Celica Forums General Discussion Engine and Performance Intermittent OBD error code 171 (System too lean) - 04 GTS

Hi, I have been picking up this intermittent OBD error code 171 (System too lean) on my 04 GTS the past few months. Check Engine Light comes on for awhile then goes off. In addition, there is also an slight odour of exhaust fumes or something in the cabin when driving more than 30 miles. Performance seems good as well as idle, here's what I've done:

Cleaned MAF
Cleaned the throttle body and IAC valve
Replaced Air Filter
Replaced PCV valve
Visually inspected vacuum hoses I could without taking anything apart
Reset the ECU, but the CEL came back shortly afterwards (300 miles)
Replaced Spark Plugs (running almost white. Grey = too lean)
Used sea foam in the gas
Running only Premium gas
No excessive oil burning or loss, average

I have spent a lot of time reviewing the forms on this on several sites and am thinking O2 Senor(s), Cat, Exhaust leak, Intake manifold leak or dirty fuel filter or faulty injectors?

Have it scheduled to go to the Toyota Dealer next week and am not looking forward to it.

Comments welcome, thanks.

I am having the same problem on my 02 GT. The only thing I haven't done is clean the throttle body. Never have time to do it(always working). I just replaced the o2 sensor, reset the ECU, and light came back on after driving 15-20 miles. I went to a local mechanic shop about 3 times this week, and nothing. I guess I'll have to do the same thing you are doing.

Do you have an aftermarket intake like a CAI or SRI?