Last winter, my buddy’s 2015 Silverado Duramax threw a P0128 code on a freezing morning haul. The truck crawled forever to warm up, heater blew lukewarm air, and fuel mileage tanked hard. Turns out both thermostats stuck open, super common on these LML engines around 100k miles. We swapped them in his driveway one afternoon, truck ran perfect after, hitting 210 steady. Saved him over a grand versus shop prices. Now dive into how you can nail this job yourself and get that beast running hot again.
Key Takeaways:
Park on level ground and let the engine cool completely before starting. Gather genuine ACDelco thermostats, fresh Dexcool coolant, basic sockets, and a drain pan. Drain coolant from the radiator petcock first to minimize mess. Remove the upper radiator hose and nearby brackets for clear access to the housing. Note positions of the two thermostats since one is primary and the other secondary with bleeder. Clean the housing surfaces thoroughly and install new seals. Torque bolts evenly to spec and refill coolant slowly while burping air. Run the engine to operating temp, top off, and check for leaks.
Gather Your Tools and Parts
First things first, round up everything before tearing into the truck. You’ll need a set of metric sockets including 10mm and 13mm, extensions, ratchet, pliers for hose clamps, torque wrench, drain pan, shop rags, and maybe a shop vac for spills. Grab two new thermostats, go with ACDelco for perfect fit, plus new o-ring seals that usually come in kits. Pick up three gallons of premixed Dexcool because you’ll lose some coolant no matter how careful.
Next, think about extras that make life better. A coolant funnel helps with filling later, and zip ties or new clamps if old ones look beat. Lay out cardboard under the front to catch drips, these trucks spill orange gold everywhere. Double check you have gloves, coolant is nasty stuff on skin.
Then, scout the engine bay. The thermostat housing sits right up front under the upper radiator hose, sandwiched between alternator and AC compressor. Pop the hood, remove any covers blocking view, and get familiar. Takes maybe 30 minutes just prepping, but rushing leads to broken clips or lost bolts.
- Socket set with 10mm, 13mm
- Torque wrench
- New ACDelco thermostats and seals
- 3 gallons Dexcool
- Drain pan and rags
Prep and Drain the Coolant
Start with the truck cold, no pressure surprises. Open the coolant reservoir cap slow to release any built up air. Place a big drain pan under the radiator petcock, usually passenger side bottom. Twist it open and let coolant flow out, catches most without huge mess. While draining, loosen the upper radiator hose clamp at housing end, twist hose off gentle to avoid cracking.
As coolant drops, stuff a rag in oil fill neck nearby, keeps drips out of engine. Remove any brackets or clips holding wires out way, label if worried about mix up. Disconnect alternator or move stuff for elbow room, LML bays tight but doable.
Once drained, close petcock and mop spills. Save old coolant if clean for reuse topping off, or dispose proper. This step avoids flooding bay when pulling housing later.
- Drain radiator fully
- Remove upper hose partially
- Protect oil fill with rag
- Clear access brackets
Access the Thermostat Housing
With coolant low, tackle housing. Locate four bolts holding it, mix of 10mm or 13mm heads. Use extensions to reach, loosen evenly cross pattern so no warp. Pull bolts out careful, set aside clean.
Next, wiggle housing off. Might stick from old gasket, pry gentle with plastic scraper, never metal to avoid gouge. As it lifts, more coolant dumps, have pan ready. Note black crossover pipe, some loosen it for space but not always need.
Then, lift housing clear, reveals two thermostats sitting in block. Front one symmetrical, rear offset with bleeder screw. Mark positions or snap photo, must go back exact.
- Loosen four housing bolts evenly
- Pry housing carefully
- Catch remaining coolant
- Note thermostat orientations
Remove the Old Thermostats
Old ones pop out easy usually. Grab jiggle pin or edge, pull straight up. If stuck, twist gentle or use pliers on base, no damage new seats. Inspect for corrosion or debris, common why they fail.
Clean block seats thorough. Use plastic scraper and rags, get every bit old gasket or sealant. Shine flashlight in, ensure no junk falls deeper. Blow out with air if have compressor.
Compare old to new, front opens around 185, rear 180 with offset and bleeder. Seals go in grooves first, then thermostats drop in, spring down.
- Pull thermostats noting positions
- Clean seats completely
- Inspect for damage
- Compare to new parts
Install the New Thermostats
Drop new seals into block grooves if separate, or most kits have on thermostats. Place rear one first, offset toward firewall, bleeder up. Then front symmetrical one forward. Wiggle ensure seated flat, no tilt.
Apply thin bead RTV sealant on housing gasket surface if needed, some use none with o-rings. Avoid excess squeezing into coolant passages, clogs bad news.
Position housing back, align bolts holes. Hand start all four bolts before snugging, prevents cross thread.
- Install seals and thermostats correctly
- Apply light sealant if required
- Align housing precisely
- Hand thread bolts
Reassemble and Torque Everything
Snug bolts cross pattern, then torque to around 18 ft-lbs, check exact for LML but that works most. Reconnect upper radiator hose, tighten clamp solid. Put back any brackets, wires, or covers removed earlier.
Reinstall oil fill tube if pulled, easy few bolts. Double check nothing left loose in bay.
Finally, close drains and prep for fill. This seals everything leak free, no comebacks.
- Torque housing bolts evenly
- Reattach hose and brackets
- Check all connections
- Prepare for coolant refill
Final Thoughts
Swapping thermostats on your 2015 Duramax brings back that proper heat, better mileage, and no more codes bugging you. Truck hits 210 quick, heater blasts hot, and engine happy longer. Do it once right, enjoy years reliable hauling. Grab a cold one after, admire that beast purring perfect again.
| Tool/Part | Description | Why You Need It | Approximate Cost |
|---|---|---|---|
| ACDelco Thermostat Kit | Includes both thermostats and seals for 2011-2016 LML | Exact OEM fit, prevents leaks | $80-120 |
| Dexcool Coolant | Premixed 50/50, 3 gallons | Refill lost coolant, maintain protection | $20 each |
| Socket Set | 10mm, 13mm deep sockets, extensions | Remove housing bolts and brackets | $50+ if buying |
| Torque Wrench | 1/2 or 3/8 drive, ft-lb | Proper bolt torque, no stripping | $40-100 |
| Drain Pan | Large capacity, 10+ quarts | Catch coolant spills safely | $15 |
| RTV Sealant | Small tube black or clear | Extra seal on housing if needed | $10 |
| Pliers/Clamp Tool | Spring clamp or hose pliers | Remove/reinstall radiator hose | $20 |
| Shop Rags/Gloves | Pack of rags, nitrile gloves | Clean up and protect hands | $10 |
Frequently Asked Questions (FAQs)
Is it hard to change the thermostat on a 2015 Duramax?
No way, most owners knock it out in a few hours with basic tools. The housing sits right up top, just drain coolant first and clear some brackets. Biggest hassle is tight space, but extensions help reach bolts. Follow positions exact for the two stats, torque even, and burp air good after. Plenty guys do it driveway style, saves huge versus dealer.
Can I drive my 2015 Duramax with a bad thermostat?
You can limp around short trips, but not smart long term. Stuck open means engine stays cold, hurts fuel economy bad, weak heater in winter, and risks more codes or regen issues. Over time, poor lubrication from cold oil wears parts faster. Fix soon, especially towing or cold weather coming.
Do I need to replace both thermostats at once on my Duramax?
Yeah, always swap both together. These trucks run dual setup, one primary, one secondary. If one fails, other close behind from same age and crud. Kits sell both anyway cheap, and you’re already in there draining coolant. Skipping one risks comeback job soon.
Is the P0128 code always the thermostat on 2015 Duramax?
Most times yes, that code means coolant temp too low too long. Stuck open thermostat classic culprit on LMLs. But check sensor or wiring rare cases. Clear code after swap, drive normal, if stays gone you’re golden. Scanner shows actual temps around 180-190 proper.
Can I use aftermarket thermostats in my 2015 Duramax?
You can, but stick OEM ACDelco best. Aftermarkets sometimes open wrong temp or fail quick, cause overheating or cold running again. Genuine ones match exact, with right bleeder and offset. Worth extra bucks for peace mind hauling heavy.
Do I have to bleed air after changing thermostat on Duramax?
Absolutely, air pockets cause hot spots or gauge flips. Fill slow with funnel in reservoir, run engine heat on high, squeeze hoses burp. Open bleeder on rear stat if have, let bubbles out. Top off cold, repeat drive till stable. Takes patience but prevents headaches.
Is torque spec important for thermostat housing bolts?
Big time, overtighten cracks housing, undertighten leaks coolant everywhere. Aim 18 ft-lbs cross pattern, snug even. Use torque wrench, cheap insurance. Clean threads first, no sealant on bolts unless spec says.
Can cold weather cause thermostat failure in Duramax?
Cold amps it up, stuck open shows worse winter with slow warmup and bad heat. But failure from age, corrosion inside usually. Many hit issues around 100k miles regardless season. Swap before deep freeze hits.
