Comprehensive diagnostic guide for OBD-II code P0123 Throttle Position Sensor A Circuit High Input

1) Code overview (what P0123 means)

• P0123 is a Powertrain (OBD-II) diagnostic trouble code that indicates the Throttle Position Sensor (TPS) A circuit is reporting a high input. In open-source code references, this is described as "TPS A - Sinal muito alto" (TPS A signal too high). This points to the throttle position sensor circuit being higher than expected for the current throttle position, which the engine control module (ECM/PCM) reads as a high voltage on the TPS A signal circuit.
• OBD-II context: P-codes are powertrain-related codes used to monitor engine and transmission control sensors, including the throttle position sensor. The general concept and code classification are described in the OBD-II overview and powertrain code sections.

2) Real-world symptom patterns

• Complaint example related to P0123: A vehicle presented with no-start symptoms after experiencing power loss and cranking problems, with P0123 logged as the fault code. This illustrates that P0123 can accompany symptoms like reduced power, hard starting, or cranking difficulties in some cases.
• Another NHTSA complaint describes a sudden vehicle shutdown during driving with a brake/parking brake engagement scenario that is not clearly tied to P0123; this highlights that vehicle symptoms with safety implications can accompany or obscure powertrain codes, and drivers may experience a range of unrelated or compounded issues. Note: This complaint does not definitively tie P0123 to the brake system or Emergency Brake; it serves as a reminder to verify code relevance to symptoms.
• Takeaway for diagnostic planning: P0123 can be associated with powertrain control issues that affect starting and drivability, but real-world complaints show a variety of symptom presentations. Use diagnostic data (freeze frame, live sensor data) to confirm relevance of P0123 to observed symptoms.

3) Technical background to guide diagnosis

• From general OBD-II reference: DTCs categorize faults detected by the vehicle's onboard monitoring systems. P0123 is a powertrain fault code related to the throttle position sensor circuit. It is important to verify whether the code is current and whether freeze-frame data show a correlation between TPS A voltage and throttle position.
• TPS basics (relevant for diagnosis): TPS A circuit high input indicates the ECM is seeing a higher-than-expected voltage on the TPS signal line, which usually means the sensor, its wiring, or the ECM input is out of expected range for the stated throttle position. The open-source TPS code definition confirms the high-input condition for the TPS A circuit.

4) Diagnostic plan: step-by-step approach

Note: Always address safety first. If the vehicle shows drivability or starting issues, perform diagnostics in a controlled, safe environment and avoid high-risk driving with suspected throttle or control issues.

Confirm and characterize the fault

• Use an OBD-II scan tool to confirm P0123 is present and note any related codes (e.g., P0122, P0120, P0121, or other sensor codes). Retrieve freeze-frame data and current/live TPS A values vs. pedal position if supported by your tool.
• Record engine status (running, cranking, or off), vehicle speed, and any drivability symptoms (stumbling, surging, idle instability).

Inspect the throttle position sensor circuit (TPS A)

• Inspect the TPS signal circuit for the A-signal wire, the 5V reference, and the ground. Look for damaged insulation, chafed wire, loose or corroded connectors, and wet or contaminated connectors.
• Verify continuity and resistance (or voltage behavior, depending on the vehicle and tool). Typical tests include checking that the signal wire changes smoothly with throttle movement and that the 5V reference is present and stable; expect the signal to vary with throttle, not stick or jump erratically.
• If the vehicle uses electronic throttle control (ETC), confirm the ECM is commanding throttle movement properly and that the TPS readings respond accordingly. If the TPS does not respond correctly, suspect sensor or wiring issues.

Evaluate sensor and sensor wiring for faults

• TPS sensor: Consider a faulty TPS or one that is out of calibration/learning. Scenarios include sensor drift, internal open or short, or a sensor that reports high input at idle or during partial throttle.
• Wiring/connector issues: Short to 5V, open circuit, or high resistance in the TPS A circuit can produce a high input reading to the ECM. Inspect for shorts to power, shorts to ground, or intermittent connections.
• ECM/PCM input: If wiring and sensor checks are clean, there may be an ECU input fault or an internal fault in the ECM reading TPS A.

Corroborate with live data and functional checks

• If available, capture live TPS voltage vs. throttle position with the engine at idle and during throttle transitions. Confirm there is a reasonable correlation and that the TPS A signal does not exceed expected ranges.
• If the vehicle can be safely tested, perform a controlled throttle movement to observe whether the ECM responds correctly and whether the fault code(s) reappear under similar conditions.

Consider common corrective actions and re-test

• If a TPS sensor issue is suspected (sensor drift, miscalibration, or failure), replace the TPS sensor and perform any required relearn or calibration procedure per the vehicle's service information.
• If wiring or connector faults are found, repair or replace damaged wiring, clean/correct connector contacts, and ensure proper seating.
• If a fault is suspected within the ECM/PCM input and all wiring/sensor checks pass, consider ECU reflash or replacement per vehicle service information. Re-test after repair to confirm resolution and clear the code(s).

Post-repair verification

• Clear codes and perform road testing. Monitor TPS readings live to confirm correlation with throttle input and absence of reoccurrence of P0123.
• If the vehicle is still showing P0123 after repairs, revisit wiring and sensor checks and consider more advanced diagnostics (e.g., oscilloscope capture of TPS A signal, high-quality current/voltage measurements, or dealer-level diagnostics).

5) Probable causes and their rough likelihood

Note: NHTSA data for direct P0123 occurrences is limited , so probabilities are approximate and should be treated as guidance rather than absolutes. General automotive field experience suggests TPS-related faults and wiring issues are common culprits for TPS high-input codes.

• TPS sensor fault or miscalibration/learning issue: 40-60%

• TPS wiring or connector issue (short to 5V, open, high resistance, corrosion): 20-40%

• ECM/PCM input fault (ECM sensing fault or internal fault): 5-15%

• Throttle body mechanical issue (sticking throttle plate or throttle-by-wire actuator problems): 5-15%

• Other sensor or system interactions or misdiagnoses: 0-5%

6) Safety considerations

• A P0123 can be associated with drivability problems that affect acceleration and throttle response. If the vehicle is driving unpredictably or stalls, avoid aggressive driving maneuvers until the fault is diagnosed and corrected.
• If the ECU is receiving a consistently high TPS input, there is a potential risk of unintended throttle opening in some configurations (especially in ETC systems). Keep clear of traffic and perform diagnostics in a controlled environment.
• If a complaint mentions no-start or sudden shutdowns, treat the issue as potentially severe and verify all safety-critical systems (engine and ignition) during diagnosis.

7) Quick reference: what to document when communicating with customers

• Exact code(s) retrieved and freeze-frame data (TPS A readings, engine RPM, vehicle speed at the time of fault).
• TPS sensor data: current voltage/position readings, changes with throttle input, and any anomalies (stuck high reading, erratic changes).
• Wiring/connector condition: appearance, corrosion, damaged insulation, connector seating, and any resistance measurements across the TPS circuit.
• Any recent service history related to throttle, sensors, or ECU (replacements, re-learns, calibrations, or software updates).
• Test steps performed and their outcomes, including any road test results after repairs.

8) Related sources for further reference

• Code definition and mapping: P0123 - Throttle Position Sensor A Circuit High Input (TPS A high input).
• OBD-II and DTC context: OBD-II diagnostic trouble codes and powertrain codes on OBD-II and diagnostic codes.
• Real-world symptom patterns: NHTSA complaints that indicate P0123 can accompany starting and drivability issues, including a case where no-start conditions occurred with P0123. Cited to Complaint 1 in , and a second complaint illustrating safety-critical symptoms not definitively tied to P0123.

This diagnostic guide was generated using verified reference data:

• NHTSA Consumer Complaints: 2 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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