Subaru air flow sensor failure?


#1

My 97 Subaru Impreza Outback has been intermittently losing power. I connected my scan tool to the car’s computer and received error code PO103, “mass or volume air flow circuit high input”. I started the engine and watched the real-time air flow display which looks OK; the indicator goes up when I step on the gas and down when I let off. So I’m guessing that the air flow sensor is intermittently failing. I also checked the power going to the sensor at the wiring harness and it looked OK even when I wiggled the cable. I was tempted to replace the sensor and see if the problem disappeared but then I checked the price on the NAPA web site: $350 rebuilt, $550 new! I don’t want to spend this kind of money if I’m not certain it’ll fix the problem, but I don’t want to be stranded if the sensor completely fails. What to do?


#2

Does the check engine light come on when your car loses power? Airflow sensor errors are ones that do tend to come up intermittently on a car in good running order. Was the error code just stored in the computer? Does it come back after you clear it?

Unless you can find a direct correlation between the losing power and the airflow sensor error, I’d look elsewhere for the problem. A fuel pressure issue or an intermittent ignition system failure would be more likely in my book and often will not be evident to an engine scanner.


#3

The error code is saying the circuit to the sensor has a problem. Before replacing the sensor I would be sure the wiring has been proper checked out.


#4

Try unplugging the MAF, drive it, and see if the problem goes away. If it does, the MAF is more than likely bad.
You could also try removing the MAF and gingerly try cleaning the delicate little wire inside with some aerosol carb cleaner.

I prefer Berryman B-12 or even the Wal Mart brand of carb cleaner because these brands appear to dry well without leaving any type of oily residue. Hope that helps.


#5

The engine computer looks at voltages and frequencies and compares them to what it expects to see, and compares them to some other inputs.
The DTC P0103 is for “circuit high input” of what? Voltage, right? The high input (sensor in) is a value higher than the computer expects during certain engine operating conditions. Which ones?
Erase the DTC engine code. Operate the car through acceleration, cruising, and deceleration while watching for the code to flash on the dash (or, on your scanner). Watch the airflow on the scanner, also, if you can. How does the air flow compare to the chart?


#6

you scan tool does not show ,REAL TIME. its buffered. hook up a graphing meter,to the feedback line,then you will have real time data.

good luck
maico


#7

www.alldata.com has DTC (Diagnostic Trouble Codes) Charts to use to check for causes to DTC codes. You can get alldata.com through your public library (which has a subscription), or pay $25 subscription for a year’s use.

It might help (can’t hurt) if you use an MAF spray cleaner on the MAF. And, use a carb/throttle body cleaner on the throttle body bore and throttle plate, IAC (Idle Air Control). Auto Zone has MAF for $207.


#8

on that note ,FIND out what is wrong first. MAF sensors do not get dirty,thats why they put them after the air filter.

maico


#9

No air filter catches every particle either, just like no oil or fuel filter weeds out every piece of contaminant.
After near 12 years and countless millions of cubic feet of air it’s possible for microscopic particles to build up on the wire; helped along by any oil vapors that may be present.


#10

No filter is perfect, especially the K&Ns.


#11

300k on 3 dirrerent vehicles,and I have never cleaned a MAF,on any of them.and they’re still being driven,with the OEM MAFS in place.so when maintaned properly,air filters do a good job.and not K/N.


#12

From a textbook: “…the MAF is tested by observing its output value in comparison to a ‘mapped’ value normalized by throttle position and engine speed. The map or table contains expected MAF output values for the engine speed/throttle set point.” alldata.com often has these tables/map.

You need to check the output of the tps (throttle position sensor) through its range, and check the vss (vehicle speed sensor, throughout the rpm range; since, the three signals are interdependent. On the tps, check the point, closely, just off idle. This is where most of the wear in the tps takes place.