P0203 on the LML Duramax indicates an electrical fault in the fuel injector circuit for cylinder 3. The LML runs a high-pressure common-rail system with precisely controlled injectors, so a circuit fault can trigger a miss or rough running. This guide explains the code and the fix.
What P0203 means
P0203 is defined as Injector Circuit/Open Cylinder 3. The LML ECM electrically drives each injector and monitors the circuit for opens, shorts, and abnormal resistance. When the cylinder 3 injector circuit falls outside expected values, it sets P0203. This is an electrical code about the circuit, so connectors and wiring are prime suspects alongside the injector.
Common causes
- Failed injector solenoid or internal open on cylinder 3
- Corroded or damaged injector connector
- Chafed or broken injector harness wiring
- Loose or dirty electrical terminal at the injector
- ECM injector driver fault (uncommon)
Symptoms
- Check engine light with stored P0203
- Misfire, rough idle, or a stumble
- Loss of power or hesitation
- Rough running that may be intermittent
- Occasional hard start
How to diagnose it
Start at the injector connector and harness for cylinder 3, checking for corrosion, chafing, or a loose terminal. Measure injector circuit resistance and compare it against the other cylinders. A wiggle test of the harness while watching for the fault can catch an intermittent open. If wiring and connectors are good and the injector coil is out of range, the injector itself is the likely cause. Reserve the ECM driver as the last suspect.
How to fix it
Repair or replace damaged wiring and clean or replace a corroded connector, ensuring a tight terminal. If the injector solenoid tests open or out of range, replace the cylinder 3 injector and have the ECM relearn injector data if required. After the repair, clear the code and verify the circuit reads correctly and the miss is resolved.
Frequently asked questions
Does P0203 mean I need new injectors?
Not necessarily. It is a single-cylinder circuit code, and wiring or connector faults are common. Only cylinder 3 is implicated, and only if the injector tests bad.
Can a connector cause P0203?
Yes. A corroded or loose injector connector can open the circuit intermittently and set the code, so inspect it before replacing the injector.
Will P0203 cause limp mode?
It can. A dead cylinder from an open injector circuit causes rough running and may trigger a power reduction until repaired.
Need parts or a second opinion? The team at Core Diesel Parts can help you match the right components to your engine. Email sales@coredieselparts.com with your VIN and symptoms and we will point you to the correct parts.
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