Damaged, corroded or loose connector terminals on the TCM-to-ECM signal circuit; Signal wire shorted to a voltage supply or chafed against other live wiring; Open circuit or high resistance in the signal wire or its ground; Underlying transmission fault that makes the TCM drive the diagnosis signal high; Faulty TCM output or internal fault; Low battery voltage or poor TCM power or earth supply; Faulty ECM input circuit.

P1596

Automatic Transmission Diagnosis Input Signal Circuit High 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
96: FaultThe specific fault in that area

What it means

On Subaru models, P1596 means the engine control unit (ECM) saw a voltage that was too high on the automatic transmission diagnosis input signal circuit. This is the line the transmission control unit (TCM) uses to report its status to the ECM. The ECM sets the code when the signal stays high outside its expected range, which points to a wiring, connector or control unit fault.

Symptoms

Malfunction indicator lamp on
Automatic transmission warning light or flashing AT lamp on some models
Harsh, delayed or flared gear changes
Transmission stuck in one gear (limp mode) on some models
Reduced performance or poor economy
Other transmission fault codes stored at the same time

Possible causes

Damaged, corroded or loose connector terminals on the TCM-to-ECM signal circuitlikely
Signal wire shorted to a voltage supply or chafed against other live wiringlikely
Open circuit or high resistance in the signal wire or its groundpossible
Underlying transmission fault that makes the TCM drive the diagnosis signal highpossible
Faulty TCM output or internal faultpossible
Low battery voltage or poor TCM power or earth supplypossible
Faulty ECM input circuitrare

How to diagnose

Read all stored codes from both the engine and transmission control units, and note the freeze frame data for when the fault set.
Check battery condition and charging voltage, as low or unstable voltage can cause false signal faults.
Inspect the TCM and ECM connectors and the harness between them for corrosion, bent or pushed-back pins, water ingress, and chafing or heat damage.
Check the TCM power and earth supplies against the wiring diagram for your model.
Using the wiring diagram, measure the voltage on the diagnosis signal wire with the ignition on and compare it with the specification for your model.
Test the signal wire for a short to power and for continuity end to end, with both control units disconnected.
Check the transmission for other stored faults or mechanical and hydraulic problems, as the correct test path depends on the model and transmission type.
Clear the code and road test, then recheck whether it returns and under what conditions.

How to fix

Clean, repair or replace faulty connectors and terminals.
Repair or replace the damaged section of wiring, including any short to power or open circuit.
Restore proper power and earth supplies to the TCM, and fix any battery or charging problem.
Repair the underlying transmission fault if another stored code points to one.
Replace or reprogram the TCM only after wiring and supplies are proven good and a fault is confirmed, following the manufacturer's procedure.
Replace the ECM only as a last resort once the circuit and TCM are proven good.

Can I keep driving?

Medium severity

The car can usually be driven carefully for a short time, but the transmission may go into a protective mode with harsh or limited shifting. Get it checked soon, and stop if the gearbox slips, shifts violently or the warning lamp flashes.

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