P0490 on the LMM Duramax is an emissions code indicating a high-voltage condition in the EGR control circuit. The 2007.5-2010 LMM was the first Duramax with a diesel particulate filter, and its EGR system works closely with that emissions hardware. An EGR control fault can cause power loss and interrupt regeneration, so it is worth diagnosing carefully.
What P0490 means
P0490 is "Exhaust Gas Recirculation Control Circuit High." The ECM operates the EGR valve and monitors its control circuit. When that circuit reads a higher voltage than expected, or shows an open or a short to power, the ECM sets P0490. On the LMM the EGR valve is electrically actuated with position feedback, so the fault can come from the valve, an open in the wiring, or a short to a voltage source.
Common causes
- Open circuit in the EGR control wiring
- Short to power (battery voltage) in the control circuit
- Failed EGR valve with an open internal winding
- Corroded or disconnected EGR valve connector
- Carbon-seized valve drawing abnormal current
- ECM driver fault (rare)
Symptoms
- Check engine light with P0490 stored
- Reduced power or limp mode
- Rough idle and hesitation
- Failed or incomplete DPF regeneration
- Increased soot output
- Additional EGR or emissions codes
How to diagnose it
Read all stored codes and freeze-frame data. Inspect the EGR valve connector and control wiring for opens, chafing, and shorts to a voltage source. Check that the connector is fully seated and not corroded. Command the valve open and closed with a scan tool while monitoring position feedback and current draw. An open circuit will usually show as no movement and a high control-side reading. Also check for excessive carbon that can jam the valve and cause erratic electrical behavior. Verify the harness routing away from hot exhaust surfaces.
How to fix it
Repair open or shorted control wiring and replace damaged connectors. If the EGR valve is internally open or seized with carbon, replace it; on the soot-heavy LMM, a badly fouled valve is often best replaced. Confirm all connections and grounds are clean. Clear the code, command the valve through its range to verify smooth operation, and road test to confirm regeneration completes and P0490 does not return.
FAQ
Does P0490 affect regeneration?
Yes. A faulty EGR circuit can prevent the LMM from completing a DPF regeneration, which leads to soot buildup and further codes.
What is the difference between P0489 and P0490?
P0489 indicates a low control-circuit voltage (often a short to ground), while P0490 indicates a high or open condition (often an open circuit or short to power).
Can carbon buildup set P0490?
Indirectly, yes. A carbon-seized valve can draw abnormal current and cause erratic feedback. Inspect for soot restriction along with the wiring.
Need parts to fix this code? The team at sales@coredieselparts.com can match the correct sensor, injector, or emissions component to your engine and model year.
Need the part?
We stock the parts these trucks need. Shop all LMM Duramax parts →