I've been working through the CEL issues associated with installing the long-tube headers and eliminating a catalytic converter. Based on some advice I got here on the forum I added a spacer on the O2 sensors. The passenger side hasn't thrown a code in weeks but the driver's side still is. I started with the sensor as close to the exhaust stream as it would go and have been slowly backing it out looking for that magic spot where it no longer gives me a code. But it's not happening. As the title says, I'm getting a P0138 code saying that there is high voltage at the sensor. My first question is what this code indicates. Does it mean that the sensor is still getting too much untcatalyzed exhaust and I should just keep backing it out? I'm starting to wonder if there is an issue with the sensor itself or the plug. One caveat that I have to mention is that I'm using an aftermarket Bosch O2 sensor on the driver's side. On one of the many times I had the old sensors out, one got damaged so I replaced it with the Bosch.

In hindsight I wish either myself, or the shop that installed the headers would have anticipated this issue. Even though it shouldn't have been a surprise, nobody saw it coming so now I'm dealing with it. I thought that spending less than half the money on the Throrley headers, compared to OEM, was a no-brainer but there was obviously more to the equations that I should have considered.

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the spacer on this side is spacing the o2 sensor out too far and you're not picking up any readings to the sensor at partial throttle which is making the ECU think the sensor is faulty. What I did when I had a similar issue to bypass a failing cat is cut down the sensor spacer a bit to get the o2 sensor a bit more down into the exhaust stream until it started reading exhaust flow at lower throttle inputs.

I followed this very guidance you gave me on another, more general thread I started. About a month ago I started with the sensor as close as I could get to the stream while still using a spacer. It gave the same code I'm getting now even though I've been backing it out little by little, and now have it as far back from the stream as I can get. That's why I'm wondering if there is something going on with the Bosch sensor I put in. I'm not really sure how to troubleshoot the sensor or it's plugin on the vehicle.

You're getting a high voltage code at the 02 sensor. The voltage signal increases with heat. Moving the sensor element closer to the steel of the pipe with these spacer tricks increases the voltage/heat signal at the element. Telling your PCM your truck is running rich. So code.. The sensor is easy to test with a voltmeter and a propane torch. You can also monitor the sensor in real time when you're driving with a voltmeter and some lead extensions hooked to the O2 input terminals at the PCM.. did urd quit making that simulator that wires directly into the PCM O2 terminals.. I'm pretty sure they use the resistor/capacitor method to simulate the signals but make it easier to install at the PCM.

So I have doug thorley short tube headers, maganflow cats, 90 degree spacers, and brand new denso O2 sensors.. and im still throwing a p0138 code.. any ideas? it’s killing me

Nothing that is currently tried and true.. without the URD simulators that used to be available there isn't a robust solution thats going to work for everyone.
I'm running stock headers, but do have a failing cat that will throw a code. After my experimenting with moving the sensor first far enough out to not read the lean condition, I then had to play with spacing to put it back towards the stream just enough to pick up the partial throttle readings it's expecting. Once I had the spacing guessed correctly and codes cleared it's been running that way ever since.. I think im going on almost 1.5 years at this point without seeing the code.

My guess is that the 90 degree adapter is doing something similar with spacing it too far out of the stream. What I would suggest is trying the standard Non-Fouler spark plug spacer that you drill out for the O2 sensor head and just use some washers to slowly start spacing the sensor back into the exhaust stream.

I imagine without any cats on this may be a bit more difficult to do than with failing cats since the factory cats, though on their way out, are still doing some work, where as in your case you don't have anything pre-filtering your exhaust before it hits the O2 sensor. I think you still have the possibility of getting the spacing just right, where it sees partial throttle exhaust flow, but is far enough out of the stream where it's not reading super rich.

URD was bullied by the EPA enough to not only stop selling their simulators, but also "officially" claim they never offered anything of its kind (or so the story goes )

I come from a mechanical background with out some basic electrical knowledge... but your comment did make me wonder though... if we know the approximate range of resistance that the ECU is anticipating seeing throughout the RPM range based off of the throttle position... (IN A STRICTLY OFFROAD USE APPLICATOIN ) would it be feasible to use some sort of a logic device that takes in the varying output from the OEM O2 sensor and coding it to put out a changing signal to feedback into the ECU that would be within the acceptable range?

I know it's unfortunately not as easy as just clamping the output signal to a certain resistance value since the ECU expects this range to fluctuate, or it will throw a faulty sensor code.. but... since it's only the upstream sensor reading is used to alter the air/fuel mixture and the downstream sensor is only in place to validate the efficiency of the catalytic converters... as long as the output can vary AND be within the acceptable range theoretically that should work..... maybe?

Thank you for the reply, I appreciate the info. I wish I could have gotten a URD sim before they became unavailable.
I do have cats though, I bought Magnaflow cats with my dt short tubes because I live in Las Vegas and need to pass a smog. I had issues forever with the exhaust and passing so that’s why I went all out and bought new everything. but even ever since I put on the new headers and welded the cats to them and new sensors, it still won’t stop throwing codes. That’s why this is still so baffling and frustrating.
Do you by any chance have a picture of your spacers? I know back in the day when I had the straight spacers someone had told me not to screw them in all the way cause the gap helped it to not throw codes.

may not be these exact ones, but this is what I'm using on mine. Originally I had 2 of them stacked, which resolved reading too rich from the failing cat, but it didn't get enough gas flow during partial throttle so it threw codes for a failing sensor. I basically went through 2 or 3 iterations of cutting down the second spacer to slowly move the o2 sensor back into the exhaust flow until the code didn't show up again. It was a bit of a tedious process, but has been running ever since.


The fact that you're running some sort of a cat should help with not having to pull the sensor too far out of the flow to get the proper reading, but running a better flowing header even on a perfectly functioning CAT is likely to throw a code so unfortunately there isn't an easy plug and play solution, at least in my experience.

If you dont already, pick up a cheap OBDII reader that reads out live data and go to work with spacing out the rear O2's and monitoring the reading during partial throttle. In my case I was throwing codes when I wouldn't see any voltage change on the o2 unless I was nearly 50% throttle. If I'm not mistaking I can see voltage start fluctuating now at about 10-15% throttle.


The fact that you're throwing a P0138 code and not P0420 means you're on the right track and confirms that taking the sensor out of the stream make the ECU believe the CAT is performing at designed efficiency level, BUT the fact that it's showing a P0138 code means that at the same time the ECU thinks there's something wrong with either your sensor or wiring to not get the feedback it anticipates as the engine runs through the RPM range.


I think one thing that could maybe be helpful is using one side of the engine as your baseline, ie: remove any spacer you have and put the sensor as it's meant to be installed and compare the voltage readings you're seeing. It will undoubtedly throw the P0420 code, but this could at least help you get a visual of what the ECU anticipates seeing for voltage changes during your partial throttle driving and can be used to slowly bring the other O2 back into the exhaust stream to keep the codes off.

Like I said, it's tedious ... but I feel like it's well worth the time. The longest part of the process really is driving around until the systems initialize after the code reset. I was lucky enough to be able to get it all done within the same long day, but depending on the driving conditions it could take a bit longer for the ECU to run all the system checks.

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Yes, you can trick the PCM by wiring in a capacitor and resistor into the circuit. I used this trick temporarily until I could diy tuner them out. The spacing "trick" seems not right to me. This should help you understand the input values expected at PCM.

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I have the same issues on an 04 4runner with Thorley Headers and no cats.
Does anyone offer a tune to remedy this?

Doug would love to include the technology to tune your PCM to it's new breathing technique. The same way a supercharger comes with the tuning tech. Unfortunately he can't because of the governments environmental bullsh1t. Yes, you'll need a tune, diy or technician. We have a shop here in NW Arkansas that does this for Toyotas specifically. Or diy. Anytime you change the inhalation or exhalation outside the PCM parameters you need to tell the PCM. The header upgrade is useless without a proper tune. Your skid ECU will be disabled and your a/f f_xked up.

that is super helpful man, I appreciate your feedback on this!
My "spacing" out of the rear O2 has worked for the past year without issues.. I'll definitely be trying the resistor/cap method to test out if/when it comes back.

Since the rear O2 sensor is literally just a health check on the CAT to ensure it's filtering properly it should have no impact on the actual anticipated A/F to keep the engine running healthy, so in the case of failing CAT's it could be the needed solution, but I agree that to take full advantage of headers you would really need to tune the upstream A/F sensor to get the most out of the setup.

No problem! I agree with your thought process in regards to the downstream O2s, that they should only monitor cat efficiency and have no affect on a/f ratio/engine performance. Unfortunately the government disagrees with that now days and requires methods to maximize the efficiency of the cat, idiotically. Even if it isn't documented in the FSM the downstream sensor is also used to fine-tune the a/f ratio up to 2% with Toyota to maximize catalyst efficiency and lifespan. It can also adjust the a/f ratio to compensate for a degraded catalyst. They don't give a sh1t about your engine running properly, which would result in very low amounts of pollution, without cats. They only care about the catalytic converter because the herd of sheep is too retarded to maintain PCM inputs or understand basic organic chemistry and complete combustion/stoichiometric ratio. Best to tune them out. I'm an not a human 👽

Did I just get lucky installing my DT short headers? Using the same OEM cats and O2 sensors I was using before and have no codes.