P201C Code - Intake Manifold Runner Control Actuator Supply Voltage Low Bank 2

On a 2015 Chevy 6.6L LML Duramax, the throttle body functions differently from traditional gasoline engines. Since it's a diesel engine, the throttle body is not responsible for controlling the amount of air going into the engine like in a gasoline engine, where it opens and closes based on throttle input. Instead, in a diesel, air is generally always allowed to flow into the engine, and fuel injection is what controls power output.

How the Throttle Body Works on the Duramax LML:

• Always Open, Except in Specific Cases: In normal operation, the throttle body (often referred to as an air valve in diesel applications) is usually open most of the time. It will only close under specific conditions, such as:
  • During engine shutdown to help facilitate a smooth shutdown and prevent engine "shudder."
  • Regeneration cycles for the Diesel Particulate Filter (DPF).
  • Exhaust Gas Recirculation (EGR) operations, where it may partially close to control the amount of exhaust gas re-entering the intake.

So, it’s normal for your throttle body to seem like it’s open most of the time. However, it shouldn’t be fully closed when the engine is running. The only time it would move toward closing is under the above circumstances (shutdown, DPF regen, or EGR operation).

Cleaning the Throttle Body and Intake System:

Cleaning the throttle body and the intake runners on the Duramax can help with performance, especially if you're seeing oil residue, soot, or carbon buildup from the EGR system.

Tips While You're Cleaning:

1. Ensure EGR Components Are Clean:

   • Since you’ve already cleaned the throttle body, it’s also a good idea to clean the EGR valve and cooler if possible. EGR systems in diesels can clog with soot over time, leading to performance issues and potential codes.

2. Inspect Intake Runner for Oil and Carbon:

   • Cleaning the intake runners can be beneficial if there’s a significant buildup of oil and soot. This is common in diesel engines that have recirculated exhaust gases mixing with oily crankcase vapors from the PCV system.
   • Use a diesel-specific intake cleaner that’s safe for aluminum components, and make sure no debris enters the intake ports.

3. Check PCV System for Oil Residue:

   • If you’re seeing excessive oil residue in the intake, inspect the Positive Crankcase Ventilation (PCV) system. An excessive amount of oil can make its way into the intake system, particularly in higher mileage engines. You may want to look into an oil catch can or separator to reduce this if it's a persistent issue.

4. Throttle Body Relearn Procedure:

   • After cleaning, the throttle body might require a relearn procedure to properly align its operation with the ECU. This can often be done with a scan tool, but on some vehicles, it happens automatically after driving a certain number of miles.

Final Advice:

• While you have things out of the way, it’s a good idea to inspect your turbocharger piping and intercooler for any signs of oil buildup or leaks. Diesel engines, especially turbocharged ones like the Duramax, can experience oil leaks in the charge air system, which can contribute to intake contamination.

• Check all sensors, such as the Mass Air Flow (MAF) sensor and Manifold Absolute Pressure (MAP) sensor, to ensure they’re clean and functioning properly.

Once everything is cleaned and put back together, monitor for any codes or drivability issues. If your throttle body is consistently staying open, and you suspect it’s malfunctioning, further diagnostics might be required with a scan tool to check if it’s operating correctly under different conditions.