Hello everyone,

I’m looking for some insight from Honda techs or experienced owners.

Vehicle details:
• 2005 Honda Civic LX
• Sedan, 1.7L D17, Automatic
• Mileage: 165,216 miles

Symptoms:
• Transmission slips and does not lock into subsequent gears
• Operates normally when cold
• Once at operating temperature, slipping begins—worse at higher speeds
• No harsh engagement or bang shifts, just flare/slip
• No MIL currently

Work already performed:
• Drained and refilled ATF (used Castrol Transmax ATF/CVT Universal)
• Cleaned transmission solenoids (linear and shift solenoids)
• Fluid level verified hot and cold

Observations:
• Cold operation feels normal (good engagement, shifts OK)
• Heat appears to worsen the issue significantly
• Problem existed before fluid change and persists after
• No external leaks

1. Could this be torque converter lock-up failure or linear solenoid degradation under heat?
2. Are clutch packs (2nd/3rd) known to fail on high-mileage D17 automatics in this way?
3. Would switching to Honda ATF-Z1 / DW-1 potentially change behavior, or is damage likely already present?