reports po 138 02 sensor high voltage. I have read all related post on this forum and other sites. Understanding there are many possible causes for the code. Where to start troubling shooting and replacing parts? Could use some help understanding these readouts.

 

Taking my Tundra in for oil change next week. They will likely recommend replacing all four O2 sensors. Should I do that?

 

If you replace the O2 sensors, only use Denso (OEM) brand sensors. Don't install or let anyone install non-Denso sensors. Don't let anyone splice wires, either. Buy them yourself if you can't trust the mechanic to use Denso, just don't buy them on scAmazon or fleaBay, or you're liable to get very authentic looking fakes.

Meanwhile, replace them all?

No. Overkill. P0138 is tied to bank 1 exhaust (driver's side) which has one O2 before the cat ("upstream"), one O2 after the cat ("downstream"). I prefer replacing in pairs, so I'd suggest just replacing the upstream and downstream on bank 1, and wait until you're tossing codes on bank 2 to replace those.

 

I echo everything @shifty` said especially the OEM sensors. Toyota engineered these 1st Gens with tight tolerances that aftermarket/knockoffs will not do as well. The O2 sensors are one of the important if not the most important component when achieving great fuel management on our Tundras IMHO. Combine O2 OEM sensors with a properly functioning, properly cleaned OEM MAF, properly cleaned throttle body, OEM plugs, ignition coils and these engines purr forever.

While I'm not disagreeing, IF another mechanic is doing your O2 work and IF you are online ordering the sensors, you might want to consider all 4 sensors which would save you on shipping and possible mechanic cost getting everything done at once, especially if you have lots of miles on your truck(say 200K +) and never been done.

 

Definitely agree here too, if the money is freely available. When one of my O2 sensors tossed at around ~67k miles, I didn't have ample funds, so replacing all four wasn't an option. Chances are you'll save money on install, and probably save money on the parts.

 

Thanks Is replacement something simple or not? I have changed the plugs and some other minor work on my Tundra. I'd like to save $ on labor. The truck has 171k on the originals, so probably should at least both bank 1 sensors Know of a good place to buy online?

 

The bank 1 upstream is a bitch to get at. The wiring harness is a pain to unclip. That's the extent of it, if being blunt.

You will bust knuckles. It won't be a fun job. But it also won't be an impossible job. It's no harder than changing your oil.

My take is, if your shop will swap the sensors for an hour of shop time, unless they're charging more than $140/hr, if your hands are old or broken and not working like a teenager like me, it could be fruitful to let them manage, but you supply the parts so you know what's going in there.

You'll want to do yourself a favor and (1) get a side-load O2 sensor socket like this AND (2) make sure you hit the O2 sensor threads with some penetrant after the next couple drive cycles.

You can order the Denso brand sensors from RockAuto (get 5% off) or from Summit Racing. Summit has really fast shipping. Both are 'clean' sources.

I checked Denso's site using this query for an '06 DC V8 Limited.

It looks like these are the Denso part numbers to get:

 

Adding to what @shifty` said....the front is a bitch but very doable even at my advanced age.
If you choose to do it, unplug a rear sensor and see how the connectors come together and in particular the tab you have to depress. With that knowledge, look for the front sensor and it's connector.
What can help is a little screw driver to somewhat gently jab into the connector to somewhat unlock the connector, then attempt to unplug. The reason being one of your hands most likely will not get to have any fun since theirs hardly no one for 2 hands blindfolded. I'd also recommend not doing it when outside temps are cold because your hands will let you know. If you're a young whipper snapper, dive on in and Get Ur Done !

 

I intend to try. In reading, Others say there could be other causes throwing the code and new sensors will not solve my problem. I can't detect any exhaust leaks, no drop in fuel usage, no bad smell from exhaust pipe. Start with both bank 1 sensors at first.

 

There is some validity in that opinion but unless someone has a scan tool that will read live data, preferably capable of graphing, they themselves would be making a WAG(wild ass guess)....speaking of oxygen sensors in this case. What they can't dispute is that Bank 1 is reporting a problem. Some will even jump all over adding a fouler to extend the rear sensor further out of the exhaust gas of CAT which may or may not stop the generation of code. You have to make the call how you wish to maintain a Tundra that can conceivably go 100's of thousands of miles properly maintained.

Proper air/fuel management can be costly especially with gas prices but improper maintenance can be costly also.

 

Not just this, but let's be real:

How much mileage is on this truck? If OP got 150k miles / 15 yrs out of an O2 sensor, he's already ahead of the game and likely exceeded the expected lifespan of the sensor itself. It wouldn't be at all wild to believe part of the sensor failed at this point.

Just replacing those two sensors, he won't be doing it again for at least another 100k-150k / 10-15 years if Denso replacements. Are there guys here on the original O2 sensors at 220k miles? Yep, I know at least one. But like I said above, I had one go before 70k, at 14yrs old.

While I'm choosy about my preventative maintenance, this is a no-brainer for me. Logical reason is simple: The chances of it actually being a failing O2 as the code indicates is much higher than some other crazy ass culprit.

 

I know when I did mine it was limited access, and I needed a lot of heat to get it to break loose.

 

Thanks to everyone for comments. Hope I can get by until better weather gets here. Then I will try to replace both drivers side sensors. I got a estimate from AAA. They want $668.00 to replace the two. Their policy does not allow them to use parts I supply. I can get the two for about $100.00 from Densoproducts.com. The po138 has displayed twice in earlier 100 miles driven. But not in last 50 miles. Fingers crossed it was a random occurrence and does not happen again.

 

Follow up: I spent about an hour today using a o2 slotted socket taring to break loose the bank 1 rear sensor, but could not get it loose. Presoaked with penetrating spray for 30 minutes and engine running to get heat up. One thing confusing to me is all the how to videos I have seen show removing the sensor mount plate with the sensor in place. I was tring to remove the sensor only. On my truck the sensor screws into the catalytic converter. I am trying to avoid removing the heat shield Should I remove the two nuts holding the mount and remove all in one piece? This would require removing heat shield. I have not purchased the sensor yet. Want to make sure I can get the old one out first. Shoppping around for sensors, some listings show a sensor and mounting plate and others just a sensor

 

Found today the heat shield does not have to be removed to get the o2 sensor out . There is no plate and studs involved. Sensor screws directly into the catalytic converter. I was able to break loose the sensor with a 18 inch breaker bar and it came out easily with fingers. Since i was waiting to order the sensor I decided to try cleaning with carb cleaner and replacing the cleaned sensor. Not expecting this to work, but why not try

 

Followup 3/20/23
Cleaning sensor did not stop the code. Got a new

$47 Denso 234-4260 Oxygen Sensor

Denso 234-4260 from Rock Auto. New sensor did not stop the code either. Not throwing any more parts for now. Engine still runs fine. Will check mpg and if thats ok, its black electrical tape over the glowing light and checking/clearing codes every few weeks. Already checked for vacuum leaks, loose/frayed wiring, cleaned maf sensor, new plugs and air filter.

 

Did you replace only the upstream, not the downstream?

I was always taught to replace both upstream and downstream any time one goes bad. I mean, if the code (what was the code again?) indicates something like a circuit failure, then you can usually get by, but if one goes out, chances are the other will follow not long after, and sometimes you'll get codes because one sensor is reading 'off', and it's marring the delta.

 

And just to confirm, you did replace the downstream, right? P0139 is Bank 1 (driver side) Sensor 2 (downstream).

Downstream sensor and upstream sensor are a different part number.

If you replaced the upstream, that's a bit of a snag...

 

Might be how this site certain items but when clicking on that Denso link from Rock Auto you posted, it takes me an item at Amacon

 

Did replace the drivers side down stream. Code being set is po138 high voltage. Will consider replacing upstream bank 1 sensor after checking mpg and some time has passed.

 

Yeah, the site auto-links on any product number recognized in scAmazon's system. It'll go so far as to hijack the link you post sometimes. I get it - the forum owner wants to capitalize on revenue, and there aren't a lot of revenue streams available other than using subscriptions and adverts, which nobody wants. Bittersweet situation.

 

Good plan. I'd have replaced both, but I realize that's not within everyone's budget. Even knowing you'd be busting the shit out of your knuckles to pull the upstream out.

Needless to say there is a free test you can try, but knuckle busting is involved. Swap the upstream sensors, left to right. If the code follows as P0158, you know it's the upstream sensor causing the issue. If not, it's more likely a wiring problem, corrosion, bad ground, spliced wire, or similar.

 

Its a glitch. I ordered and received the part from Rock Auto





More Information for DENSO 2344260
{Alternate Inventory Numbers: 234-4260}
Continue Shopping


DENSO 2344260




$43.79
Price

 

That's the correct p/n for sure as you see here. Upstream is 234--9051 which is definitely the more-spendy of the two.

 

Well, still have the code po 158 high voltage bank 1 sensor 2.. To recap here is what I have done. Truck has 171 k on the odom. It runs fine Gas mileage is good. air filter, new plugs, cleaned the maf sensor. First inspected as best I could for vacuum leak, cracked manifold loose wiring, fuel cap. I do have a minor valve cover leak. Don' know if that would throw the code?
Replaced downstream (no 2) sensor with Denso on bank 1. Cleared code Code returns at ~50 miles. Took all four sensors out and reversed their position(s making sure they were installed in correct new positions, a/f in upstream and sniffers in down stream. Cleared code it returns at ~50 miles. I was thinking if I have a bad sensor the code would follow to bank 2, but it stayed on bank 1 sensor 2. More ideas appreciated. Possibilities bad cat, valve cover gaskets, bad injector,

A learned lesson for me. New parts don't always solve code problems. O2 sensors just report a problem. They don't tell me the sensor is the problem. You wouldn't replace a code scanner because it read a code.

BTW The down stream sensors broke loose easily with a 12" breaker bar. The upstreams broke easier using just a small ratchet. The hardest part was laying on my back underneath the truck. The connectors are difficult to get loose. You have to use a flat head screw driver to press the release clip. Passenger side upstream is the hardest to get to.
About ready to use the black electrical tape to out of sight out of mind

Last scan results

 

Rewind a sec.

You just said you're now throwing a P0158 which is passenger downstream sensor circuit high (B2S2)

The start of this thread you were throwing P0138 which was driver downstream sensor high (B1S2)

If you reversed sensors between banks, this makes sense: The code should've changed from left to right, respectively, and the (now in passenger downstream position) needs to be replaced.

 

My error.. Need to get my eyes checked and pay more attention . The problem did jump to bank 2 sensor 2. Its now po158. It was on bank 1 sensor 2 code po138. Strange thing is The po138 code was popping on before and after I put a new Denso sensor in bank 1 sensor 2. When I reversed sides with the sensors, the new Denso is now the sensor throwing the po158 code. My golf buddy thinks its a fuel injector problem. If its a bad injector, would not the code stay on the bank the bad injector is on. Don't think Denso bought at Rock Auto would arrive faulty .

 

Have you changed the front o2 sensors? I had p0138 and p0158 and changed the front o2 sensors to fix it.

 

Black tape over the light is not the solution for O2 sensor problems. Eventually the open loop rich running all the time will burn your cats out and cost you more money. Cheapest initial route would be testing all the sensors to determine which ones are out of spec and need replacing. Easiest route would be replacing the sensors on the DTC indicated trouble side in pairs. Even better, replace all 4 and be done with it. OEM Denso is a no brainer here due to other aftermarket options not performing well. I’ve had good luck with the Bosch sensors, but would replace OEM myself if a code popped up today. Good luck with this. Oh, and your valve cover leak has nothing to do with this code. Your golf buddy is spit balling about injectors, but a leaky injector could add additional fuel to a cylinder that wouldn’t all burn or be clogged and not inject enough fuel creating issues due to the AFR being off in that cylinder. That should create a different code for issues in that cylinder. I’ve not heard of that being the cause of anyone’s O2 sensor codes, so I wouldn’t chase that down as a priority to fix this issue.

 

FIXED (fingers crossed) I did swap the sensors and the code followed to bank 2 sensor 2 (po158) The previous code was po138 . Remember, I had already put a new Denso in bank 1 downstream and the code still came. Then I replaced the bank 2 upstream. I took a 150 mile trip with no codes.. Now both sensors on bank 2 are new and the sensors on bank 1 are the old ones moved over from bank 2 So, a upstream sensor can blame the problem on its downstream partner. You and others are right