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Cooling SystemQuestionCommunity Starter

2018 Honda Civic — a/C is cold on the highway but warm at stoplights

BrakeHauler786Community StarterMay 26, 20262018 Honda Civic SiNorth Jersey
2018 Honda Civic Si, about 156,000 miles. The A/C works well with road speed but gets noticeably warmer at long lights, then cools again almost immediately once moving. Does that point more toward airflow/fan behavior than refrigerant level? If I figure it out I will update the thread. What would you check next?
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5 replies

FlatSixTunerCommunity StarterCommunity StarterMay 26, 2026permalink
Ask what changed right before it started. Move a coil or plug on the 2018 Honda Civic if it's cylinder-specific and see whether the code follows. Save the big job until that test fails. Happy to compare notes if your setup is different. I'll poke at it this weekend.
DealBuyer603Community StarterCommunity StarterMay 26, 2026permalink
If it were in my driveway, swap the cheap/easy piece first on the 2018 Honda Civic and see if the fault moves. That's the branch I'd chase this week. Not gospel, but it saved me a headache. I'd rather be boring and correct. No warning lights, which is why I'm documenting it.
ApexTuner773Community StarterCommunity StarterMay 27, 2026permalink
I wouldn't throw parts yet. Get a real battery/charging test on the 2018 Honda Civic, not just resting voltage, before blaming a module. Save the big job until that test fails. Had a similar mess a couple years ago and that's what finally sorted it. Your commute and mine probably aren't the same, but the test still applies. I'll poke at it this weekend.
GarageMechanicish119Community StarterCommunity StarterMay 27, 2026permalink
Agree — and I'd add this: Capture voltage while the 2018 Honda Civic is actually failing — after it restarts you've lost the clue. Save the big job until that test fails. On “a/c is cold on the highway but,” that was the fork that mattered for me. That's the whole play. This is the filter I use when the thread starts repeating itself. Next check is the cheap one I can do in a driveway.
BrakeHauler786Community StarterCommunity StarterMay 27, 2026permalink
OP update: scanned the 2018 Honda Civic again and still only have the one pending code, no new lights. I'm in no rush to be a hero on it.
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