Rough Idle and White Smoke on Cold Start — 6.7 Cummins (SCR / DEF)

A concise diagnostic reference for 6.7 Cummins (SCR/DEF) 2013–present showing rough idle and white smoke on cold start. Covers intake air grid heater…

Updated October 15, 2026

What’s going on

On the 6.7 Cummins (SCR/DEF) 2013–present, rough idle with white smoke on a cold start usually means incomplete combustion while the engine is cold. The platform uses an intake-air grid heater (no glow plugs) for cold start aid, and white smoke on cold start is commonly linked to a non-functioning grid heater, an injector that is not firing cleanly, or the engine running too cold.¹

Most likely causes

1. Intake air grid heater not heating (P2609) — The 6.7 uses an intake-air grid heater, not glow plugs. A failed heater element, bad relay, corroded heavy cables, or low battery voltage during heater operation prevents intake air warming, leading to rough idle and white smoke on cold start. This condition commonly sets P2609.¹²
2. Injector not firing cleanly (P0263) — A leaking or slow-to-respond common-rail solenoid injector can cause a cold misfire and white smoke until combustion stabilizes. On this platform injector electrical faults are in the P0263 family.¹
3. Engine running too cold (P0128) — A stuck-open mechanical wax thermostat or low coolant temperature keeps the engine cold, prolonging white smoke and rough idle on start. This commonly sets P0128.¹

How to narrow it down

1. Read stored trouble codes first; note P2609, P0263, P0128 if present.
2. Check intake air grid heater operation on a cold soak: verify relay command, heavy cable integrity, element resistance, and battery voltage while the heater is active.
3. Perform a cold-start injector balance / contribution test and inspect injector return flow for a leaking injector; correlate with P0263 if set.
4. Verify engine coolant temperature rise from cold start and confirm thermostat function; compare actual ECT to expected warm-up curve for P0128.

Specs and test values

Parts and tools you may need

• Digital multimeter
• Scan tool with live data
• Infrared thermometer
• Injector balance test adapter

Tools you’ll need

• digital multimeter — Measures grid heater element resistance and relay command voltage to confirm the intake air heater is energizing during a cold soak.
• OBD-II scan tool — Reads P2609, P0263, and P0128 and provides live ECT and injector balance data for cold-start diagnostics.
• infrared thermometer — Checks intake manifold temperature rise after grid heater activation and verifies engine warm-up rate for thermostat assessment.
• fuel injector test kit — Performs cold-start injector contribution/balance and return-flow tests to identify a leaking or non-firing injector causing white smoke.

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 high-pressure fuel injection work is required, have a qualified diesel technician perform diagnosis and repair. Do not attempt high-pressure fuel system service without proper training and safety 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.

Related trouble codes

• P2609 — Intake Air Heater System Performance
• P0263 — Cylinder 1 Contribution/Balance Fault
• P0128 — Coolant Thermostat Below Regulating Temperature

Sources