Can't pass smog because of engine light always goes on

I have a 99' 4runner and I have been getting a continuous PO172 code (bank 1 running rich). I have changed everything that I can think of: spark plugs, sp wires, MAF sensor, front and rear O2 sensors, fuel pump, fuel injectors, coils ...I believe that's all of it. But the check engine light always goes back on after I clear the code. Normally, it would go back on when it is cleared and then when I shut off the truck, then restart it. It doesn't matter if it's a short or long distance, it will go back on reading PO172. Any ideas?

 

I found a talented auto electrical specialist who had a shop near my home years ago.
He helped me get a GMC Sierra 6.0 working. His OBD2 instrument reader was professional grade and did a deep exam of components plus he had vast knowledge.
Have you considered asking around your area for a top notch independent electric repair shop?
Maybe the issue is in the ECM?

 

Have you tried resetting the ECU by disconnecting the battery for a couple minutes? I had one once in my Subaru that would come back after being cleared, but stayed away after an ECU reset.

 

Might be a bad catalytic converter? Or an exhaust leak? O2 sensors will go bad but usually show a lean condition. Otherwise it could be a few things like a bad injector, bad fuel regulator...

 

I would see about getting an advanced scanner and investigating teh switching rate between front and rear O2 sensor to see if you have a failing catalytic converter

 

They'd fail you after noticing that it doesn't illuminate when the car is started

 

They scan for codes anyways. Check engine light is meaningless.

 

They hook the computer up to the port under the hood. Prob still fail.

 

Your almost antique? Is it 25 years old there?