P0172 Code Diagnosis

I’ve got a 2000 SR5 T4R, P0172 Error Code “System Running Rich, Bank 1” keeps popping up, I’ve cleaned the MAF, and replaced the downstream Air/Fuel sensor. Examined the throttle body and it doesn’t seem to have become stuck open to any degree (I was advised to check that by someone who used to own a same gen 4Runner), what else might be causing this trouble code? Performance has not suffered, rig runs and drives perfectly fine, but I just want to figure it out.

 

Reset the code and see if it comes back. I get a code for my downstream A/F sensor every 6-12 months. It's been doing it for years. I reset the code and go another 6-12 months.

What do your Fuel Trims look like?

Sometimes it is a bad injector that the ECU can't compensate for. You see it when guys buy non OEM injectors on Amazon or Ebay. Or OEM injectors that were "flow tested". No one on ebay or amazon flow tests a set of injectors.

I believe 2000 4Runners were NOT A/F sensors. A/F sensors were in 2001-2002 but there may have been some crossover (for Cali cars?).

Did you replace both O2 sensors or just the downstream one and did you use a TOYOTA one? Again, non-OEM sensors simply do not work reliably.

The upstream O2 sensor I believe is just as important as the downstream one in order for the ECU to know whether you are running rich or lean.

Did you check your air filter? Saw a thread with an absolutely clogged air intake throwing a rich code.

 

Yeah it is a Cali car I did the proper research when I got the part. Denso A/F sensor, just the downstream. Air filter seems fine, and I’ll check the fuel trim later.