Honda Universal Transmission Cap Breather 21396-P20-000

The Honda Transmission Cap Breather 21396-P20-000 (also listed without hyphens as 21396P20000) is the small but critical vent cap that sits on top of your transmission case. It lets built-up pressure escape while blocking moisture, dirt, and road grime from working their way inside. Without it, or with a cracked or missing one, you get condensation buildup inside the transmission, contaminated fluid, and over time, accelerated wear on internal components. This is a genuine OEM Honda part, not an aftermarket substitute - same factory spec, same fit, same material as what came on your vehicle originally.

Here is what the cap breather actually does: as your transmission heats up during normal driving, it generates internal pressure. That pressure needs somewhere to go. The breather cap vents that pressure in a controlled way while the mesh or valve inside it filters out contaminants. If the cap is absent, cracked, or clogged, pressure has no clean exit. The result is either forced seepage past seals (leading to fluid leaks) or negative pressure on cool-down that pulls in whatever is sitting around the vent opening - usually moisture-laden air that degrades fluid quality and promotes internal corrosion.

This part fits a wide range of Honda and Acura manual and automatic transmissions. It is the same cap used across the Civic, Accord, CR-V, Odyssey, Pilot, Element, Fit, and their Acura counterparts including the Integra, RSX, TSX, TL, and MDX, among others. Check your fitment data below or verify against your existing cap if you are replacing one that is present but damaged.

Because this is a genuine OEM Honda part, it drops in without any modification. No trimming, no adapters, no hoping the aftermarket tolerances are close enough. The part number stamped on the cap will match your factory service manual spec - 21396-P20-000 or equivalently 21396P20000 depending on where you sourced the reference. Both formats refer to the same physical part.

If you are doing a transmission service, a clutch job, or any work that involves removing components near the top of the case, this is a smart add-on to the order. They are inexpensive, easy to lose during a job, and a missing one will quietly cause problems that are expensive to trace later.