Comprehensive diagnostic guide for OBD-II code P1123

Topic focus: Throttle Position Sensor - High Value (TPS high input)

What This Code Means

• P1123 is a powertrain OBD-II diagnostic trouble code signifying a throttle position sensor
• In OBD-II nomenclature, P1123 is classified as a powertrain code (part of the broader set of DTCs).
• OBD-II DTCs are diagnostic trouble codes used by on-board diagnostics to indicate faults detected by the vehicle's control modules.

Symptoms

• Check Engine Light (CEL) illumination accompanied by P1123 on a scan tool.
• Irregular throttle response: throttle feels overly sensitive or responds inconsistently to pedal input.
• Idle behavior changes: potential for high idle, fluctuating idle, or idle stability issues.
• Hesitation or surges during acceleration as the PCM reacts to an unexpectedly high TPS reading.
• In some cases, other related P-codes or fuel trim indicators may appear if the TPS fault is affecting fueling calculations.

Notes on symptoms:

• Symptoms are strongly influenced by how the vehicle's PCM interprets a high TPS signal and whether there are compensating faults (fuel trim, idle air control, etc.). The code's primary meaning is the TPS signal is too high; resulting symptoms will reflect how the PCM adapts to that signal.

Probable Causes

Because do not include NHTSA complaint patterns for P1123, probability estimates below are grounded in general field experience with TPS-related faults and the interpretation of a "high TPS value." They are rough, vehicle- and model-dependent, and should be treated as starting points:

• TPS sensor fault (sensor itself or its internal circuitry) - 40% to 60%
• Wiring/connector problems (short to high, open circuit, or poor connection in the TPS signal/reference circuit) - 20% to 40%
• PCM/ECU input fault or incorrect sensor calibration/adaptation (ECU misinterpretation or failing TPS learn) - 5% to 15%
• Mechanical throttle body issues or throttle linkage sticking (if the high reading is caused by a mechanical condition that prevents the TPS reading from reflecting pedal position accurately) - 5% to 15%

Diagnostic Approach

Baseline and safety notes

• Work safely with electrical components; disconnect battery only if required by your procedure and after noting parasitic drain implications. Always follow shop safety protocols and the vehicle's service manual for sensor replacement and throttle body procedures.
• Document all findings, including freeze-frame data, live data, and any related DTCs.
• This guide uses general OBD-II and TPS concepts. Specific vehicle designs may vary (e.g., analogue TPS with a separate pedal sensor, or drive-by-wire systems with calibrated learn procedures).

Step 1 - Confirm the fault and gather data

• Use an appropriate scan tool to confirm P1123 is active and to retrieve freeze-frame data (RPM, vehicle speed, engine load, throttle position, etc.) at the time the fault occurred.
• Review live TPS data (engine drivetrain data stream) while moving the throttle manually (if pedal and throttle are accessible) or during a road test to verify how the TPS value tracks pedal movement.
• Look for consistency: a TPS reading that remains abnormally high regardless of pedal position strongly suggests sensor or harness fault; a reading that spikes or oscillates erratically suggests wiring or sensor integrity issues.

Step 2 - Visual inspection

• Inspect TPS wiring and connectors for damage, corrosion, bent pins, pushed-out seals, or friction with moving components.
• Check for obvious harness wear at routing points (grommets, near the throttle body, near the pedal assembly, or where the harness runs against sharp edges).
• Inspect the throttle body and linkage for signs of sticking or binding (if mechanical throttle) or issues that might cause the pedal position shown by the TPS not to reflect actual throttle plate position.

Step 3 - Electrical tests on the TPS circuit

• Identify the TPS wire scheme (common configurations: 5V reference, signal, ground). Check the reference voltage at the TPS connector with the key on or while the engine is running, depending on your vehicle and tool capability.
• Verify:
  • 5V reference is present and stable (no significant voltage drop or fluctuation).
  • Signal circuit shows a smooth, proportionate voltage increase as the throttle is opened; at closed throttle, the signal should be near the low end of the range; at full throttle, near the high end.
  • Ground circuit is solid with minimal resistance to chassis ground.
  • There are no shorts to 12V or excessive pull on the signal line (which could produce a falsely high TPS reading).
• If readings are inconsistent, erratic, or consistently high regardless of pedal position, the TPS or its wiring is suspect.

Step 4 - Electrical tests on the pedal/throttle system (if applicable)

• For drive-by-wire systems, verify pedal position sensor signals (PED1/PED2 or equivalent) and ECU tolerance. If pedal signals correlate poorly with TPS readings, there may be an issue with the pedal sensor, the CAN network, or the ECU calibration.
• Confirm there are no software or calibration issues that would cause the PCM to interpret an incorrect TPS value. Some systems require a TPS relearn or throttle-adaptation procedure after replacement.

Step 5 - Mechanical inspection (throttle body and related components)

• If the throttle body is dirty or the throttle plate is sticking, the PCM may interpret throttle movement differently than the TPS sensor reports. Clean or service as per manufacturer guidelines, and perform idle/adaptation procedures as required post-service.
• Check for any mechanical interference, binding, or debris that could elevate the sensor readings indirectly.

Step 6 - Component replacement or service actions

• If the TPS is found faulty (sensor output stuck high, non-linear response, or consistent out-of-range readings with good wiring), replace the TPS and recheck.
• If wiring/connectors are defective (damaged insulation, corrosion, loose connectors, or shorts), repair or replace the wiring harness and connectors, and re-test.
• If the ECU is suspected (rare, but possible in some cases where the sensor is fine yet the ECU misreads), consult vehicle service information for possible ECU reflash or calibration/learn procedures.

Step 7 - Adaptation, relearn, and software considerations

• After replacing a TPS or correcting a sensor/wiring issue, some vehicles require a throttle-position sensor adaptation or "learn" procedure to allow the PCM to re-sync with the new sensor. Follow the vehicle manufacturer's procedure for idle relearn or throttle body/idle adaptation as applicable.
• Clear the codes after repair and perform a road test to verify that P1123 does not reappear and that the TPS readings track properly with pedal input.

Step 8 - Verification test drive and recheck

• Take the vehicle on a controlled test drive to verify proper throttle response across low, mid, and full-throttle ranges.
• Confirm the TPS reading tracks smoothly with pedal movement, and that the PCM's fuel trims and idle behavior return to normal ranges (where applicable).
• Re-scan after the test drive to ensure P1123 does not reappear and to check for related codes that may have been cleared or newly triggered.

Interpreting test results and decision paths

• If TPS reading is consistently high with pedal movement and wiring is solid, suspect TPS sensor fault first; replacement with proper relearn/adaptation is recommended.
• If the TPS sensor reading is high, but the signal wire and sensor ground test good, then wiring or ECU impedance issues might be involved; repair wiring or inspect ECU inputs as appropriate.
• If readings are erratic or intermittent, suspect wiring harness damage, poor connector tension, or a failing sensor; perform targeted wiring repair and retest.
• If a relearn/adaptation procedure is required and not performed, the new sensor may not operate correctly; ensure the relearn is completed as per manufacturer requirements.

Safety Considerations

• Avoid exposure to high-voltage or hot components when testing with living engine data. Use proper PPE and isolate power as needed when performing electrical tests.
• Ensure the engine is in a safe state during testing to prevent unexpected throttle movement; if necessary, have an assistant ready, particularly during road tests.

Notes

• The exact, vehicle-specific TPS voltage ranges and wiring schemes vary by manufacturer and model. The general approach here emphasizes confirming a consistently high TPS reading and verifying sensor/wiring integrity, then performing replacement and relearn only when supported by the vehicle's service procedures.
• do not include NHTSA complaint patterns for P1123, so probability estimates for causes are rather than formal complaint data. Where applicable, probability ranges are provided as field-based estimates rather than statistically derived figures.

Documentation tips for technicians

• Record all measured values (TPS reference voltage, input signal, observed range, and correlation with pedal position) and preserve freeze-frame data for future reference.
• Note any required relearn or adaptation steps performed after sensor replacement.
• If replacement is performed, verify that the ECU no longer stores P1123 after road testing and scanning again.

In summary

• P1123 indicates a high throttle position sensor value as seen by the PCM. Use a systematic approach: confirm with live data, inspect wiring and sensor, test the TPS circuit, consider mechanical causes, replace the TPS if the sensor is at fault, re-learn if required, and verify with a road test. This aligns with the general concepts of OBD-II DTCs and powertrain codes, and specifically with the TPS high value definition provided by the Open Source code definition.

This diagnostic guide was generated using verified reference data:

• 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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