Well, since you asked... I made a file which I've probably posted before with many of the known LLY problems... Here goes:
I usually warn that while these are GREAT trucks, they have a LOT of potentially big problems. If you do your own work, then they can be manageable. That said, I took mine on a 13k mile trip in 2014, several 3300 mile trips, and my truck was flawless (until 2017 when injector #2 connector went bad). But, I’ve had it since new, have never overheated (and added an aux radiator to make sure it doesn’t), have never ran a tuner (which can also lead to transmission problems), and I live in southern California so there is NO rust under the truck. At 107k miles, I replaced the water pump, serpentine belt (and idlers and tensioner) . . . not because there was a problem, but because I don’t want to wait for the problem to show up when I’m on a trip.
Unless you need the diesel to tow or haul a load, then it’s hard to justify the diesel. If you drive 60 miles a day and want better mileage, then a small economy car (I’m partial to Japanese makes for this) is the way to go. That said, here’s a partial list of issues with the 2004.5/2005 LLY Duramax trucks . . .
Short or Long bed? Standard, Extended, or Crew cab? Short bed has a 26 gal. tank; long has a 34 gal. tank. Crew cab with long bed has a 2-piece driveshaft and the carrier bearing/mounts can be a problem.
Wiring harness – may rub through where it goes over the alternator (near FICM) bracket on front of motor, passenger side. It needs to be protected where it rubs, usually with a piece of heater hose. Other potential chafing spots exist, and some connectors can have contact problems.
Injector connectors - #2 and #7 are common culprits, but it can happen on any cylinder. They can be replaced or “ice picked” to make sure they make a good connection. I finally had #2 go bad while on a trip 2 years ago. I bought cheap connectors on Amazon (about $16 for 4) and soldered in 2-way connectors so I can easily replace a connector on the road with a new one. My cheap connectors (just on #2 and #7) have worked great for 2 years now.
Overheating – worse on long, hot climbs (especially in the west). Some trucks are worse than others. An auxiliary radiator kit is available.
Head gaskets – mostly a problem if it’s been overheated or had a tuner. Expensive to fix ($6K?) and should include replacing torque-to-yield bolts with ARP studs.
4x4? – If so, magnesium transfer case can get a pinhole from “pump rub”, leak out the ATF, and ruin the transfer case.
Water pump – failure-wise, no worse than any other vehicle, i.e., will probably have a problem sometime after 100k miles. More difficult to change than most (must remove harmonic balancer on crankshaft; o-ring on tube from pump to thermostat housing can be tricky).
Serpentine belt idlers – usually make noise or bearing gets sloppy before going bad. Not expensive or hard to change (probably should do tensioner, too, along with serpentine belt). This is no worse than any other vehicle on the road.
Fuel filter head – pump may leak, or need new o-rings for pump to work, and some have cracked. BTW, fuel filters are expensive and can be tricky to replace. Some have had problems with certain brands/types of filters, especially the ones with plastic housings. I use Racor.
Some problems will be worse if the truck has resided in the rust belt, e.g., rusting brake lines, abnormal brake wear, speed sensors affecting ABS (truck could be hard to stop).
Automatic Climate control system (if so equipped) is, um, maybe too smart and does stupid things when it thinks an actuator door is “out of range”. It may be reset by turning off/on (mine does, and has since new), or it may require changing an actuator door which is a difficult task.
Allison 5-spd automatic – a great transmission, but can have failures if engine has power tune added. Also, it originally used Dexron-III ATF which is no longer available (or hard to find) with that spec. GM replaced with Dexron –VI ATF but there may be a seal problem if used. If the truck has been serviced at a dealer, it will almost certainly have Dexron-VI. It should be replaced with a “synthetic Dex-III” (e.g., Mobil 1 Fully Synthetic ATF “for 2005 and older GM”) or TES-295 (Castrol Transynd, Mobil Delvac ATF, etc.).
Lift Pump and auxiliary fuel filtering – highly recommended!!
Cluster – stepper motors go bad and needles go crazy. While often the speedometer, they are all the same so any of them go. Both speedometer and fuel gauge went on mine, probably 10 years ago.
2004.5 Silverado 3500 SRW 4x4, D/A (LLY), Ext. Cab, Long Bed
engineer837 aux rad, TxC CAI, LBZ MP, Scangauge II, Nicktane aux filter head, KD lift pump, 2008 PS pump, 2011 hydroboost, Frederico Swaybars, Rancho 9000XL, Torklift StableLoads, BFG AT/KO2, Lance 835 Camper often in the bed.