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DTC codes / Honda codes / P1790

P1790 · HondaTCM A/T TP Sensor Circuit Fault

P1790 on Honda vehicles means the Transmission Control Module (TCM) has detected a fault in the Automatic Transmission (A/T) Throttle Position (TP) sensor circuit. The TCM monitors the throttle position signal it receives and flags this code when the voltage or signal behavior falls outside the expected range during key-on or running conditions. This circuit tells the transmission how far the accelerator is pressed so shift points and line pressure can be scheduled correctly.

SystemPowertrain
Code typeHonda specific
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • The Check Engine Light or Transmission warning light illuminates on the dashboard.
  • Harsh or delayed shifting because the TCM defaults to a limp or fixed shift schedule.
  • Erratic shift points that feel disconnected from accelerator pedal input.
  • The transmission may stay in a single gear or refuse to downshift during acceleration.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.
  • Reduced fuel economy or a noticeable lack of power during normal driving.

Common causes, most likely first

  1. A dirty or failing throttle position sensor on the throttle body sending an erratic signal.
  2. Damaged, chafed, or corroded wiring between the TP sensor and the TCM.
  3. Loose or corroded electrical connectors at the throttle body or TCM harness.
  4. A faulty TCM that is misreading a valid throttle position input.
  5. Poor ground connection shared by the TP sensor and TCM circuits.
  6. Throttle body carbon buildup causing the sensor to bind or sweep unevenly.
  7. Aftermarket or incorrectly installed electronic throttle components altering the signal.

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data for P1790 before clearing anything.
  2. Inspect the throttle body, TP sensor connector, and TCM harness for corrosion, chafing, or loose pins.
  3. Back-probe the TP sensor signal wire and compare voltage to the manufacturer's specification through the full pedal sweep.
  4. Check sensor ground and reference voltage at the TP sensor connector with a multimeter under load.
  5. Clean the throttle body and perform a throttle position relearn or idle relearn procedure if carbon buildup is found.
  6. If the signal is out of range at the sensor, replace the TP sensor or throttle body assembly and recheck live data.
  7. If the sensor and wiring test good but the TCM still sets P1790, diagnose the TCM power, ground, and internal fault before replacement.

What Honda techs check before replacing the TCM

  • A throttle body carbon cleaning and relearn fixes many P1790 cases on high-mileage Hondas because the sensor sweep becomes erratic near closed throttle.
  • The TCM and Engine Control Module (ECM) share throttle position data on many Honda models, so a fault in the engine-side sensor circuit can set P1790 even when the transmission itself is healthy.
  • Intermittent P1790 codes often trace to a chafed harness near the throttle body or a corroded connector pin that only acts up with engine vibration or heat.
  • Do not replace the TCM based on P1790 alone; verify the TP sensor signal at the TCM connector with a scope or multimeter first, because TCM replacement requires programming and is rarely the real cause.
  • A weak battery or poor ground can cause voltage glitches that the TCM interprets as a TP sensor circuit fault, so check charging system health before condemning parts.

Can I keep driving with P1790?

You can usually drive the vehicle to a shop, but expect harsh or unpredictable shifting and reduced performance because the TCM is in a default mode. Avoid aggressive acceleration, towing, or long highway trips until the fault is diagnosed, since limp mode can leave the transmission in a single gear.

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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