AutoDiag

P0121

Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance

High· Fix promptlyIntermediatePowertrainGeneric, all makes

What it means

The throttle position sensor A (TPS A) is sending a signal that is electrically valid but does not make sense compared with other sensors. The throttle position sensor A (TPS A) tells the ECU how far open the throttle plate is. This usually points to a sensor that is reading wrong, or to a real mechanical problem it is reporting correctly, rather than a broken wire.

Is it safe to drive?

Drive with caution. On electronic throttle cars the ECU may cut power to a "limp mode", which makes merging and overtaking dangerous.

Symptoms

  • Check engine light on
  • Hesitation or flat spots on acceleration
  • High or unstable idle
  • Reduced engine power or "limp mode" on drive-by-wire cars
  • Harsh or erratic automatic transmission shifts

Probable causes

Most likely first

  1. 1Carbon build-up holding the throttle plate off its stop
  2. 2Worn sensor track giving dead spots
  3. 3Throttle body needs relearning after cleaning or battery disconnect
  4. 4Failing sensor giving an inaccurate reading
  5. 5Poor connection at the sensor connector

How to diagnose it

Work through these in order before buying parts

  1. 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
  2. 2Inspect the throttle position sensor A (TPS A) connector and wiring (on the side of the throttle body). Look for corrosion, bent or pushed-back pins, oil contamination and chafed insulation.
  3. 3Watch the throttle position sensor A (TPS A) reading on live data. Reading should rise smoothly from about 0.5 V (closed) to about 4.5 V (wide open) with no jumps or dropouts as you press the pedal (key on, engine off).
  4. 4Compare the throttle position sensor A (TPS A) reading against related sensors and the real-world condition. If they disagree, find out which one is telling the truth before buying parts.
  5. 5If the wiring checks out, test the sensor itself against the specification in a service manual.
  6. 6Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

  • On drive-by-wire engines the TPS is usually part of the throttle body, so the whole body is replaced and a relearn is done
  • Fix the underlying problem the sensor is reporting, if real
  • Replace the throttle position sensor A (TPS A) if it reads wrong against a known-good reference

This information is general guidance for all makes. Exact causes, specifications and procedures vary by vehicle, so confirm with your service manual or a qualified mechanic before replacing parts.