2003 Subaru WRX - New MAF Sensor

I'm very new to logging. I'm using a bluetooth adapter in the OBDII port and Torque Pro on an Android.

I've been logging the A/F ratio, MAF g/s, short & long term fuel trims, and a few other values for a few days to get a feel for the baseline. I cleaned the old MAF sensor and immediately got several more MPGs. Short + Long term trims mostly added up to 5-6. But I had already ordered a new MAF sensor. I figured, hey, why not put it in and see what happens? Certainly not what I expected. I didn't reset anything. I was thinking let the computer take the new readings and do what it wants to get A/F to 14.7. I thought doing it that way might tell me more about whether I achieved anything with the new sensor. I drove 35 miles to 'work'. I could see the MAF grams/second changing with the throttle. I could see the fuel trims going up and down. But once the car warmed up (entered closed loop?) the air/fuel ratio displayed 15.03 and stayed there for the rest of the trip. I can't reconcile this. If the computer was seeing that a/f ratio wouldn't it just keep adding fuel until it finally threw a code? I'm thinking I need to pull the battery cable on the car, reboot my phone, and try again. Or maybe it's idiotic and not do a reset after replacing the MAF sensor and I'm just not understanding the basics.

I need to go look through some old Subaru data to confirm, but I'm pretty sure that air/fuel ratio PID is a product of the Torque app and not the car.

If it is, then I'd wonder what the app is using to calculate that, and if you can truly rely on it or not.

One way to prove this would be to force a rich or lean condition and look for that PID to respond rationally. As in, get that PID to say 15:1 then pull a vacuum line. Look for the PID to swing lean.

Nice. Thanks.

I rebooted the phone and the a/f started reading sensibly again. I think that was my problem. At idle it settled on 14.7. I pulled a small vacuum hose and it jumped to almost 20, so pretty lean as I understand it.

I should have fiddled with the setup before I posted. But sometimes you don't know what you don't know.

No worries! Part of the fun of diagnostics is figuring out what data you can and can't rely on. AND calling BS on a data PID when you think it's lying to you.

Does your WRX use an upstream air/fuel ratio sensor, or a conventional O2? That might explain where Torque is getting it's information from.

I see people call it the front a/f sensor and the front O2. I thought they were both O2 sensors and that the cat was checked by testing O2 before and after, and that the mixture was somehow inferred by the amount of O2 in the upstream. Let me see what I can find out.

As you can see. I am a noob in this area.

Subaru calls it Sensor Assembly-air/fuel ratio.

bit.ly/2HJ8ObJ

Torque looks very flashy but is perhaps not the most useful OBD2 Android app you might InCarDoc Pro (used to be called ObdCarDoc Pro) more useful

Looking at processed data from A/F sensors can be misleading and varies between manufacturers Torque looks very flashy but is perhaps not the most OBD2 Android app you might InCarDoc Pro (used to be called ObdCarDoc Pro) more useful

Testing A/F sensors is usually a case of watching the sensors response to a snap throttle and how quickly it settles back.

Well, $2.99 isn't much to risk. I'll try incardoc pro.

After I reset my phone I logged again on the way home. Assuming what I was seeing is real, the fuel trim numbers were lower and the a/f ratio stayed much closer to 14.7 (steady cruise). I think that the new MAF sensor was needed, even if I bought it before I had sufficient evidence.