So your car was running fine, but suddenly it's acting completely weird, and the "check engine" light (CEL) is on. It's stalling, jerking, idling poorly, surging, and just generally running poorly.
There are so many things that could register a trouble code and illuminate the CEL, but not too many have these kinds of symptoms. Let's go ahead and virtually connect a code reader, and it indicates...
An issue with your throttle position sensor!
What does the throttle position sensor do?
The throttle position sensor sends information back to the engine control module (ECM), indicating how open your engine's throttle is at any given moment. The ECM uses this information to adjust fuel metering, ignition timing, and emissions. Since it contains both electrical and mechanical parts, the throttle position sensor can be prone to failure over time and will send voltage readings to the ECM that are out of spec, illuminating the CEL. At this point, the ECM doesn't "know."
Failed throttle position sensor symptoms
- Jerks and bucks while the engine is under moderate load
- Surges in idle speed
- Engine stalls without warning and for no apparent reason
- Poor power, poor fuel economy
- Hesitation while accelerating
- Sudden surges in speed on the highway
- Delayed shifts or difficulty in changing gears (information from the TPS informs the ECM's shift points for the transmission)
- Illuminated or flashing CEL
In some cases, these symptoms may come and go before the sensor fails completely. Also, depending on the make/model you drive, it may require a few bad readings before the ECM will store a trouble code such as P0122 or P0123 and illuminate the CEL.
Diagnosis and Repair
In some instances, problems with a TPS might originate from a bad electrical connection (if you're lucky) or corroded contacts on the connector. Be sure to check this before going any further.
Use your multimeter for diagnosis—the readings from the TPS should be between 5 volts at WOT and under 1 volt at idle, depending on your vehicle (check a service manual for the exact parameters of these voltages). The throttle position sensor is located near the throttle body itself. You may need to move or remove some parts to get easy access to it. On some vehicles, it may be necessary to reprogram the ECM so that it can "relearn" the proper shift points for the transmission.