Back here again, just recently went through and replaced my knock sensors and intake manifold gaskets, o2 sensors, many vaccumn lines and such, bringing my number of CEL codes from 9 to only 1, leaving me with “system too lean bank 1” I know it’s common for this to be thrown from air leakage pretty much anywhere from past MAF to downstream o2s. Haven’t seem to come across any so far but trying to see if anyone knows other causes of said code. Thanks!

99 gs4

edit: forgot to mention car is catless, wondering if that could effect my situation

 

Hello,

What parts did you use?
Was the Lean code present before the mentioned repair?

Lean mixture can be caused by a variety of factors, here you can learn more about Fuel Trims, mixtures, and causes for those conditions.

Starting with the Vacuum Leak, a proper way would be to use a Smoke Machine, but to at least have an idea whether there is a leak at all, you can start the car up and block the intake, the car should die almost immediately, if it hesitates or there is a hissing sound somewhere on the engine, chances are you have a Vacuum Leak somewhere.

Other cause for Lean condition can be faulty sensors, specifically MAF and A/F sensors. You wrote that A/F sensors were replaced, but it's always important to make sure that new ones behave exactly as they should. Here is a thread on how to do it, in short, check that the readings are close, but not stuck on 3.2-ish volts, if that's good, floor the car for a split second, the readouts should immediately go to Rich side, and then level out, if there is a considerable delay, the sensors are bad.

MAF sensor should read about 3.5 - 4 g/sec at idle, if it is more, the sensor is likely getting old and either needs to be cleaned, or replaced altogether. If it is less, there is a chance you have a vacuum leak, as the air finds its way around the sensor, the readouts drop as well.

Hope this helps and best of luck!