2001 - 2004 GMC Sierra, Chevrolet Silverado
The Duramax diesel was introduced for the 2001 model year. The first generation Duramax was dubbed the LB7. Horsepower and torque peaked for the LB7 in 2004, generating 300 hp and 520 lb-ft. The lack of emissions equipment contributed to relatively high fuel economy figures for these model years. Reliability overall has not been a huge concern, but GM was forced to recall the injectors for these engines due to a considerable number of problems. This is not a serious concern for prospective buyers today since most owners followed GM's protocol and had them replaced under warranty (though this is worth asking the seller). A manual transmission was optional for these model years, though the 5 speed Allison 1000 automatic was far more popular and it's quite rare to come across a stick shift.
2004 - 2006 GMC Sierra, Chevrolet Silverado
For 2004, the LLY was introduced alongside the LB7 before replacing it indefinitely. 2004 - 2006 models often suffered from overheating issues while towing. The aftermarket has responded with a variety of fixes, though buyers should be aware of the issues when searching. For 2006, the LLY underwent many changes, including a stronger cylinder head design, newly designed fuel injectors, a new VVT turbocharger, reduced engine noise, and performance was boosted to 310 hp and 605 lb-ft. For 2004 models, the 5 speed Allison automatic was available, while the Allison 1000 was upgraded to a 6 speed unit in 2005 (the 6 speed version being more desirable).
2006 - 2007 GMC Sierra, Chevrolet Silverado
The LBZ was introduced for 2006, eventually phasing out the LLY. With it came a huge leap in performance, offering 360 hp and 650 lb-ft. The 6 speed Allison 1000 automatic or a 6 speed manual transmission was available mated to the engine. The 2006-2007 model year pickups did not have any major reliability concerns and simply continued the progression of the Duramax engine family.
2007.5 - 2010 GMC Sierra, Chevrolet Silverado
Midway through the 2007 model year, GM launched the LMM Duramax. The goal of this engine was to meet strict emissions regulations, while offering up a small performance increase as collateral. The LMM produced 365 hp and 660 lb-ft. Fuel economy suffered severely, caused by the adoption of a diesel particulate filter and active regeneration. To be fair, both Dodge and Ford trucks experienced similar complaints regarding poor fuel economy for the same reasons. Beyond that, the trucks are reliable and the DPFs are not typically prone to clogging issues.
2011 - Current GMC Sierra, Chevrolet Silverado
The Duramax became more complicated for 2011 to meet even stricter emissions. There seem to be no major reliability concerns. The trucks employ SCR (selective catalytic reduction) technology with the use of DEF (urea injection) in order to combat NOx emissions. A 5 gallon DEF tank is required to remain filled, though the use of exhaust after-treatments actually helped the 2011+ model trucks achieve an 11% + increase in fuel economy versus the 2007.5 - 2010 models. These trucks transmit 397 hp and 765 lb-ft of torque through a stronger-than-ever Allison 1000 6 speed automatic (a manual transmission option was retired).
• If the price is right, look past the overheating issues of the LLY, but be prepared to invest in upgrades if you plan on doing a lot of towing.
• The 2001 - 2004 model years are great on fuel, have the least emissions components, and shouldn't give you any problems if the injectors were replaced under warranty.
• Avoid the 2007.5 - 2010 model year trucks if you are concerned with fuel economy. The DPF can be removed, but this technically renders the truck illegal for highway use. If you come upon an offer you can't refuse, there are some options for improving fuel economy.
• 2001 - 2007 model years are great if you're looking for a truck to tinker with. There is lots of aftermarket support, the engines are relatively simple, and there are many around.
• A truck that has been well maintained is worth significantly more than one that has been neglected - service records should be a big bonus for prospective buyers.