P0204 — Injector Circuit/Open - Cylinder 4 — LB7 Duramax

P0204 indicates the ECM has detected an electrical fault in the fuel injector circuit for cylinder 4 on the LB7 Duramax.

Updated October 15, 2026

What it means

The ECM has detected an electrical fault in the fuel injector circuit for cylinder number four.¹

Most likely causes

1. Open or high-resistance in the injector harness or connector at cylinder 4 — Inspect the under-valve-cover injector harness and external engine harness for chafing, corrosion, or loose pins at the cylinder 4 injector connector.¹
2. Failed injector solenoid/electrical open inside the cylinder 4 injector — LB7 injectors are mounted under the valve covers and are a known weak point; an internal electrical open can set P0201–P0208.¹²³⁴
3. ECM driver fault for cylinder 4 injector circuit — A fault in the ECM output driver for injector 4 can set the code; verify wiring before condemning the ECM.¹
4. Poor ground or power supply to the injector bank affecting cylinder 4 — Check the injector power/ground circuits and related fuses/relays for voltage drop or intermittent loss.¹

How to diagnose

1. Verify code and freeze frame; confirm P0204 is present and repeatable.
2. Visually inspect the engine harness and under-valve-cover injector harness for chafing, oil contamination, and connector integrity at cylinder 4.
3. Perform a pin drag test and continuity check of the cylinder 4 injector control circuit from ECM connector to injector connector.
4. Measure injector solenoid resistance at the injector connector and compare to spec; an open or out-of-range reading indicates injector electrical failure.
5. Swap the cylinder 4 injector electrical connector with an adjacent cylinder (if harness allows) and retest to isolate harness vs injector.
6. Check ECM power, ground, and injector driver supply circuits for proper voltage and no intermittent opens.
7. Clear codes and road test under load to confirm the fault is resolved.

Specs and test values

Common mistake to avoid

Buying injectors on balance rates alone. Compression, rail pressure and the pressure regulator all move balance rates; confirm a suspect injector with a return-flow test before replacing it. Source: BD Diesel — LB7 injectors: balance rates and return tests.

Parts and tools you may need

• Digital multimeter
• Scan tool with live data
• Wiring diagram for LB7 injector circuits
• Terminal release tools
• Dielectric grease

Tools you’ll need

• digital multimeter — Measures injector solenoid resistance and continuity of the cylinder 4 control circuit to detect opens or high resistance.
• OBD-II scan tool — Reads and clears P0204 and freeze frame data to confirm the fault is repeatable and monitor after repairs.
• fuel injector harness breakout box — Allows safe pin drag testing and continuity checks from ECM to cylinder 4 injector without damaging connectors.
• inspection light — Provides clear illumination to inspect under-valve-cover harness for chafing, oil contamination, and connector integrity at cylinder 4.

Rough cost and difficulty

Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If the injector tests electrically good but the code returns, or if you need to remove the valve cover/injector on a high-pressure common-rail system, have a qualified diesel technician perform the work to avoid fuel leaks and high-pressure hazards.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources