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DTC codes / Subaru codes / P1594

P1594 · SubaruAutomatic Transmission Diagnosis Input Signal Circuit Malfunction

P1594 on Subaru vehicles indicates that the Transmission Control Module (TCM) has detected an abnormal signal from the automatic transmission diagnosis input circuit, which typically carries data from the transmission's internal sensors or the inhibitor switch to the TCM. The TCM monitors this circuit for expected voltage or continuity and sets the code when the signal is missing, out of range, or erratic, often resulting in the transmission entering a limp-home mode.

SystemPowertrain
Code typeSubaru specific
SeverityMedium
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • Illumination of the Check Engine Light or Automatic Transmission warning light on the dashboard.
  • Transmission may shift harshly or fail to shift out of a single gear (limp mode).
  • The vehicle may feel sluggish or lack power due to restricted gear selection.
  • In some cases, the transmission may slip or hesitate when accelerating.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.

Common causes, most likely first

  1. Faulty or contaminated transmission fluid causing sensor signal interference.
  2. Damaged wiring or corroded connectors in the transmission diagnosis input circuit.
  3. Malfunctioning inhibitor switch (also known as neutral safety switch) sending incorrect signals.
  4. Failed transmission internal sensor, such as the vehicle speed sensor or turbine speed sensor.
  5. Poor electrical ground connection at the transmission or TCM.
  6. Defective Transmission Control Module (TCM) itself, though this is less common.

How to diagnose and fix it

  1. Check transmission fluid level and condition; top off or replace if contaminated.
  2. Inspect the inhibitor switch and its wiring for corrosion, damage, or loose connections; clean or replace as needed.
  3. Perform a visual inspection of the transmission wiring harness and connectors for chafing or breaks; repair or replace damaged sections.
  4. Test the transmission diagnosis input circuit for continuity and voltage drops using a multimeter; compare to factory specifications.
  5. Check and clean all ground points related to the transmission and TCM.
  6. If the circuit tests good, test the internal transmission sensors (e.g., speed sensors) for resistance and signal output; replace faulty sensors.
  7. As a last resort, substitute a known-good TCM to rule out internal failure; replace if necessary.

Don't overlook the inhibitor switch

  • A misadjusted or failing inhibitor switch is a frequent culprit for P1594 on Subaru models, often causing intermittent no-start or gear indicator issues that owners miss.
  • The code can be set by a simple loose ground bolt on the transmission, which is easy to overlook during diagnosis.
  • Transmission fluid that is overdue for a change can cause sensor signal degradation, leading to this code without any mechanical failure.
  • P1594 does not always mean the transmission needs a rebuild; many cases are resolved with wiring repairs or switch replacement.
  • On some Subaru models, a weak battery or charging system fault can cause voltage spikes that trigger this code, so check the charging system first.

Can I keep driving with P1594?

If the transmission is in limp mode, driving is not recommended because the vehicle may be limited to a single gear and could overheat the transmission. If the only symptom is the warning light and the car drives normally, you can drive cautiously to a repair shop, but avoid heavy acceleration or towing.

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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