OBD-II: Throttle Position Sensor A Circuit Low Input

P0122 Diagnostic Guide (OBD-II): Throttle Position Sensor A Circuit Low Input

Summary

• Code meaning: P0122 denotes a fault in the Throttle Position Sensor (TPS) - Sensor A circuit with a low input to the Powertrain Control Module (PCM). In most vehicles, the PCM uses the TPS A signal (often one channel of a multi-channel TPS) to determine throttle position. A "low input" typically means the PCM is seeing an unexpectedly low voltage from the TPS A circuit across the throttle range.
• Relevance to TAC: Some engines use throttle actuator control (TAC/ACCEL or electronic throttle) in conjunction with TPS input. If the TPS A signal is consistently low, TAC/ECU calculations can be affected, contributing to drivability issues such as rough idle, hesitation, stalling, or sudden changes in engine behavior. This aligns with reports where a P0122 code is observed alongside TAC-related drivability concerns.
• Supporting data: In real-world NHTSA complaint data, a complaint describing rough idle and drivability concerns associated with P0122 helps illustrate how a TPS A circuit fault can manifest in symptoms like sputtering, jerking, and even engine shutdown.

What this code is telling you (high-level)

• The PCM expects a valid, varying TPS A signal (0-5 V reference) as throttle position changes. A low input indicates the signal is staying abnormally low (near the lower end of the scale), or the PCM is reading a fault condition on that circuit.
• This can be caused by a faulty TPS A sensor, wiring/connector problems, issues with the 5 V reference or ground, PCM input faults, or in rarer cases, mechanical throttle issues that influence the TPS reading indirectly.
• The problem typically impacts driveability and can produce symptoms such as rough idle, hesitation, acceleration issues, stalling, or abrupt engine shutdown if the PCM cannot correctly interpret throttle input.

Common Causes

• TPS A sensor fault or wear (Most Likely)

  • The TPS itself has failed or its wiper/contact is degraded, causing an abnormally low output signal.
  • Expect symptoms to track throttle position improperly (e.g., engine thinks throttle is near closed even when open).
  • Source support: TPS A circuit low input definition exists in open-source TPS circuit references; this aligns with the P0122 fault class.

• TPS A wiring/connector issue (Very Likely)

  • Damaged, chafed, corroded, or loose wiring, harness abrasion near the throttle body, or a poor connector connection can pull the signal low or intermittently lose signal.
  • Commonly presents as intermittent P0122 or intermittent drivability symptoms.

• 5 V reference or ground problems to the TPS (Likely)

  • The PCM provides a 5 V reference to the TPS. If the reference is weak, noisy, or shorted to ground, the TPS A signal can appear too low.
  • Could stem from a general electrical system issue, battery/charging problems, or PCM grounding.

• PCM/ECU input fault (Possible)

  • A fault in the PCM input stage or a bad reference to the TPS A circuit can cause a false low reading.
  • Less common than sensor or wiring faults, but possible in some vehicles.

• Mechanical throttle body or throttle plate binding (Possible, but less direct for P0122)

  • If the throttle body binds or the throttle mechanism sticks, the TPS output may not reflect actual throttle position smoothly; however, P0122 is specifically a low-input condition on the TPS A circuit, so mechanical issues are usually accompanied by other codes or symptoms and are less likely to be the primary cause of a pure low-input fault.

Diagnostic Approach

1) Gather data and confirm

• Confirm the DTC: P0122 (Throttle Position Sensor A Circuit Low Input) and note any other codes (e.g., P0120, P0121, P0123, P021 something for the same sensor family). Check freeze-frame data and VIN-specific service information if available.
• Correlate symptoms: rough idle, hesitation, stalling, jerking, or "engine may stall" events align with a TPS A signal problem.

2) Visual inspection

• Inspect the TPS sensor and its harness at the throttle body:
  • Look for damaged insulation, worn wires, chafing, or signs of oil/contamination around the TPS and harness.
  • Check the electrical connector for bent pins, corrosion, or a loose latch. Reseat if necessary.
• Inspect the 5 V reference and ground paths to the TPS circuit for obvious issues (loose grounds, damaged ground straps, or corroded battery/alternator connections can affect sensor circuits).

3) Verify power and grounds

• With ignition on (engine off) or engine running as appropriate, back-probe the TPS A circuit:
  • Confirm there is a stable 5 V reference to the TPS A sensor (and that it remains present under load changes).
  • Confirm a good ground path on the TPS A circuit.
  • Note: voltage should be within the expected reference window; any significant drop indicates a supply/ground issue. (General TPS testing approach; TPS A uses 5 V reference)

4) Live data testing (throttle position sweep)

• With a scan tool, observe TPS A signal live while slowly moving the throttle from closed to WOT (wide-open throttle).
  • Expect a smooth, monotonic increase in voltage from near the low end toward near 5 V as throttle opens.
  • If the signal is consistently low, or there are dead spots, glitches, or abrupt jumps, suspect TPS A sensor or its wiring.
• If your scan tool can graph the TPS A signal, look for a clean slope rather than intermittent dips or flat spots. (General TPS diagnostic method; aligns with TPS A low input concept)

5) Electrical test on the TPS A circuit

• Measure TPS A output voltage directly with the sensor connected, while manipulating the throttle and watching for voltage range consistency.
• Verify continuity of the TPS A wire from the TPS connector to the PCM harness connector (back-probe or harness test). Look for high or intermittent resistance that could indicate a wiring issue.
• Check for short to ground or short to 5 V on the TPS A line. A short can pull the signal down toward ground.

6) If the sensor or wiring tests indicate fault

• TPS A replacement: If the TPS A sensor is found defective or its output is out of spec, replace the TPS sensor. After replacement, verify proper operation with a throttle sweep and confirm P0122 clears after road test/drive cycle.
• Wiring/connector repair: Repair or replace damaged wiring, and clean/replace the connector as needed. Ensure a solid connection and proper routing to avoid future chafing.
• Relearn/adaptation: Some vehicles require throttle plate or ECU adaptation after TPS replacement. Confirm and perform any required relearn procedures or throttle body calibration (per vehicle-specific service information).

7) If sensor and wiring test good, investigate other sources

• 5 V reference/ECU supply integrity: Check the vehicle battery, alternator, and ground circuits; verify the PCM 5 V reference supply is stable under load.
• ECU/PCM fault: If everything checks clean but P0122 persists, consider PCM/ECU fault or a need for firmware reflash-only after consulting vehicle-specific service information and considering other symptoms and fault codes.
• Mechanical throttle issues: If other symptoms or codes point to throttle mechanism binding, inspect the throttle body and linkage; correct mechanical binding and re-check TPS output and P0122 status.

8) Verification and test drive

• Clear the codes, perform a road test, and monitor live TPS A data during normal driving and ramp/stop maneuvers.
• Confirm that P0122 does not reappear and that drivability symptoms (rough idle, stalling, hesitation) have improved or resolved.
• If symptoms persist or reappear with P0122, re-evaluate following the diagnostic loop, considering alternate causes or a deeper ECU check.

Tools Needed

• OBD-II scan tool capable of live data and freeze-frame capture
• Digital multimeter (DVOM) or oscilloscope for TPS A voltage and reference checks
• Back-probe leads or wiring probes for safe, accurate measurements
• Vehicle service information for TPS A specifications and any required relearn/calibration procedures
• Safety equipment: fire extinguisher, eye protection, and gloves; follow standard safe-work practices around electrical systems and the throttle body area

Documentation

• P0122 indicates a low input on the TPS A circuit, which impacts throttle position data the PCM uses to control the engine. The most common fixes are replacing a faulty TPS A sensor or repairing a damaged wiring/connector. In some cases, a faulty PCM or a need for calibration/relearn can be involved. A careful inspection of the sensor, its harness, and the 5 V reference/ground is essential.

This diagnostic guide was generated using verified reference data:

• NHTSA Consumer Complaints: 1 real-world reports analyzed
• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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