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Duramax LLY-LBZ Emissions


The 2004.5 and 2005 Duramax LLY engines are physically identical. All 2006 LLY and LBZ along with the 2007 “Classic” body style trucks are identical within their own special group. For simplicity sake from here on out I will refer to these engines as two separate groups: LLY and LBZ. Remember that the 2006 LLY groups with the LBZ. All engines utilize a cooled EGR (exhaust gas recirculation) system, catalytic converter a.k.a. soot trap, and a closed crankcase ventilation (a.k.a. PCV) system. The key difference between these vehicles is the emissions group. They can be identified by the RPO code label inside the glove box. The group listings are as follows:

  • FE9 is Federal Emissions
  • NE1 is Northeast Emissions this covers NY, VT, MA, or ME
  • YF5 is California Emissions
  • VCL is a CFF (Clean Fuel Fleet) designation required to qualify for certain tax incentives

Operational Differences:

The key operational difference among these groups is how emission related DTC’s are processed. In short, the FE9 trucks treat EGR system DTC’s as “non emissions” and simply stores a record of the fault with no corrective action. The NE1, YF5, and VCL trucks typically require 2 trips of the code and then trigger an SES light and some form of reduced performance. The setting of codes and SES lights can be corrected and will be covered in a later paragraph.


Before I go further, I feel that it is important to note that not all vehicles react or respond the same to alterations, modifications, or conditions. This is true for both emissions and performance modifications. Some trucks even been known to set DTC’s (Diagnostic Trouble Codes) and trip the SES (Service Engine Soon) light in factory stock condition. I should also note that tampering with emission control devices is against the law regardless of whether your State or Municipality performs periodic emissions checks or not. That being said, I feel that there are far more negatives rather than positives to having these systems active on a diesel engine. Catalytic converters can cause excessive heat loading in the engine, turbo and exhaust system. EGR systems can load or potentially even plug the intake tract with soot. Even when operating under normal conditions EGR systems redirect abrasive soot and carbon back into the combustion chamber and crankcase. Closed crankcase ventilation systems draw oil vapors in from the crankcase via the turbo and intercooler system. These oil vapors coat the intake tract decreasing efficiency. Combine the oil and exhaust soot and you get a gooey tar like substance that can build up and severely restrict airflow. Both EGR and PVC systems replace a percentage of clean, oxygen laden air with a lesser quality air, oil vapor and exhaust gas mix that has less performance potential.


This is generally one of the first areas that people begin to modify, and it just makes good sense. Relieving exhaust back pressure enhances flow through the engine and can lower combustion and exhaust gas and temperatures. My opinion on the matter is that the best way to go about improving the exhaust is to go to what is called a turbo back 4” exhaust system. Altering parts of the OE system like cat back, muffler only, or straight piping with the OE size pipe will only provide marginal gains. Adding a full 4” system has produced Dyno Proven results. https://www.kennedydiesel.com/photogal/pages/4-5exhaust_lly.htm The downside to altering the exhaust is the potential to set emissions related DTC’s. The common codes set are P0401 (Exhaust Gas Recirculation Flow Insufficient), and P0404 (Exhaust Gas Recirculation Open Performance). These codes (and SES/reduced power on NE1, YF5, and VCL vehicles) are tripped because there is insufficient back pressure to push the gasses from exhaust to the intake as a result of the free flowing exhaust system. This can occur with cat back OR full exhaust systems.

EGR System:

There are two common methods of defeating this system. The first is to simply unplug the EGR electrical connector. The second is to install a plate to physically obstruct the flow of EGR gasses. Both of these alterations will typically result in the P0401, P0404, plus P1404 (Exhaust Gas Recirculation Closed Position Performance) and P0405 (EGR Position Sensor Circuit Low voltage) will typically set with the EGR connector unplugged.

Why do the Codes set?

When the ECM activates the EGR valve it is looking for a certain response on the various channels associated with the system. Unplugging the valve’s electrical connector also eliminates the electrical voltage feedback from the valve. When the EGR system operates, the ECM is essentially looking for a change in the MAF (mass air flow) signal to verify that a part of the engine’s combustion “air” is coming from another source than the air box. As an example, an idling Duramax will flow approximately 37 g/sec of mass air with the EGR off. When the EGR activates (say 10% duty cycle) the resultant change in flow will show approximately 27 g/sec of mass air. This is because the engine is now receiving part of its air flow from the recirculation of exhaust.

Dealing with the Codes:

There are really two basic ways to handle this. The cheap way and the permanent way.

The cheap way: The LLY EGR Blocker Package or or LBZ EGR Blocker Package are kits that includes a blocker plate to eliminate the flow of EGR gasses into the engine, and the “Fingers Stick” device that can be used to emulate the altered MAF signal during periods where the ECM is calling for EGR operation. This setup when installed properly can be highly effective and reliable, BUT if the EGR motor (prone to fail) quits it will need to be replaced as the Fingers Stick requires an accurate and functional position sensor in order to make it’s commands.

The permanent way: My Kennedy Custom Tuning has been built using my own personal trucks as test subjects. It can be set up to modify the ECM’s diagnostic routine, eliminate specific DTCs from the program altogether, as well as modifying or eliminating the actual operation of the EGR system eliminating the need to physically block the system. Clean, simple, effective, and foolproof.