P0123 is a relatively common OBD-II code. It’s generic, which means it has the same meaning for the Subaru Legacy as it would any other vehicle. It’s an OBDII code that references issues with the Throttle Position Sensor (TPS). It is relatively serious problem since it can leave your vehicle stranded or severely limited.
The Throttle Position Sensor monitors the angle of the throttle blade. When the throttle is closed, the signal that is sent back to the ECM is around .45 volts. When the throttle is wide open, the signal will be around 5 volts. P0123 occurs when the voltage from your Legacy’s TPS is higher than the upper operational range.
P0123 occurs when the voltage from your Legacy’s TPS is higher than the upper operational range.
Subaru Legacy: P0123 Symptoms
The symptoms of P0123 are going to vary by year and model, but they certainly are going to cause noticeable changes in the way that your Legacy runs. Without a proper signal from the TPS, your Legacy will compensate in a number of ways.
Here are the most common symptoms of P0123:
- Throttle Response– Your mayLegacy hardly respond to the throttle at all. This limited power is a failsafe to keep the vehicle able to move, but not super fast. Since the ECM won’t really know how much throttle response is actually happening when you push the gas.
- Idle Speed Issues– Without knowing exactly where the throttle is, the Legacy’s computer may have a difficult time getting the idle speed just right.
Different model year and engine combinations will experience different symptoms.
P0123 Diagnosis: Subaru Legacy
OBDII code P0123 is a serious problem that requires serious attention. Diagnosing it can be tricky without a good scanner and the knowledge of what you are doing. Here is a YouTube video that shows how to go about diagnosing this code with a scanner.
A good mechanic will utilize a scanner to diagnose when the codes were set and reset them. They’ll then see if the problem comes back.
If it all checks out and the code comes back, they may go ahead and replace the TPS sensor in question. Most of the time, the problem when P0123 is thrown will be the TPS sensor itself. It is no longer tracking how hard the fuel pedal is being pushed.
P0123 Causes- Legacy
There are a few issues that cause P0123. They are:
- Wiring Issues– Since the TPS is so accessible, a great place to start is to do a visual inspection of the wiring and harness coming to it from the engine’s computer. Is there any damage? Here’s a great article on diagnosing a short circuit.
- Bad TPS– There is a YouTube video above on how to diagnose a TPS sensor. If you need one they are pretty inexpensive on Amazon.
- TPS not properly mounted– While not very common, it is possible the TPS sensor was not mounted properly. If you have recently replaced it and are getting P0123, it’s a good idea to check the replacement sensor instructions and see if they mention this at all.
Subaru Legacy P0123 Conclusion
Diagnosing the issue with a scanner or having a mechanic take a look can save the expense of buying a TPS sensor that your Legacy does not actually need.
Good luck diagnosing P0123 in your Subaru Legacy. If there is anything that you would like to add, please leave a comment below. Thanks for reading.