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DTC codes / Subaru codes / P1742

P1742 · SubaruAuto Trans Diagnosis Input Signal Malfunction

P1742 on Subaru vehicles means the Transmission Control Module (TCM) has detected an abnormal signal from the automatic transmission diagnosis input circuit, which typically monitors the transmission's internal sensors and switches. The TCM sets this code when the voltage or signal pattern from that input does not match expected values, often due to a wiring fault, poor connection, or a failing component within the transmission.

SystemPowertrain
Code typeSubaru specific
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • Illumination of the Check Engine Light or Automatic Transmission Warning Light
  • Often no symptoms besides the warning light, especially in early stages
  • Harsh or delayed shifting, or the transmission entering limp mode
  • Inconsistent gear engagement or slipping sensations
  • Reduced fuel economy due to improper shift scheduling
  • Occasional stalling or rough idle when the transmission signal interferes with engine load calculations

Common causes, most likely first

  1. Corroded, loose, or damaged electrical connector at the transmission diagnosis input sensor or solenoid
  2. Chafed or open wiring in the harness between the transmission and the TCM
  3. Failed transmission input speed sensor or diagnosis signal sensor
  4. Internal transmission fault such as a faulty valve body or solenoid pack affecting the diagnosis circuit
  5. Contaminated transmission fluid causing sensor signal interference
  6. Faulty TCM or poor ground connection to the TCM
  7. Aftermarket electrical accessories improperly tapped into the transmission harness

How to diagnose and fix it

  1. Verify battery voltage and inspect the transmission fluid level and condition; low or dirty fluid can cause signal anomalies
  2. Locate and inspect the transmission diagnosis input connector and sensor for corrosion, tightness, and damage; clean and reseat as needed
  3. Perform a wiggle test on the harness while monitoring live data for the diagnosis input signal to identify intermittent wiring faults
  4. Check resistance and continuity of the diagnosis input circuit from the sensor to the TCM using a multimeter and the factory wiring diagram
  5. Replace the diagnosis input sensor or solenoid if resistance is out of specification or signal is erratic
  6. Inspect and repair any damaged wiring or connectors, then clear the code and perform a relearn or adaptation drive cycle
  7. If the code persists, diagnose the TCM and internal transmission components, including valve body inspection, before replacing expensive parts

Don't overlook the diagnosis input circuit

  • A common misdiagnosis is replacing the transmission assembly when the actual fault is a $20 sensor or a chafed wire in the harness near the transmission mount.
  • The diagnosis input signal is often shared with other transmission sensors, so a single bad ground can set P1742 along with other codes, leading technicians to chase multiple faults.
  • Aftermarket remote starters or alarm systems tapped into the transmission wiring can introduce noise that mimics a failed sensor, so always check for non-factory wiring first.
  • On some Subaru models, the TCM is integrated into the valve body, and a faulty TCM can set P1742 without any external wiring issue, requiring a dealer-level scan tool to confirm.
  • Driving with P1742 for extended periods can cause the transmission to stay in limp mode, accelerating clutch wear and turning a minor repair into a major overhaul.

Can I keep driving with P1742?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration, towing, or highway speeds because the transmission may be in limp mode and shift unpredictably. If the vehicle enters limp mode or shifting becomes severely harsh, stop driving and have it towed to prevent further damage.

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 Subaru vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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