I am having an issue with the P0138 not clearing Problem originally started started with a P420 code. Replaced the upper O2 sensor and that code cleared and never came back. But right after that, I got the P0138 that won't clear. Replaced the lower O2 sensor (that I replaced 6 months earlier) and that did not clear it Monitors also not completing are Evap (most the time) O2 (some of the time) and HTR O2 (some of the time). Not sure if it is all the same issue or a couple issues. Any thoughts? Someone said something about something that heats the O2 at start up so it gets to temp quick. Is there a module for that I can check?
A fuse? I have been Googling for a few weeks and can't come up with anything.

Thanks in advance.

__________________

 

 

Thanks. That is one of the things I found in my Google search. Was hoping someone who has had this issue could advise what worked for them.

To add more info, other than the Check Engine Light I am having no other symptoms. No foul odors, no driveability issues, no misfires..... just the P0138 code (and sometimes the incomplete monitors).
I checked and cleaned connectors with electrical cleaner last night and did not see any issue there. Let the car idle on the driveway for 30 minutes and no codes. Code came back after 10 min of driving this morning.

Hoping to avoid a diagnosis charge at the dealership

__________________

 

When you replaced your O2 sensors did you use OEM, because when buying aftermarket was you have to be really careful, as they don't always work. OEM is usually the best when buying O2 sensors even though they may be expensive.

https://www.700r4transmissionhq.com/p0138-scion-tc/

• Bad Oxygen Sensor– The O2 sensor itself is one of the most likely reasons that P0138 is triggering your service engine soon light. Although it is a temptation to replace it right away, it wouldn’t hurt to check out the wiring harness around it beforehand.
• Wiring Harness Issues– The wiring that goes to the front and rear oxygen sensors is very susceptible to damage. This is due to the fact that O2 sensors are under the chassis where they are subject to punishment from road debris, and they are subjected to a lot of exhaust heat.
• The downstream sensor is especially susceptible to this problem.
• Damage to the Terminal– The terminal that the oxygen sensor connects to (the plug) is also relatively susceptible to damage. You’ll want to make sure that there is voltage going to it. There’s more on that directly below.
• Wrong O2 Sensor– If you are getting P0138 after having recently replaced your Oxygen sensor, it may be that the replacement is wrong or bad.

 

I took it to a shop that specializes in Honda and Toyota. The only thing they found was a minor exhaust leak on the top of the muffler. Did not feel that was the issue and recommended I take it to the dealership for in depth diagnosis. Both downstream sensors were ones I bout from eBay. Starting to think maybe the O2 sensor is the problem. I think I am going to swap back in the original o2 sensor that I think I changed for a code other than P0138. If I get a different code and/or it takes longer to trip a code (was a few days as opposed to the current 10 min of driving) I would guess it is the sensor. RockAuto has Bosch, Denso and ATK. I would lean toward Bosch, but does anyone have another recommendation? Also (on a side note) why is the upstream sensor so much more than the downstream?

__________________

 

I would recommend Denso as that is the OE Part/Manufacturer for most Toyota parts. The difference between the OE part and OEM part it really just price mark up, but there are differences between parts as Toyota has its own standards. So, I would say go with Denso or OEM. Upstream sensors control the air to fuel ratio, downstream sensors only purpose is to check catalyst performance/effeciency.

 

So tonight I put the original downstream sensor in. No change. Checked voltage on pins at harness and got 12 volts on one of 4 the pins. Not sure if 2 of the 4 pins should show 12 volts. Off to the dealer tomorrow to see what they say. Out of curiosity, anyone know where the wiring goes after the it plugs into the harness? Does it go to another junction or into the cabin or something? Couldn’t find anything with Google. Thanks in advance

 

Also one last thing........could a cheap or incorrect upstream sensor give the P0138 on the downstream sensor?

__________________

 

Yes, a cheap or incorrect upstream sensor could cause the code. What the code means is that the Engine Control Module (ECM) detects that the rear O2 sensor signal voltage remains excessively high for an extended period of time.

How to test an O2 Sensor -
https://itstillruns.com/test-sensor-5193043.html

Possible Causes and Symptoms -
https://www.yourmechanic.com/article...by-jay-safford
https://www.700r4transmissionhq.com/p0138-scion-tc/

 

So put the original upstream back in and as of 30 min of highway driving to dealer with one stop......no codes (when I would get the P0138 within 10min).
So skipping diagnostics for now and will monitor. Any change is good when tracing the issue. No codes or a different code is good.

Will see what happens and might get a Denso upstream if needed

__________________