Hello everyone,

Hoping to tap the collective knowledge to figure out this enigma. I appreciate any advice.

June 26th, 2019 I got my first P0175 and P0172 codes, at the same time. Rich in left and right bank. Clearly caused by something fuel control/emissions related that effects both banks of this '05 V8.

I have the doug thorley shorties and running all 4x magnaflow high-flow cats (installed in 2017). Original O2 sensors... all of which continue to pass all sensor tests, and always have (first device we wanted to blame). I have the SAIS bypass kit as well. I contacted the manufacturer and they provided a set of instructions to test the unit to see if it may be causing the code. Unit passed testing, and connected or not, makes no difference.

At the time I likely had around 300,000km

I had the intermittent stalling problem around that time, eventually figured out it was the throttle body, and replaced that. Stalling issues disappeared but not the P0175 and P0172 codes showing up.

Back in the summer of '19, I replaced the spark plugs, cleaned the (original/broken) throttle body, new OEM MAF sensor, new air filter; all of which had no effect on the CEL.

Both codes would appear and disappear intermittently, problems undetectable with smoke tests, on-board systems diagnostics, and external testing. Whatever was causing it wasn't easily found. At one point my fuel trims were out over 30%, but still no direct cause could be found. All systems registered and measured as normal, all devices would operate within normal perimeters during testing & test driving. I spent some big money having the guys drive it around with equipment connected trying to figure it out...

I got fed up in Dec 2020 and decided to throw parts at it. The shop's Toyota Master tech said that the three major things that could cause it are:

• Thermostat
• Coolant Temp sensor
• Fuel Pump/Filter
• (for good measure) injector service

So we did all those... and it's run great since. Until Friday, May 21st. I had my summer tires installed and the CEL came on after about 30 minutes of driving. Of course it's my old friends, P0175 and P0172. I don't understand how that can be related, seems to me to be a pure coincidence. My summer set (BFGs) are significantly heavier than my winter set, likely around 30% heavier.

Anyway, just as strangely, the CEL has disappeared once again, indicating it is running within acceptable parameters, but it'll just come back later.

I was hoping others have had this issue and may have some insight into what could be causing it. When it runs good, it runs perfectly great. Smooth as butter and pulls all the gears hard.

Thanks in advance.

-Don