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DTC codes / Honda codes / P1121

P1121 · HondaThrottle Position Lower Than Expected

P1121 on Honda vehicles means the powertrain control module (PCM) has detected that the throttle position sensor (TPS) signal is lower than the value it expects for the current operating conditions, such as engine speed and load. The PCM compares the TPS voltage against a calculated model and sets the code when the actual reading falls below the minimum threshold for a calibrated time.

SystemPowertrain
Code typeHonda specific
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Hesitation or stumble when accelerating from a stop
  • Reduced engine power or limp mode in some cases
  • Poor fuel economy or erratic idle quality
  • Delayed or harsh transmission shifts on automatic models

Common causes, most likely first

  1. Faulty throttle position sensor (TPS) or its internal wear
  2. Damaged or corroded TPS wiring harness or connector
  3. Poor electrical connection at the TPS or PCM
  4. Throttle body carbon buildup causing incorrect throttle plate position
  5. PCM software calibration issue or internal fault
  6. Mechanical binding in the throttle linkage or pedal assembly

How to diagnose and fix it

  1. Inspect the TPS connector and wiring for corrosion, looseness, or damage
  2. Check TPS reference voltage and signal voltage with a multimeter while moving the throttle
  3. Clean the throttle body and perform a throttle position relearn procedure
  4. Compare TPS readings with the accelerator pedal position sensor (APPS) using a scan tool
  5. Replace the TPS if voltage output is erratic or out of specification
  6. Repair or replace damaged wiring between the TPS and PCM
  7. Update the PCM calibration or replace the PCM if all other checks pass

Common misdiagnosis traps for P1121

  • A dirty throttle body often causes a lower-than-expected TPS signal because the plate does not return to the correct rest position, so cleaning and relearning can resolve the code without parts replacement.
  • The TPS and accelerator pedal position sensor (APPS) are separate components on many Hondas, and a faulty APPS can set P1121 indirectly by causing the PCM to expect a different throttle angle.
  • Intermittent TPS wiring faults may not appear during a static voltage test, so a wiggle test of the harness while monitoring live data is essential.
  • After replacing a TPS or throttle body, a relearn procedure is required; skipping it can cause the code to return immediately even with good parts.
  • P1121 can be stored by a low battery or charging system issue that causes voltage drops affecting sensor reference signals, so check battery and ground integrity first.

Can I keep driving with P1121?

You can usually drive the vehicle to a repair shop, but avoid aggressive acceleration and highway merging until the fault is fixed because the engine may enter limp mode without warning. If the check engine light flashes or the engine runs poorly, stop driving and have it towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Honda vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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