P0121 OBD-II Trouble Code (Throttle Position Sensor "A" Circuit Problem)

You probably know that the illuminated Check Engine light (CEL) on your dashboard means that a sensor has reported a reading that's out of spec, registering a trouble code in the vehicle's powertrain control module (PCM). You probably also know that the trouble codes are accessible using a scanner/code reader device that hooks up to the diagnostic connector, usually found under the dashboard.

The introduction of OBD-II back in the mid '90s (which standardized trouble codes across all makes and models) meant that the technician's troubleshooting process suddenly got a lot easier. Rather than relying on intuition and experience, a tech could just access the codes for a push in the right direction and get a head start on diagnosis and repair.

Note: Retrieving diagnostic codes is one of the free services Advance offers.

It's important to remember, though, that the sensor's trouble code can often be misleading and can actually result from a related problem and not the problem indicated by the code. In addition, a single problem might trigger multiple codes and the technician has to know how to interpret those codes to arrive at the right solution.

check engine light

P0121 — Throttle Position Sensor "A" Circuit Range Performance Problem: What It Means

A fuel-injected engine has a throttle position sensor (TPS) near the butterfly of the throttle body. The TPS's job is to inform the PCM of the exact position of the throttle at all times. The PCM then takes this information and readings from other systems to calculate timing and fuel delivery. The TPS sensor can be a potentiometer-type, Hall-effect type, inductive, or magnetoresistive design, but any type of TPS sensor can be susceptible to wear and fatigue, especially considering the difficult environment under the hood.

Regardless of design, the reading sent to the PCM from the sensor goes up in voltage as the throttle opens. The PCM supplies a reference voltage to the sensor as well, and in most cases it also serves as a ground for the sensor. At idle, the voltage from the sensor should be 0.5 volts, and at wide-open throttle (WOT) the reading should be 4.5-5 volts. If the PCM detects a reading for a specific throttle angle that doesn't correspond to the RPMs from the engine, a trouble code will be set.


  • Intermittent surging, miss or stumble on acceleration or deceleration
  • Hard starting or no-start condition
  • Rich-running with black smoke from exhaust
  • Poor fuel economy
  • Sluggish acceleration
  • Stalling

What Happens If I Ignore It?

A throttle position sensor code is pretty serious and could, eventually, result in a vehicle that won't start at all. On some vehicles, the PCM might put the engine into limp-in mode, limiting top speed to about 35 mph.

Possible Fixes

  • Check all TPS wiring and connectors for loose, open or corroded condition.
  • Using a multimeter, check the voltages from the TPS at idle and at WOT and compare to spec.
  • If no problems are noticed, perform a wiggle test by jiggling the wires to and from the TPS. See if the reading fluctuates.
  • If you can't detect a reading from the TPS, check for a 5-volt reference voltage from the PCM. If you detect a reference voltage, check the TPS for open circuit or shorts.
  • Make sure the voltage to the TPS isn't 12V. Full battery voltage should never supply this sensor.
  • Replace the TPS sensor as necessary, clear any trouble codes from the PCM, and test drive.

Note: P0120, P0122, P0123 and P0124 are all related trouble codes.

Have you come across any TPS-related trouble codes? Tells us about your experiences in the comments.

Last updated October 25, 2019