What to Do Before Changing Transmission Fluid

Before changing transmission fluid, verify the correct fluid and service method for your specific transmission, inspect for problems, gather the right parts and tools, plan the fill-level procedure at the proper temperature, and address any leaks or fault codes first. This preparation helps prevent damage, ensures accurate fluid level, and avoids voiding warranties. Below is a detailed, step-by-step guide to do it right.

Confirm Whether and How the Service Should Be Done

Start by confirming the manufacturer’s guidance and any special procedures. Modern automatic, CVT, and dual‑clutch transmissions often require precise fluids, temperatures, and fill levels—and some are “sealed” with specific service intervals and methods.

• Read the owner’s manual and factory service information for intervals, approved fluid (e.g., Toyota WS, ZF Lifeguard 8/9, Ford/GM ULV, Nissan NS-3 for CVT, Honda HCF-2), capacity, and method (drain‑and‑fill, pan service, or exchange).
• Identify the transmission type: traditional automatic, CVT, dual‑clutch (DCT/DSG), or manual—each has distinct procedures and fluids.
• Check for TSBs or recalls about shift quality, software updates, or revised fill specs or gaskets.
• Review warranty/extended coverage; using non‑approved fluid or off‑spec procedures can void coverage.
• Note fill/level requirements: many units must be checked at a specific fluid temperature via a level plug or scan‑tool guided “service mode.”
• Gather torque specs, fill/level plug locations, pan bolt sequence, and any one‑time‑use parts (crush washers, pan bolts, filter seals).
• Confirm whether a filter change is required or even possible (some filters are internal and not serviceable without splitting the case).

Clarifying the official procedure up front reduces the risk of overfilling, underfilling, or introducing the wrong fluid—top causes of post‑service issues.

Inspect for Problems First

A pre‑service inspection can reveal issues that fluid changes won’t fix and may prevent making a marginal transmission worse. Addressing faults before service improves outcomes and diagnostics.

• Perform a road test: note flare, slip, harsh shifts, delayed engagement, shudder, or noise.
• Scan the TCM/ECM for diagnostic trouble codes and live data (shift solenoid performance, temps, adaptation values); resolve faults first.
• Check for leaks at the pan, cooler lines, axle seals, and the transmission cooler (including radiator-integrated coolers) to rule out cross‑contamination.
• Assess fluid condition correctly: use the manufacturer’s method (hot/cold, engine running/not). Look for burnt odor, excessive darkening, metal shavings, or clutch material.
• Inspect mounts and driveline components that can mimic transmission symptoms.
• Verify any software updates that address shift quality or calibration.

If you find significant metal, a strong burnt smell, or active codes, diagnose first; a fluid change alone won’t repair internal wear and may unmask pre‑existing problems.

Gather Correct Parts and Tools

Having the exact fluid and equipment on hand ensures a clean, accurate service. Many modern transmissions require temperature monitoring and specific tools for refill and level checks.

• Correct OEM‑approved fluid: match the spec precisely (e.g., Dexron VI vs. Mercon ULV are not interchangeable; CVTs require CVT‑specific fluids).
• Filter and gasket or pan with integrated filter (common on some ZF units); new O‑rings, seals, crush washers, and any one‑time‑use bolts.
• Fluid pump or syringe for bottom‑fill transmissions; long funnel for dipstick‑fill designs.
• Scan tool or thermometer to monitor fluid temperature during level set; some vehicles require “service mode.”
• Torque wrench, sockets, and bits matching pan/plug fasteners; brake‑cleaner, lint‑free rags, drain pan, and a way to measure drained volume.
• Floor jack, stands or ramps, wheel chocks, gloves, and eye protection.

The right fluids and tools prevent contamination, leaks, and incorrect levels—key to preserving transmission life.

Plan the Procedure

Mapping the sequence reduces mistakes. A careful drain, refill, and level‑set at the correct temperature is critical, especially for “sealed” transmissions with no dipstick.

1. Warm the vehicle to operating temperature per the service manual; park level and secure it safely.
2. Document baseline: note fluid temperature and level method; if applicable, check current level before draining.
3. Place a clean catch pan and measure all fluid removed to inform initial refill quantity.
4. Decide service scope: simple drain‑and‑fill, pan drop with filter/magnet cleaning, or machine‑assisted exchange using only OEM‑approved fluid.
5. If removing the pan, clean it and the magnets thoroughly; replace the filter and gasket, torque bolts in sequence to spec.
6. Refill with the measured amount of new fluid, then follow the OEM’s temperature‑based level procedure (often engine running, cycling through gears, then setting level at a target temp via overflow/level plug).
7. Check carefully for leaks; clear any stored codes if repairs were made.
8. Perform a road test through a range of speeds and gears; recheck the level at the specified temperature and adjust as needed.

Accurate leveling at the prescribed temperature is non‑negotiable; even small deviations can cause shift issues or internal damage.

Special Cases and Cautions

Some transmissions have unique requirements or risks. Following the exact OEM procedure—and exercising caution with high‑mileage, never‑serviced units—can prevent trouble.

• CVTs: Use only the specified CVT fluid (e.g., NS‑3, HCF‑2). Overfill/underfill can quickly cause belt/chain slip and failure. Many require a temperature‑specific level check.
• DCT/DSG: These may have separate circuits for gears and clutches; observe fluid type and dual‑circuit service steps, plus adaptation resets if required.
• Modern 8/9/10‑speed automatics (e.g., ZF8HP, GM/Ford 10‑speed): Level set is temperature‑critical and often requires a scan tool; some pans integrate the filter.
• “Sealed” units: Lack a dipstick by design; never guess the level—follow the overflow/standpipe method at the specified temp.
• High‑mileage, never‑serviced: A change won’t fix worn clutches; fresh fluid can reveal pre‑existing slip. Do a careful inspection and road test first.
• Flush vs. exchange vs. drain‑and‑fill: Power flushing at high pressure is generally discouraged; gentle, machine‑assisted exchanges that follow OEM pressure/flow specs and use approved fluid are typically safe.
• Hybrids/eCVTs: Procedures and fluids differ; some use ATF, others dedicated fluids; always consult OEM guidance.
• Additives: Avoid aftermarket additives or friction modifiers unless explicitly approved by the manufacturer.
• Environmental: Dispose of used fluid at a licensed facility; never mix with other waste; follow local regulations.

Respecting these exceptions protects complex hardware and keeps you aligned with manufacturer standards.

When to Seek Professional Service

Certain scenarios are better handled by a qualified shop with the right tools, software, and liability coverage.

• No dipstick/complex level procedure requiring a scan tool or service mode.
• Active trouble codes, slip/shudder, or harsh shifts that need diagnostic work before service.
• Units with integrated pans/filters, one‑time‑use fasteners, or corroded hardware.
• Vehicles under warranty or those with TSBs requiring software updates or adaptations after service.
• DCT/DSG or CVT services that require precise adaptations and temperature windows.

Professional service can save time and prevent costly mistakes, especially on high‑complexity transmissions.

Bottom Line

Before changing transmission fluid, confirm the exact OEM procedure and fluid, inspect and correct underlying issues, assemble the right parts and tools, and plan a temperature‑accurate level set. Taking these steps ensures the service helps your transmission rather than harms it, preserving shift quality and longevity.

Summary

Verify the manufacturer’s specs and method, inspect for faults and leaks, gather approved fluid, filter, and tools, and follow a temperature‑based leveling process after refill. Be cautious with CVTs, DCTs, and sealed units, and consider professional help if specialized procedures or diagnostics are required.

What are some things to do when changing transmission fluid?

The proper way to change transmission fluid is to remove the oil pan which drains the fluid. Replace the filter and reinstall the oil pan with a new gasket. Refill with fresh fluid. The flush leaves the old filter in place and mixes the new fluid with the old dirty stuff while the flush is happening.

Is it better to flush or just change transmission fluid?

A transmission fluid change involves a simple gravity drain and refill, leaving some old fluid and debris behind, while a transmission fluid flush uses a machine to remove nearly all old fluid and contaminants, but risks dislodging problematic deposits in neglected or high-mileage transmissions. A fluid change is gentler and less expensive, whereas a flush is more comprehensive but carries a higher cost and potential risk if the transmission is old or poorly maintained. The best choice depends on the vehicle’s maintenance history, mileage, and manufacturer recommendations.
 
This video explains the difference between transmission fluid change and flush: 25sFixITYouTube · May 25, 2025
Transmission Fluid Change

• What it is: A standard drain and fill procedure where old fluid is drained from the pan, and new fluid is added. The transmission filter, if present, is also typically replaced. 
• Pros:
  • Less expensive than a flush. 
  • Lower risk of causing damage by not dislodging hidden deposits. 
  • Gentler on the transmission system. 
• Cons:
  • A significant portion of the old fluid (up to 60%) remains in the transmission, including in the torque converter. 
  • Does not remove accumulated sludge or contaminants from the entire system. 

Transmission Fluid Flush

• What it is: A procedure using a specialized machine that circulates new fluid through the system to remove virtually all old fluid and any attached debris. 
• Pros:
  • Removes nearly all the old, contaminated fluid. 
  • Can provide a more thorough clean-out of the transmission’s internal components. 
  • May improve performance and fuel economy. 
• Cons:
  • More expensive than a fluid change. 
  • Higher risk of damage, as the pressure can dislodge particles and deposits that may then clog delicate passages in the valve body or other parts, especially in older, neglected transmissions. 

When to Choose Which

• Choose a Change:
  • For routine maintenance on a vehicle with good fluid condition. 
  • For a high-mileage vehicle with a known, but not critical, maintenance history. 
  • To avoid the risks associated with a flush on an older or neglected transmission. 
• Choose a Flush: 
  • When the manufacturer recommends it for a vehicle with a good maintenance history. 
  • If you want the most complete removal of old fluid, and you are confident in the condition of your transmission. 

Recommendation 
Always check your owner’s manual for manufacturer recommendations regarding transmission service intervals. A qualified technician can assess your vehicle’s condition and recommend the best approach.

Why do people say never change transmission fluid?

If you’ve neglected an automatic transmission oil and filter changes, then leave it alone. The reasoning for this is transmissions develop a varnish inside of them, from the oil film. By changing the oil and filter, you’re placing a new corrosive oil that’s designed to keep the oil galleys clean.

How long should you wait before changing transmission fluid?

Change transmission fluid according to your vehicle manufacturer’s specific guidelines, found in the owner’s manual, typically every 30,000 to 60,000 miles or 2-5 years. Inspect fluid color for signs of breakdown (dark brown/black instead of red), and watch for performance issues like hard shifting, slipping, or grinding as indicators of a necessary change. 
Manufacturer Guidelines are Key

• Consult your owner’s manual Opens in new tabfor the most accurate service intervals and fluid recommendations for your specific vehicle. 
• Consider driving conditions: Opens in new tabExtreme use, such as towing or constant driving in harsh climates, may require more frequent changes. 

This video explains how often to change transmission fluid, including the importance of the manufacturer’s recommendation and how to check the fluid yourself: 1mBudget MechanicYouTube · Jan 20, 2024
Inspect the Fluid

• Color: Fresh, healthy transmission fluid is typically bright red. 
• Signs of degradation: If the fluid appears dark brown or black, it indicates the fluid is breaking down and needs to be changed. 
• Smell: A burning smell can also signal that the fluid is old and overheating. 

Watch for Performance Problems 

• Difficulty shifting: Hard slams into gear or the transmission hesitating to shift. 
• Slipping gears: The engine revs, but the vehicle doesn’t move as it should. 
• Unusual noises: Grinding sounds while shifting. 
• Surging: Unexpected forward or backward movement. 
• Warning lights: Illuminating warning lights for high transmission temperature or the check engine light. 

This video demonstrates how to check transmission fluid for signs of damage and when to change it: 1mAlex The Car DoctorYouTube · Mar 13, 2025
Important Considerations

• High-mileage vehicles: Opens in new tabIf your car is over 100,000 miles and hasn’t had regular maintenance, a simple fluid change may be recommended instead of a flush to avoid dislodging accumulated debris and potentially causing slippage. 
• Fluid changes prevent problems: Opens in new tabRegular fluid changes keep your transmission lubricated and cool, reducing wear on parts and preventing costly repairs.