So your check engine light is on. As you probably know, the check engine light (CEL) can light up for all sorts of reasons. But since the introduction of OBD-II back in the 1990s, many of those reasons are stored in a standardized set of diagnostic trouble codes that can be accessed with a generic code reader.
The good thing about the CEL and the entire OBD-II engine management and diagnostic system is that it takes a lot of the guesswork out of what's actually going on with a vehicle's engine and drivetrain. A tech can connect a code reader to the OBD-II port and quickly access any trouble codes that are stored in the engine computer, pointing him or her in the right direction for diagnosis and repair.
Where it can get tricky, though, is interpreting what the trouble codes mean. At times, there's a certain amount of reading between the lines needed to come up with an informed conclusion on why a certain code might be stored—and a single problem might cause a cascade of trouble codes to all be stored.
To help you learn more about these codes and decide whether the issue is one you can tackle yourself or one that you should take to a trained technician, Advance Auto Parts will be breaking down common codes. In this article, we cover P0123 and what it means.
Trouble Code P0123: Throttle Position Sensor Switch A Circuit High Input —What It Means
The throttle position sensor is usually mounted on the shaft of the butterfly, on your engine's throttle body. Readings from the TPS monitor the position of the throttle in real time and figure into fuel metering calculations, and can also signal the transmission to "kick down" into a lower gear for passing or wide-open throttle acceleration. Some units also have a wide-open throttle sensor and closed throttle sensor, usually found on drive-by-wire systems with no throttle cable.
The TPS is a potentiometer design with a variable resistor. Potentiometers use three wires: a ground, a 5-volt reference, and the signal wire. As the throttle plate opens, the voltage signal increases, and then decreases as the throttle closes. The PCM uses this reading to calculate fuel delivery and timing. Readings should range from .45 volts at idle to about 4.5 at wide-open throttle.
A P0123 trouble code means that the PCM has recorded a reading from the TPS that exceeds the normal 4.5 volts for more than two seconds. A fault like this means that the sensor either has a short or has failed.
- Failsafe or "limp-in" mode, with the PCM limiting fuel injection and timing to control speed
- Limited speed
- Poor acceleration
- Bucking or jerking
- Hard starting
- High idle speed
Note: symptoms can vary as different makes'/models' failsafe mode can vary.
What Happens If I Ignore It?
You'll have a vehicle that runs so poorly that you won't really be able to drive it. A P0123 code is something you'll need to fix right away.
- Inspect the TPS sensor's wiring for loose connections, loose wires, improper mounting, damage or corrosion.
- Remove the TPS electrical connector, spray the contacts with contact cleaner and reinstall.
- Clear codes in the PCM and test-drive the vehicle to see if the problem or code returns.
- If the code returns and you don't find any damage or loose connections on the wiring, it's time to test the TPS:
- Connect a multimeter set to read DC voltage between the signal and ground wires.
- Slowly press the accelerator pedal and watch for the voltage to steadily increase. If it doesn't increase or there are "dead spots" that prevent smooth voltage change, replace the sensor.
- Repeat by letting off the pedal slowly to go from wide open throttle to idle. The reading should be similarly smooth as it decreases, and the voltage at idle should be within the manufacturer's spec. If not, replace the sensor.
There have been instances when a high reading from the TPS is due to a faulty alternator which is supplying too much voltage system-wide. This will usually be accompanied by other trouble codes, though. A P0123 code might also be accompanied by trouble codes P0120 through P0124, or P0220 through P0229, all of which pertain to problems in TPS A, B or C circuit.
Have you come across TPS issues before? Let us know in the comments.