For most vehicles made in the last 30 years, you should check the automatic transmission fluid with the engine running, the transmission in Park, and the engine at its normal operating temperature. This is the standard procedure because the fluid circulates through the transmission when the engine is on, providing an accurate reading on the dipstick. Checking it with the engine off can result in a falsely high reading, leading you to believe there's enough fluid when there actually isn't, which can cause serious transmission damage.
The process is straightforward but must be done correctly. First, park your car on a level surface and engage the parking brake. Start the engine and let it idle. Pull the transmission dipstick (usually located near the back of the engine bay, with a red or yellow handle), wipe it clean with a lint-free rag, reinsert it fully, and then pull it out again to get the true reading. The fluid level should be between the "Full" and "Add" marks on the dipstick.
The fluid's condition is just as important as its level. Healthy fluid is typically a translucent red color and has a slightly sweet smell. If it appears dark brown or black, has a burnt odor, or contains visible metal particles, it indicates internal wear and the need for a fluid change or professional inspection. Always consult your owner's manual for the exact procedure, as some modern vehicles, particularly those with sealed transmissions or certain manual transmissions, may not have a user-serviceable dipstick at all.
| Transmission Type | Common Checking Procedure | Key Considerations |
|---|
| Automatic (Most Cars) | Engine ON, Transmission in Park, Warm | Ensures fluid is circulated and expanded for an accurate level. |
| Older Manual | Engine OFF | Fluid is not pressurized; level is static in the gearbox. |
| CVT (Continuously Variable) | Engine ON, Specific position (e.g., Neutral) | Often requires a precise procedure; consult the manual. |
| Modern Sealed Units | Professional scan tool required | No dipstick; level is checked via an overflow plug by a technician. |
| Dual-Clutch (DSG/DCT) | Often requires a specific temperature range | A complex procedure best left to a professional mechanic. |
