Damaged, chafed or corroded wiring in the signal circuit between the engine ECU and transmission control unit; Loose, corroded or contaminated connector at the ECU, transmission control unit or transmission harness plug; Fault within the transmission control unit or its internal circuitry; Low or poor battery voltage, or a weak earth connection, upsetting communication between units; Internal transmission fault or sensor problem that causes the transmission to send an invalid diagnosis signal; Fault within the engine ECU input circuit.

P1594

Automatic Transmission Diagnosis Input Signal Circuit Malfunction.

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
94: FaultThe specific fault in that area

What it means

On Subaru, P1594 means the engine control unit has detected a fault in the circuit carrying the automatic transmission diagnosis input signal. The engine ECU and the transmission control unit share information, and the ECU saw a signal that was missing, out of range or not as expected. The exact signal and wiring path depend on the model, engine and transmission fitted.

Symptoms

Check engine light on
Transmission warning light or gearbox fault indication on the dash
Harsh, delayed or erratic gear changes
Transmission stuck in a fixed gear (limp mode)
Reduced power or poor response
Other transmission-related codes stored alongside

Possible causes

Damaged, chafed or corroded wiring in the signal circuit between the engine ECU and transmission control unitlikely
Loose, corroded or contaminated connector at the ECU, transmission control unit or transmission harness pluglikely
Fault within the transmission control unit or its internal circuitrypossible
Low or poor battery voltage, or a weak earth connection, upsetting communication between unitspossible
Internal transmission fault or sensor problem that causes the transmission to send an invalid diagnosis signalpossible
Fault within the engine ECU input circuitrare

How to diagnose

Read all stored codes and freeze frame data, and note whether any other transmission or communication codes are present
Check battery condition and charging voltage, and inspect main earth connections
Check the transmission fluid level and condition, following the procedure for the specific model
Visually inspect the wiring harness between the engine ECU and transmission control unit for chafing, heat damage or previous repairs
Disconnect and inspect the relevant connectors for corrosion, bent pins, moisture and poor terminal tension
Using the manufacturer wiring diagram for the model, test the signal circuit for continuity, short to earth and short to supply
Check the live data for the transmission control unit and see whether it communicates and responds normally
Clear the code and road test to see if it returns, then recheck the freeze frame conditions

How to fix

Repair or replace damaged wiring in the signal circuit, using proper soldered or crimped sealed joints
Clean, repair or replace faulty connectors and terminals
Correct battery, charging or earth faults
Repair any transmission fault or faulty sensor identified by further diagnosis
Replace or have the transmission control unit repaired or reprogrammed only after wiring and supply faults are ruled out and an internal fault is confirmed

Can I keep driving?

Medium severity

The car may be driveable, but gear changes may be poor or the transmission may be in limp mode, and a continuing fault could cause further transmission damage. Have it checked soon and avoid heavy loads or towing until it is diagnosed.

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