o2 senor issuse and running to lean

[RSdriftPrncss]

Hey guys I’m needing some of your expertise on my subie. My check engine light keeps coming on. I hooked up my code reader and the first code I get is the o2 Sensor and the second code is she’s running to lean.

I went to Checker to price the o2 sensor and well there’s 2 sensors and I don’t know which one is having issues or if I even need to replace both at the same time or just one.

Also what does it mean that she’s running to lean?

[impreza_GC8]

I think if its a P0420 it is the rear sensor. Perhaps someone else can verify this. They will both go bad after enough time. I think I've replaced both of mine by now.

[KevinWelker]

The rear sensor really doesn't do much for fuel control, simply ensures the cats are working properly. The front O2 sensor is the important one that the ECU uses to control fuel trims and is likely the culprit here. They are not cheap, but you don't want to skimp on them either. You can drive it like that for a little while, but take it easy on the engine and change it out as soon as you can. Once the sensor goes bad it will usually read lean, give a lean code, so the engine dumps more fuel in, the problem is that the engine is not necessarily lean, it can actually be too rich, which is a problem also and will give you crappy mileage (that is a sure sign of longer term too rich conditions).

Pay the $100+ and get a good replacement and an O2 sensor socket and change that out. DIY instructions on Scoobymods.com

[MConte05]

Make sure first you don't have an exhaust leak before the first O2 sensor. Either at the header-manifold location or where the header connects to the cat. I chased down a "running too lean" CEL for the longest time. Originally thought it was loose vacuum lines or a bad O2 sensor, turned out I had a bad exhaust leak at the header-cat connection. Got a thicker gasket and some exhaust gasket goo stuff, and the problem was solved completely.

What it means when it is running too lean is that the ECU is detecting that there is too much oxygen in the exhaust flow, so it tries to compensate by dumping in more fuel, which in turn causes you to run extremely rich. I know I ignored my "running too lean" problem for a while, and you can run for a while with it, I went for a month or two. However when it starts to get really bad, the engine will have a hard time holding idle and die sometimes because there is simply too much fuel in the system. Typically the problem comes up from improper intake installations, but as I learned the hard way, also pre O2 sensor exhaust leaks. As air from the outside makes it into the exhaust system, and then the O2 sensor reads that, tricking it into thinking that the engine really is running too lean when in reality it's not.

[409industries]

Id put money down that eventually that o2 sensor will start throwing the code "sensor malfunction". Its best just to replace the sensor all together. They are not hard to do, you just need the sensor socket that allows the cable to come out of the side while you tighten the new one down.

I had to do both my O2 sensors, and a knock sensor on my 01 RS.

[RSdriftPrncss]

Thanks guys for all your info. I'll contact my Uncle in So Cal to see if he can maybe get the part cheaper. Then its bagging my dad for the install!

[bigjoshsubyfan]

mine recently did the very same thing but I just happened to change the right one and it went of.

[RSdriftPrncss]

mine recently did the very same thing but I just happened to change the right one and it went of.

Which one did you change? So I know what one I need to do. ;)

[bigjoshsubyfan]

It was the before Cat one and everything is fine now. Mine is a GC though so it might be different but I don't know. Mine was a wide band and yours probably it too.

[bigjoshsubyfan]

I think if its a P0420 it is the rear sensor. Perhaps someone else can verify this. They will both go bad after enough time. I think I've replaced both of mine by now.

So mine came back on and now its P0420 and thats the after cat so the rear sensor. I always go to autozone its usually cheaper on a lot of things the rear one is 100+ unless you want to do some wiring and then you can get the same thing you just have to wire it yourself its not to hard and thats what I'm going to do.

Also if it says its running lean the means that the 02 sensor is reading that and that there is not the proper air fuel ratio. Then your engine is going to be using more fuel so you might notice a little bit of performance drop off, and it will also affect your gas mileage so I would replace the back one and if it still doesn't go off then do the others.