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Cooling SystemDiscussionCommunity Starter

Need advice: Coolant level slowly drops with no puddle

TrailTech93Community StarterMay 21, 20262023 GMC Sierra 1500 AT4DMV / Maryland
2023 GMC Sierra 1500 AT4, about 4,800 miles. Reservoir moves from full to low over a few weeks. No puddle, no obvious hose leak, and the oil looks normal. Is a pressure test the next logical step or is there something simpler I should inspect first? If I fix it I'll come back and update the thread.
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6 replies

QuietBuilder89Community StarterCommunity StarterMay 22, 2026permalink
Log the conditions before you spend. Loaded-suspension checks on the 2023 GMC Sierra 1500 catch play that a wheels-hanging inspection misses. Save the big job until that test fails. On “coolant level slowly drops with no puddle,” that was the fork that mattered for me. That's the whole play. Wrote this up on my lunch break.
TurboBuyer66Community StarterCommunity StarterMay 23, 2026permalink
That's a good start, but I wouldn't stop there. Swap the cheap/easy piece first on the 2023 GMC Sierra 1500 and see if the fault moves. If the cheap test changes it, stop and reassess. This is from a 2023 GMC Sierra 1500 in DMV / Maryland with 4,800 miles. The universe charges extra for guessing. If this is nothing, great. I want that in data.
TrackDaily71Community StarterCommunity StarterMay 22, 2026permalink
Start with the test that can prove you wrong. Move a coil or plug on the 2023 GMC Sierra 1500 if it's cylinder-specific and see whether the code follows. Save the big job until that test fails. If they can't show the line item, I assume it's hiding. Worth doing even if the car feels fine right now. I'd rather pay for diagnosis than a second guess-repair.
TorqueWrench20Community StarterCommunity StarterMay 22, 2026permalink
If it were in my driveway, if a shop can't duplicate it on the 2023 GMC Sierra 1500, bring a video plus freeze-frame data, not just a story. That's the branch I'd chase this week. Parts cannon is a personality type I try to avoid. I'll update if anything new shows up.
MileageHunterCommunity StarterCommunity StarterMay 23, 2026permalink
Yes, with one caveat: If a shop can't duplicate it on the 2023 GMC Sierra 1500, bring a video plus freeze-frame data, not just a story. Save the big job until that test fails. On “coolant level slowly drops with no puddle,” that was the fork that mattered for me. If they can't show the line item, I assume it's hiding. If your situation is the opposite, ignore me. Might be overthinking it, but I'd rather overthink than overspend.
NorthFan11Community StarterCommunity StarterMay 23, 2026permalink
Ask what changed right before it started. Get a real battery/charging test on the 2023 GMC Sierra 1500, not just resting voltage, before blaming a module. If the cheap test changes it, stop and reassess. I'd want a dollar figure or a measured spec, not 'should be fine.' I'm in no rush to be a hero on it.
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