Still getting a PO135 On my 2005 GT

[Squiggle05GT]

Hey everyone,

I recently swapped out my stock headers for a set of BBK Shorty Tuned headers, along with new O2 sensors—two for the upstream and two for the downstream. I'd been getting a code for a while, so I figured this was the perfect time to replace the rusty, old headers. Let me tell you, those things were completely shot!

I thought everything was good to go after the install, but as soon as I cleared the code and drove home, it came right back. I took it to VPS Racing here in Florida, a well-known shop in the drag racing scene that specializes in custom work with superchargers, turbos, nitrous, and more. They did a fantastic job, so I have to give them props. If you're into performance work, check them out on Instagram: [VPS Racing Instagram](https://www.instagram.com/vps_racing/?hl=en).

Anyway, I was pretty bummed to see the code return after dropping all that cash. After doing some research, here are a few possibilities I'm considering for why the code came back:

1. One of my PCM relays might have gone bad.
2. My stock catalytic converters could be on their last legs (though I'm not entirely sure about this one).
3. One of the wires for the O2 sensors might be pinched or not fully connected.

Any thoughts or suggestions from those who've dealt with this issue before?

 

[AHaze]

I can tell you it has nothing to do with the cats. New O2 sensors should eliminate those as a possible source of the issue but if you want to be absolutely sure, you could swap the upstream sensors side to side and see if the code changes to a P0155. If the P0135 remains, you'll likely have to check for continuity in the harness going to the passenger side upstream sensor.

 

[86GT351]

 

[Squiggle05GT]

I can tell you it has nothing to do with the cats. New O2 sensors should eliminate those as a possible source of the issue but if you want to be absolutely sure, you could swap the upstream sensors side to side and see if the code changes to a P0155. If the P0135 remains, you'll likely have to check for continuity in the harness going to the passenger side upstream sensor.

Thanks for the advice! I'll try swapping the upstream sensors to see if the code changes to P0155, which would rule out other issues. Interestingly, when the P0135 code appeared, I also started noticing other issues. The parking brake light stays on during cold starts but goes away after I start driving and restart the car. I've also had a low oil pressure warning come up during longer drives but it also goes away after I turn off the car and turn it on again.

I plan on checking the PCM relay and fuse to see if it's causing these problems. I'm also tackling the low oil pressure issue soon. I just bought a new OEM oil pressure switch, which I'll install during my next oil change along with new spark plugs, coils, and a set of Ford Performance valve covers. The coil plug bolt areas were cracked from the previous owner (which I didn't realize until after buying the car—lesson learned!), so this should hopefully clear up a few things. Thanks again for the help.

 

[AHaze]

If you're getting multiple seemingly unexplainable electrical glitches, it might be worth checking for a failing alternator and/or battery as well.
I've had some weird stuff caused by both.

 

[JC SSP]

I second that… rule out the basics before changing any parts.

 

[Squiggle05GT]

If you're getting multiple seemingly unexplainable electrical glitches, it might be worth checking for a failing alternator and/or battery as well.
I've had some weird stuff caused by both.

When I first bought the car, one of the first things I replaced was the alternator, and I made sure not to cut corners. I went with an SR Performance 200 Amp Alternator along with their Premium Power Wire Kit, a Ford Performance SVT cooling fan, and a new condenser to fix a failing alternator and a leaking condenser for the AC system. Most of the parts I've purchased have been from American Muscle, prioritizing ones with the best reviews, and I've also sourced parts directly from Ford dealerships. However, I'm not entirely sure how long the battery has been in the car, so I'll be checking into that as well.

 

[Squiggle05GT]

I realized I never actually checked out what the old O2 sensors looked like when they were removed from my car. I couldn't be there for the job since I was in classes that day, so I just trusted it was done right. Today, though, I took a thorough look at the old sensors and noticed something strange: the heads on the old upstream sensors look different from the new ones that were installed.
Here's what the old upstream sensors looked like which were NGK from the branding on them:



And here's what the new ones look like (I bought 2 Bosch "Upstream" and 2 from Motorcraft "Downstream":



Has anyone else had a similar experience with replacing O2 sensors? Could this difference in design impact performance, or did I end up with the wrong type of sensor?

Thanks in advance!

 

[86GT351]

I realized I never actually checked out what the old O2 sensors looked like when they were removed from my car. I couldn't be there for the job since I was in classes that day, so I just trusted it was done right. Today, though, I took a thorough look at the old sensors and noticed something strange: the heads on the old upstream sensors look different from the new ones that were installed.
Here's what the old upstream sensors looked like which were NGK from the branding on them:

View attachment 101588View attachment 101589

And here's what the new ones look like (I bought 2 Bosch "Upstream" and 2 from Motorcraft "Downstream":

View attachment 101587View attachment 101590

Has anyone else had a similar experience with replacing O2 sensors? Could this difference in design impact performance, or did I end up with the wrong type of sensor?

Thanks in advance!

The difference in design is due to legal issues and patents. If a product has a patent on it and another company start producing they need to change something about it.

 

[Squiggle05GT]

The difference in design is due to legal issues and patents. If a product has a patent on it and another company start producing they need to change something about it.

This O2 sensor issue has been driving me nuts. I finally picked up a floor jack from Harbor Freight—the Daytona low profile with long reach and 3-ton capacity—to get under the car and check the O2 sensors. Between school and midterms, I haven't had much time to tackle it, but this Saturday I'm putting my full attention into the problem. I also got me a can of CRC QD Electronic Cleaner in case the connections are dirty, causing a weak signal to the O2 sensors, and a circuit tester to check the wiring. On top of that, I ended up ordering a new O2 sensor with the different design just to see if it's the correct one for the car. Also when I bought the car the owner had told me he added a Bama tune, and I think I remembered reading somewhere that some tunes can affect the O2 sensors, which might be why I'm getting that PO135 code. I read somewhere that if the sensors aren't the right ones, it could mess with the computer. Hopefully, all these steps will get me closer to solving it.

 

[JC SSP]

PLEASE don’t forgot to use jack stands when working under the car. DO NOT solely rely on the floor jack!

 

[Squiggle05GT]

PLEASE don’t forgot to use jack stands when working under the car. DO NOT solely rely on the floor jack!

Yea I got that in check too, my dad got a pair for free rated 4 tons each from a yard he worked at

 

[Squiggle05GT]

Recently found out what was giving the problems, it was my PCM. Now I need to find a replacement for it, I read Around and someone said they would recommend these guys:

Flagship One

Ford PCM | Ford ECM | Ford ECU

Our Ford PCM are specifically programmed to your VIN and flashed with the latest software update from Ford. We offer an immense variety of PCM for all engine sizes and emissions.

www.fs1inc.com

Has anyone else had any experience with this company or used another service/company to get it replaced?
Please let me know.