Comprehensive diagnostic guide for OBD-II code P1122

Code reference

• P1122 generally relates to throttle position sensor
• In OBD-II references, P1122 falls under Powertrain Codes as part of the standardized set of DTCs used by modern vehicles with onboard diagnostics.

What this guide covers

• This guide provides a safety-focused, step-by-step diagnostic approach to P1122, with symptom patterns, likely causes and estimated probabilities, testing procedures, and repair flow. It aligns with general OBD-II diagnostic practices described in Wikipedia's OBD-II sections and uses the official-ish code definition from the open-source repository.

Important safety and preparatory notes

• Vehicle should be parked in a safe, level area. Engage parking brake if performing measurements with the vehicle on.
• When working with the throttle/TPS circuits, avoid shorting or disconnecting power sources inappropriately. Follow manufacturer service procedures for any sensor replacement or calibration.
• If you need to disconnect electrical connectors, do so with the ignition off and the battery disconnect if required by service manuals to prevent ECU damage or shorts.
• For drive-by-wire systems, be aware that the throttle response can be active even with ignition off in some setups; always follow OEM safety guidelines when performing bench tests or sensor swaps.

Symptoms

• Intermittent or erratic engine idle and/or throttle response.
• Hesitation or surges during acceleration or cruise, especially when throttle position changes abruptly.
• Check Engine Light (CEL) may be illuminated or intermittently on.
• In some cases, vehicle may feel normal at times and "drop" to a low-throttle reading, causing under- or over-fueling conditions if the ECU re-averages fuel trims.

Probable Causes

• TPS sensor fault or intermittent TPS signal (including a failing sensor, internal intermittent open/short): ~40-50%
• Wiring/connector issues (loose, corroded, damaged harness, failed pin seals, moisture intrusion, poor ground or 5V reference wiring): ~25-30%
• ECU/PCM input or calibration issue (faulty reference voltage, ECU internal fault, needs reflash/calibration after sensor replacement): ~10-15%
• Electrical supply/ground issues affecting sensor signal integrity (battery voltage drop, blown fuse, bad ground path): ~5-10%
• Mechanical throttle body issues (sticking/throttle plate binding that affects sensor readings indirectly): ~5-10%

Notes:

• These percentages are intended as practical guidance for diagnostic prioritization in the absence of NHTSA complaint data. They reflect common field observations for intermittent TPS signals and are not OEM-spec or vehicle-specific.

Step-by-Step Diagnosis

1) Confirm the fault and collect data

• Use an OBD-II scanner to confirm P1122 and review any freeze-frame data or live data corresponding to the TPS signal(s).
• Note whether the code is current or history; gather other codes (P0120/P0121 and related TPS/Pedal Position codes) that may indicate a pattern across the A/B circuits.
• Observe engine data related to throttle position (TPS A and/or TPS B, if a vehicle provides dual circuits). If available, log data over a drive to capture intermittent dips to low signal.

2) Visual and mechanical inspection

• Inspect TPS/pedal position sensor connectors for corrosion, bent pins, worn locking tabs, and signs of moisture intrusion.
• Inspect the TPS harness and connector routing for pinched wires, exposed insulation, or chafing against moving parts.
• Check for any aftermarket wiring modifications that could affect the TPS signal or 5V reference.
• Confirm there are no obvious vacuum leaks or intake manifold issues that could influence throttle position readings indirectly (general engine response, though not a root cause of a low TPS signal).

3) Electrical checks (TPS circuit integrity)

• Vehicle ignition off, access the TPS connector and perform resistance/continuity checks on the TPS signal circuit and ground:
  • Verify the 5V reference supply from the ECU to the TPS is present when ignition is on (some vehicles require engine off for certain tests; follow OEM procedure).
  • Check that the TPS signal return to the ECU is continuous, with no short to ground or open circuits.
  • Inspect the ground path and ensure a solid ground connection on the TPS circuit and ECU ground.
• Wiggle test the wiring harness around the TPS connector while monitoring the live TPS signal with a scan tool to detect intermittent opens/contacts.

4) Functional TPS testing (live data and sensor response)

• With the engine running or as per vehicle procedure, observe the TPS A/B sensor signals on the scanner:
  • The TPS signal should exhibit a smooth, monotonic change with pedal position or throttle plate movement (depending on the system). Look for intermittent dips or low readings that correspond to the P1122 condition.
  • Confirm that the 5V reference is stable and within expected range (generally 4.5-5.5V on many vehicles, but verify via service data for the specific vehicle).
• If the readings show intermittent low signals that coincide with the symptom occurrences, this supports a TPS or wiring fault.

5) Cross-check with pedal position sensors and related systems

• If the vehicle uses a pedal-position sensor (PPS) in addition to or instead of a TPS, verify PPS signal integrity and its interaction with the ECU. In some systems, PPS and TPS signals are cross-referenced by the PCM; a fault in one can cause intermittent readings in the other.
• Check for any software calibration updates or reset procedures that might be required after sensor replacement. Some systems require TPS calibration after replacement.

6) Component testing / replacement path

• If wiring and connectors pass inspection and live data consistently shows intermittent TPS signals, replace the TPS sensor.
• After replacement:
  • Reconnect all wiring, ensure proper torque on connector tabs, and clear codes.
  • Re-scan to verify the P1122 code is cleared and monitor live data during a test drive.
  • If the problem persists after a known-good TPS replacement, re-check wiring for intermittent faults and test the ECU/PCM reference circuits; consider ECU calibration or software updates per manufacturer procedures.

7) Additional troubleshooting aids (advanced)

• Use a known-good power supply to the ECU or a bench test as appropriate for the model if you suspect a weak 5V reference due to ECU power issues.
• For vehicles with drive-by-wire systems, consider using a diagnostic flow that also checks throttle plate control motor operation if accessible, especially if replaceable TPS did not resolve the issue.

Documentation

• Vehicle make/model/year, engine type, and transmission type.

• Specific DTC status (current vs. history) and any freeze-frame data.

• TPS live data readings (A and B), 5V reference status, and any observed intermittent dips to low signals.

• Wiring harness inspection results, connector condition, and any repaired/cleaned connections.

• Sensor replacement details (part number, calibration reference, procedure followed) and testing after replacement.

• Test drive results, including driving conditions when the fault occurs (idle, light throttle, heavy load, cold start, etc.).

• Code context and classification: OBD-II diagnostic trouble codes are part of standardized powertrain codes. These sources describe how DTCs are generated and categorized by the onboard diagnostic system.

• Specific code definition used for this guide: The OBD2 code definition repository identifies P1122 as This provides the targeted interpretation for this diagnostic task.

• General diagnostic approach and the concept of powertrain codes: The OBD-II overview and related sections emphasize that modern vehicles monitor various parameters and generate DTCs when issues are detected, underscoring the systematic approach to diagnosing issues like P1122.

Appendix: Quick reference for P1122

• Definition: Throttle Position Sensor - intermittent signal low (TPS signal intermittently reads low)
• Category: Powertrain Code (OBD-II)
• Common symptoms to watch for: intermittent idle instability, hesitation or surges, CEL illumination
• Primary diagnostic focus: TPS sensor condition, wiring/connectors, 5V reference and ground integrity, ECU input behavior
• Primary repair action: TPS replacement or repair of wiring/connector; ensure calibration if required by the vehicle

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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