Damaged, chafed or shorted-to-earth wiring in the transmission diagnosis signal circuit between the ECU and the transmission control unit; Corroded, loose or pushed-back pins in the ECU, transmission control unit or transmission harness connectors; Fault in the transmission control unit itself pulling the signal line low; Underlying transmission fault logged by the transmission control unit, causing it to output a fault signal; Low battery voltage or poor earth connections affecting the control units' signal levels; Internal fault in the engine control unit input circuit.

P1595

Automatic Transmission Diagnosis Input Signal Circuit Low Input.

Subaru
Powertrain
Manufacturer code
Medium severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
5: AreaVehicle speed, idle control and auxiliary inputs
95: FaultThe specific fault in that area

What it means

On Subaru, P1595 means the engine control unit has seen a low voltage on the automatic transmission diagnosis input signal circuit. This is the line used for communication or fault-status exchange between the transmission control unit and the engine control unit. The ECU set the code because the signal sat below its expected range, which points to a circuit, connector or control unit problem rather than a mechanical fault.

Symptoms

Check engine light on, and possibly the transmission warning light or AT OIL TEMP light flashing
Harsh, delayed or erratic gear changes
Transmission stuck in one gear (limp mode) in some cases
Poor fuel economy or reduced performance
No noticeable symptoms other than the warning light

Possible causes

Damaged, chafed or shorted-to-earth wiring in the transmission diagnosis signal circuit between the ECU and the transmission control unitlikely
Corroded, loose or pushed-back pins in the ECU, transmission control unit or transmission harness connectorslikely
Fault in the transmission control unit itself pulling the signal line lowpossible
Underlying transmission fault logged by the transmission control unit, causing it to output a fault signalpossible
Low battery voltage or poor earth connections affecting the control units' signal levelspossible
Internal fault in the engine control unit input circuitrare

How to diagnose

Read all stored codes from both the engine and transmission control units and note any transmission codes, as these should be dealt with first
Review freeze frame data and check whether the code is current or intermittent after clearing and road testing
Check battery condition and charging voltage, and inspect the main earth connections
Visually inspect the transmission and ECU harness for chafing, heat damage, water ingress and previous repairs
Disconnect and inspect the relevant connectors for corrosion, bent or loose pins and poor terminal tension
Check the signal circuit for continuity, short to earth and short to other wires using the wiring diagram for the specific model and year
Wiggle-test the harness with the circuit monitored to find intermittent faults
Check for a technical bulletin or software update for the specific model, and only consider control unit replacement once wiring and power supplies are proven good

How to fix

Repair or replace the damaged section of wiring, using proper soldered or crimped sealed joints
Clean or replace corroded connectors and restore correct terminal tension
Repair any underlying transmission fault that is setting a fault signal, following the transmission codes
Restore good battery condition and clean or tighten earth connections
Repair, replace or reprogram the transmission or engine control unit only if testing proves it faulty, or if an updated calibration is a known fix for the model

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but gear changes may be harsh or the transmission may go into limp mode. Get it diagnosed soon to avoid possible transmission damage.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.