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DTC codes / Subaru codes / P1595

P1595 · SubaruAutomatic Transmission Diagnosis Input Signal Circuit Low Input

P1595 on a Subaru means the Transmission Control Module (TCM) has detected a low input voltage on the automatic transmission diagnosis input signal circuit, which is the data line the TCM uses to receive diagnostic and status information from the transmission. The TCM compares the expected signal voltage against what it actually sees and sets this code when the circuit stays below its normal threshold for a calibrated period.

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

Symptoms you may notice

  • The Check Engine Light or AT OIL TEMP light illuminates steadily on the dashboard.
  • Often no symptoms besides the warning light, with the transmission shifting normally.
  • Harsh or delayed shifting if the TCM enters a limited-operation strategy.
  • The transmission may stay in a single gear or refuse to downshift under certain conditions.
  • Intermittent warning light that comes and goes with vibration or moisture.
  • A stored code that returns immediately after clearing if the circuit fault is hard.

Common causes, most likely first

  1. A chafed, corroded, or broken wire in the diagnosis input signal circuit between the TCM and the transmission connector.
  2. Water or oil intrusion into the transmission solenoid harness connector or the TCM connector, causing a low-resistance path to ground.
  3. A failed transmission internal harness or solenoid pack that pulls the diagnosis signal line low.
  4. A poor ground connection at the transmission case or TCM mounting point, skewing the reference voltage.
  5. A faulty TCM that misreads a correct signal or fails to supply the expected pull-up voltage.
  6. Aftermarket electrical accessories or a poor splice tapping into the transmission harness.
  7. A short to ground on the signal wire where it passes through a body grommet or near a heat source.

How to diagnose and fix it

  1. Connect a scan tool and record all stored and pending codes, then verify the P1595 is current and not a ghost from a previous repair.
  2. Inspect the transmission harness connector and TCM connector for corrosion, water intrusion, bent pins, and loose terminals.
  3. Measure resistance and voltage on the diagnosis input signal wire from the TCM to the transmission connector, comparing against the factory wiring diagram.
  4. Wiggle-test the harness while watching live data or a multimeter to catch intermittent opens or shorts.
  5. Check and clean the transmission case ground and the TCM ground, then re-test the circuit.
  6. If the wiring and grounds are good, substitute a known-good TCM or transmission harness to isolate the failed component.
  7. Clear the code and perform a relearn or adaptation drive cycle, then confirm the code does not return.

Why this code is often a wiring fault

  • P1595 is frequently set by a chafed wire in the harness near the transmission mount rather than by a failed transmission, so a visual inspection of the loom often finds the fault before any parts are replaced.
  • A low input signal can be caused by a short to ground anywhere on the circuit, including an aftermarket accessory spliced into the wrong wire, so check for non-factory wiring before condemning the TCM.
  • The code can appear after a transmission fluid service if the harness connector was not fully seated or if fluid wicked into the connector pins.
  • On some Subaru models the diagnosis input signal shares a path with other diagnostic data, so a single bad terminal can set companion communication codes that mislead the diagnosis.
  • Clearing the code without repairing the circuit often results in a return within a few drive cycles, and repeated clearing can erase freeze-frame data that would have pointed to the conditions that triggered it.

Can I keep driving with P1595?

The vehicle is generally safe to drive short distances if the transmission shifts normally, but avoid towing, heavy loads, and aggressive driving until the circuit is repaired. If the transmission enters limp mode or shifts harshly, stop driving and have it diagnosed to prevent internal 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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