My check engine light is on and the dealership has reset the computer but it continues to come back on. They have told me it is a computer glitch and Chevy is writing a new code to eliminate the problem. I have also been told by Chevy that it is an additive that is put in the diesel fuel that is causing the problem. Has anyone heard of this or had it happen to them?? Need help!

What modifications do you have, and what is the code?

04 - 05 LLY 6.6L Diesel EGR DTCs P0401 P0404 Under Investgation - kw idle rough #PIP3256A - (Dec 17, 2004)
04-05 LLY 6.6L Diesel EGR DTCs P0401 P0404 Under Investigation.
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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in the PI.

Condition/Concern:
Vehicle may be brought in for SES or MIL light on. Upon inspection of the vehicle, the technician may find EGR DTCs P0401 or P0404.

Recommendation/Instructions:
Complete the proper SI diagnostics for the DTCs found.
If normal SI diagnostics have been completed and were inconclusive, inspect for any exhaust, air intake, or wiring harness modifications made to the vehicle. Modifications may induce EGR codes.
Monitor the five volt reference to the EGR valve. A chafe to ground on a five volt reference circuit may pull the reference voltage low and induce EGR codes. Inspect and wiggle test the complete engine wiring harness. Inspect all terminal pin tensions at connector C108.
Please document as much of the following information from the freeze frame data and the customer description as possible:
• Engine run time:

• Engine temperature:

• MAF sensor G/S:

• MAF sensor Volts:

• EGR position sensor volts:

• EGR position sensor position %:

• EGR solenoid command:

• Desired turbo vane position %:

• Turbo vane position sensor %:

• Coolant Temperature:

• Vehicle speed:

These readings will be used for diagnostics and an engineering investigation. This PI will be updated with any new information when it becomes available.
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Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

Models:
(04 05 LLY 6.6 C2 C3 K2 K3 Chevrolet Silverado Kodiak GMC Topkick, Sierra)

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No modifications made. They said this happens occasionally when the exhaust has been modified. Same code as the above post P0401 and P0404

After experiencing what I thought was turbo bark on several occasions (under medium acceleration immdiately following the shift fro about 3000 rpm a slight cough is heard from the engine bay and a momentary loss of power is felt) I thought I would check the computer for codes, and sure enough I found the P0401 and P0404 codes. I have about 2200 miles on the truck, and no modifications. It even did it on the way to the dealer to drop off the truck.

Here is a quote from the Dealer work order:
Code scan PCM data to find codes P0401 and P0404 in history, one time, 1586 miles ago. No updates found for PCM add stanadyne treatment to fuel.

The service writer said that they had no other solution and that there was someone working there who had experienced the same thing, and that it had happened while the writer was riding in his truck.

I guess we build a history and they may do something???

Finger sticks....