Open or high-resistance wiring in the IAC signal 1 circuit, such as a broken wire or corroded connector terminal; Failed idle air control solenoid winding or internal electrical fault; Poor connection at the IAC valve connector or at the ECM harness connector; Short to voltage on the signal 1 wire, often from chafed insulation against a power wire; Contaminated or carbon-fouled IAC valve causing the pintle to stick and the solenoid to draw abnormal current; ECM internal driver fault, though this is less common than wiring or valve issues

DTC codes / Subaru codes / P1511

P1511 · SubaruIdle Air Control Solenoid Signal 1 Circuit High Input

Q: What does code P1511 mean?

Idle Air Control Solenoid Signal 1 Circuit High Input. P1511 on a Subaru means the engine control module (ECM) has detected an abnormally high voltage on the signal circuit for the idle air control (IAC) solenoid, specifically signal 1. The ECM expects a certain voltage range from the IAC valve as it commands idle speed, and a high input tells it the circuit is open, shorted to power, or the solenoid winding is failing.

P1511 on a Subaru means the engine control module (ECM) has detected an abnormally high voltage on the signal circuit for the idle air control (IAC) solenoid, specifically signal 1. The ECM expects a certain voltage range from the IAC valve as it commands idle speed, and a high input tells it the circuit is open, shorted to power, or the solenoid winding is failing.

SystemPowertrain

Code typeSubaru specific

SeverityMedium

Fix difficultyIntermediate

AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Unstable or fluctuating idle speed, sometimes surging or hanging high
Engine may stall when coming to a stop or when the throttle is released
Rough idle quality, especially when cold or when electrical loads are applied
In rare cases, the engine may idle too low and feel like it wants to cut out

Common causes, most likely first

Open or high-resistance wiring in the IAC signal 1 circuit, such as a broken wire or corroded connector terminal
Failed idle air control solenoid winding or internal electrical fault
Poor connection at the IAC valve connector or at the ECM harness connector
Short to voltage on the signal 1 wire, often from chafed insulation against a power wire
Contaminated or carbon-fouled IAC valve causing the pintle to stick and the solenoid to draw abnormal current
ECM internal driver fault, though this is less common than wiring or valve issues

How to diagnose and fix it

Connect a scan tool and verify P1511 is current, then check for any related idle control or throttle codes
Inspect the IAC valve connector and wiring for corrosion, pushed-out pins, or chafed insulation
Measure resistance across the IAC solenoid windings and compare to Subaru specifications
Back-probe the signal 1 circuit at the ECM and check voltage with the key on and engine off
Perform a wiggle test on the harness while watching live IAC data to catch intermittent opens
Clean the IAC valve passages and pintle if carbon buildup is present, then recheck operation
If wiring and valve test good, substitute a known-good IAC valve or ECM to isolate the fault

What P1511 on a Subaru really points to

A high input on signal 1 often means the circuit is open rather than shorted, so a broken wire inside the harness loom is a common find on higher-mileage Subarus.
Carbon buildup on the IAC pintle can make the solenoid draw less current and set a high input code even when the winding tests fine with a meter.
Technicians sometimes replace the IAC valve only to find a corroded terminal at the ECM connector, so always back-probe before condemning parts.
This code can be set by a weak battery or charging system fault that causes the ECM to see an out-of-range signal during cranking, so check system voltage first.
On some Subaru models, a dirty throttle body or a misadjusted throttle stop can mimic IAC faults, so verify base idle and throttle plate cleanliness before chasing wiring.

Can I keep driving with P1511?

You can usually drive the car, but expect an erratic idle that may cause stalling at stops, so avoid heavy traffic and keep your foot ready on the brake. If the engine stalls repeatedly or idles dangerously low, park it and have the fault diagnosed before continuing.

Tools that help

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Guidance is general and written for Subaru vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.