A failing starter switch (ignition switch) internal contact, common on higher-mileage Subarus.; Corroded or loose electrical connectors at the starter solenoid or ignition switch harness.; A damaged or chafed wire in the starter switch circuit, often near the steering column or firewall.; A faulty starter solenoid that back-feeds voltage into the switch circuit.; A weak or failing battery or poor ground connection causing voltage drops during cranking.; An aftermarket alarm or remote start system improperly spliced into the starter circuit.; Rarely, an internal fault in the ECM itself, but this should only be suspected after all wiring checks pass.

DTC codes / Subaru codes / P1100

P1100 · SubaruStarter Switch Circuit Malfunction

Q: What does code P1100 mean?

Starter Switch Circuit Malfunction. P1100 on a Subaru means the engine control module (ECM) has detected an unexpected signal on the starter switch circuit, which is the electrical path that tells the computer when the key is turned to the crank position. The ECM expects a specific voltage only during cranking, and it flags this code when the circuit stays active, drops out, or shows a voltage outside the normal range during a start attempt.

P1100 on a Subaru means the engine control module (ECM) has detected an unexpected signal on the starter switch circuit, which is the electrical path that tells the computer when the key is turned to the crank position. The ECM expects a specific voltage only during cranking, and it flags this code when the circuit stays active, drops out, or shows a voltage outside the normal range during a start attempt.

SystemPowertrain

Code typeSubaru specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

The engine may crank but not start, or may not crank at all when the key is turned.
The check engine light illuminates and stays on while the engine is running.
Intermittent no-start conditions where the engine starts normally on the next attempt.
The starter may continue to run briefly after the key is released, or may fail to engage.
Often no symptoms besides the warning light if the fault is a brief glitch in the circuit.
The security or immobilizer indicator may flash unexpectedly on some models.

Common causes, most likely first

A failing starter switch (ignition switch) internal contact, common on higher-mileage Subarus.
Corroded or loose electrical connectors at the starter solenoid or ignition switch harness.
A damaged or chafed wire in the starter switch circuit, often near the steering column or firewall.
A faulty starter solenoid that back-feeds voltage into the switch circuit.
A weak or failing battery or poor ground connection causing voltage drops during cranking.
An aftermarket alarm or remote start system improperly spliced into the starter circuit.
Rarely, an internal fault in the ECM itself, but this should only be suspected after all wiring checks pass.

How to diagnose and fix it

Check battery voltage and condition, including terminals and grounds, before anything else.
Inspect the starter switch circuit wiring for chafing, corrosion, or loose connectors from the ignition switch to the starter solenoid.
Test the ignition switch contacts with a multimeter for continuity and voltage drop in the crank position.
Verify the starter solenoid operation and check for voltage back-feed on the signal wire during and after cranking.
If an aftermarket alarm or remote start is present, disconnect it temporarily to rule out interference.
Clear the code and perform a test drive with multiple start cycles to confirm the fault is gone.
If all wiring and components test good, follow the factory diagnostic flow for the ECM before replacing it.

Starter switch circuit faults that fool technicians

A common misdiagnosis is replacing the starter motor when the real problem is a worn ignition switch contact that only acts up in cold weather or after the car sits.
Aftermarket remote start systems are a frequent culprit because they splice into the starter signal wire and can introduce voltage spikes that the ECM interprets as a switch malfunction.
A weak battery can set P1100 without any starter switch fault, because cranking voltage drops below the threshold the ECM expects to see on the circuit.
On some Subaru models, the starter switch circuit shares a ground with the immobilizer system, so a poor ground can trigger P1100 along with security-related symptoms.
The code can be stored by a single failed start attempt, so always check freeze-frame data and clear it to see if it returns before condemning any part.

Can I keep driving with P1100?

You can usually drive the car if it starts and runs normally, but avoid long trips or shutting the engine off in remote locations until the fault is repaired. If the engine cranks but will not start or the starter behaves erratically, stop driving and have the circuit diagnosed.

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.