Diesel Trouble Codes

P0203 — Injector Circuit/Open - Cylinder 3 — LB7 Duramax

P0203 on the LB7 Duramax means the ECM has detected an electrical fault in the injector circuit for cylinder 3. The LB7 uses Bosch injectors mounted under the…

Updated October 15, 2026

What it means

The engine control module has detected an electrical fault in the injector circuit for cylinder number three. On this platform the LB7 uses Bosch injectors mounted under the valve covers, a well-known weak point for cracked bodies and return-flow/balance-rate drift.1

Most likely causes

  1. Open or high resistance in the cylinder 3 injector harness/connector under the valve cover — Inspect the under-valve-cover injector sub-harness and connector for chafing, oil intrusion, or terminal fretting that creates an open circuit to cylinder 3.1234
  2. Failed injector solenoid/electrical open inside cylinder 3 Bosch injector — A failed injector solenoid or internal open in the Bosch injector mounted under the valve cover will set an injector circuit/open code for that cylinder.1234
  3. ECM injector driver fault for cylinder 3 — An internal ECM driver fault or wiring back to the ECM can present as an injector circuit/open for cylinder 3.1234

How to diagnose

  1. Verify code P0203 is present and check for accompanying misfire codes P0300/P0303 and injector balance-rate data.
  2. Inspect the under-valve-cover injector harness and connector for oil, chafing, or corrosion; check continuity and resistance of the cylinder 3 injector control circuit from ECM to injector.
  3. Measure injector solenoid resistance for cylinder 3 and compare to spec and other cylinders; look for open or out-of-range reading.
  4. Check ECM connector and related grounds/power; confirm no wiring damage between ECM and valve cover pass-through.
  5. If electrical tests pass, evaluate injector mechanical health via balance rates and return flow; LB7 injectors are a known weak point for cracked bodies and return-flow/balance-rate drift.
  6. Clear codes and retest after repairs; monitor for return of P0203 and misfire behavior.

Specs and test values

CheckNormal readingWhen to measureSource
Injector balance ratesHealthy engines sit near ±2 mm³; values near ±4 mm³ or one cylinder far from the rest need a closer lookHot idle in Park, A/C offBD Diesel

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

Tools you’ll need

Rough cost and difficulty

Likely repairPartsShop laborWho usually does it
Injector wiring harness / under-valve-cover harness$50–$3002–5 hrsShop
Fuel injector (each)$250–$4502–4 hrsShop
ECM/PCM replacement (programmed)$600–$2,0001–3 hrsShop

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 circuit tests good but misfire/balance-rate issues persist, or if fuel-in-oil/white smoke is present, have a diesel specialist evaluate injector mechanical condition and high-pressure fuel system integrity. Do not perform step-by-step repair of high-pressure fuel lines or safety-critical systems without proper training and equipment.

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

  1. Curated DTC seed — LB7 Duramax (curated) ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — LB7 Duramax (curated) ↩︎ ↩︎ ↩︎

  3. Curated DTC seed — LB7 Duramax (curated) ↩︎ ↩︎ ↩︎

  4. Curated DTC seed — LB7 Duramax (curated) ↩︎ ↩︎ ↩︎