P0103 is a common generic OBD2 trouble code that can occur in the Subaru Forester. In plain English, it means that the MAF sensor voltage signal is outside of the accepted range (too high).
It’s important to understand what the MAF sensor does. It measures the air density and volume between the air filter and the intake manifold. It’s a sensor that a modern engine really needs to operate effectively.
The ECM uses this reading to determine the exact amount of fuel that it needs for optimal combustion. When this signal is too high, P0103 is stored and the check engine light will come on.
With P0103 one or more of these symptoms will occur in your Subaru Forester:
- Service engine soon light
- Poor idle
- Hard starts
- Poor performance
- Improper air/fuel mixture
Subaru Forester P0103 Causes
Here are the most common causes of P0103 in the Subaru Forester. Before starting though, it’s common to see other codes with P0103. Cross referencing their common symptoms and causes can help you narrow down the potential causes of this code.
Dirty MAF Sensor
Over the course of its life, the MAF sensor will often get so covered in grime that it’ll no longer get an accurate reading. They can be cleaned, but it’s easy to mess one up by handling it wrong, and you don’t want to use a wire brush. Here’s a great article on exactly how to clean one from Family Handyman: MAF Sensor Cleaning
MAF Sensor Failure
While it is very likely that the MAF sensor itself could be the problem, it’s absolutely necessary to test it before you replace it. Here’s a video on how to test one without pulling it.
This is important for two reasons, the first is that they are expensive, and the second is that it only takes a few minutes to test one with a multimeter. If you do need one, they are pretty reasonably priced at the local parts store.
Try and get one made by Subaru if you at all possible.
Intake Manifold/Vacuum Leak
If there is a leak in the intake manifold, it will be impossible for the MAF sensor to get an accurate reading. Without knowing how much air is really getting into the engine, P0103 will be thrown.
MAF Sensor Wiring
Make sure to check around the MAF sensor for opens or shorts. Start by inspecting the wiring harness for obvious damage. If you don’t see any you can use a multimeter to test the wiring.
At this point if you use the YouTube video above in the MAF sensor dirty section, and there turns out to be no voltage at all, try checking out How to Find an Electrical Short on Most Any Vehicle from BackYardMechanic.
Clogged Catalytic Converter
If you are getting P0103 in your Forester accompanied by a noticeable lack in power at speed, it may very well be that you have a clogged catalytic converter. If that were the case, you should have a converter related code as well.
You should consider P0103 a high priority fix, as the MAF sensor not reading properly can really make an impact on the performance of the vehicle.
Whatever the reason for P0103 in your Subaru Forester, ignoring it is a bad idea. Good luck figuring it out, and if there is anything you’d like to add, please leave a comment below!