How to Use the AUTOOL AS502 Brake Fluid Tester Step by Step

The AUTOOL AS502 checks the water content indicated in brake fluid. Identify the fluid first, wipe the probe clean, choose the matching Normal or supported DOT 4 Expert setting, immerse the sensing end, press TEST and read the percentage together with the on-screen status.

Updated August 27, 2026.

Choose the correct AS502 mode

What is in the vehicle?	AS502 choice	When to stop
Identified DOT 3 fluid	Normal → DOT 3	Stop if the cap or service information does not confirm the fluid
Identified DOT 4 fluid	Normal → DOT 4	Stop if the fluid identity is uncertain
Identified DOT 5.1 fluid	Normal → DOT 5.1	Do not select DOT 5.1 for DOT 5
Identified DOT 4 from a brand shown in the Expert menu	Expert → exact listed brand	Stop if the brand is absent or the formulation cannot be identified

DOT 5 and DOT 5.1 are different fluids. The current AS502 manual lists DOT 3, DOT 4 and DOT 5.1 in Normal mode; it does not list DOT 5. Expert mode is for the selected DOT 4 brands actually shown by the tester, not a universal higher-accuracy mode.

Before you open the reservoir

Have four 1.5 V AAA batteries, a clean lint-free wipe, suitable eye and hand protection, the vehicle service information and the exact fluid specification from the reservoir cap. Park securely on a stable surface, switch the engine off and clean dirt from around the cap before opening it.

Stop if the probe or cable is damaged, the fluid cannot be identified, the reservoir cannot be opened without introducing dirt, or the vehicle requires a service procedure you cannot follow safely. Brake fluid can damage paint, so keep it controlled and follow the fluid manufacturer’s handling instructions.

AUTOOL AS502 step-by-step procedure

Step 1: Confirm the tester, fluid and task

Verify that the device is an AS502 and that the planned task is a water-content check on identified DOT 3, DOT 4 or DOT 5.1 brake fluid. Read the reservoir cap and vehicle service information instead of choosing from color, age or memory.

Step 2: Inspect the probe and housing

Check the flexible probe, sensing end, display, buttons and battery cover. The sensing end should be intact and free of residue; the outer hose should not be split, crushed or loose.

Step 3: Install four AAA batteries

Open the rear battery cover and install four 1.5 V AAA batteries in the polarity shown inside the compartment. Refit the cover securely.

Step 4: Prepare clean reservoir access

Secure the vehicle, switch the engine off and make the reservoir area visibly clean. Keep the cap closed until the tester is ready so the fluid is exposed for the shortest practical time.

Step 5: Clean the probe before it reaches the reservoir

Wipe the sensing end with a clean, lint-free material. Remove any residue from storage or a previous sample without adding solvent or another fluid.

Step 6: Power on the AS502

Hold the power key for more than about two seconds. Confirm that the display starts normally. The unit can switch itself off after five minutes without activity, so restart it if the screen has gone dark before testing.

Step 7: Select Normal or Expert mode

Use Normal mode for identified DOT 3, DOT 4 or DOT 5.1 fluid. Use Expert mode only when the fluid is DOT 4 and its exact brand appears in the delivered AS502 menu. The current manual lists MOTUL, ATE, FERODO, MOBIL, BREMBO, TRW, BOSCH, CASTROL, TOTAL, VALEO, FUCHS and TEXTAR.

Step 8: Confirm the fluid selection on screen

Use the fluid-type control to move to the required selection, then pause and read the screen before opening the cap. Do not assume the previous test’s selection is still correct.

Step 9: Turn on the backlight if needed

Use the backlight key when ambient light makes the display difficult to read. A clear screen helps prevent a wrong mode or misread result, but it does not change the measurement.

Step 10: Immerse the sensing end

Open the clean reservoir cap and guide the flexible probe into the fluid. Immerse the sensing end sufficiently for contact without forcing the hose against the reservoir, seals or nearby components.

Step 11: Press TEST and hold the probe steady

Press the TEST key once and keep the sensing end in stable contact with the fluid. Avoid moving it against the reservoir wall while the AS502 responds; the manual gives an approximate response time of ten seconds.

Step 12: Read the percentage and status together

The manual groups 0–2% as good, 2–3% as warning and above 3% as replacement guidance. Because the printed bands share the exact 2% boundary, use the status actually shown on the AS502 and the vehicle’s service information for a boundary result.

Do not convert this one indication into a claim about boiling point or the entire braking system. A warning, a result above 3%, an overdue maintenance interval or other brake concern should be handled through the vehicle’s specified service process.

Step 13: Verify an unexpected result once

If the reading looks implausible, remove the probe, wipe it clean, confirm the fluid selection and repeat the test once with stable immersion. Compare the two results without averaging away a warning.

Step 14: Remove and wipe the probe

Lift the sensing end out without touching painted surfaces. Wipe it clean immediately with a lint-free material and keep removed fluid away from the reservoir.

Step 15: Close the reservoir and power off

Refit the correct reservoir cap securely, then hold the AS502 power key for about two seconds to switch it off. Check the work area before leaving the vehicle.

Step 16: Store the tester dry

Let the clean probe and housing remain dry, then store the AS502 at room temperature away from moisture and contamination. Remove the batteries when the tester will be stored for a long period.

What a complete AS502 test looks like

A complete test leaves more than a number on the screen. The fluid was identified before mode selection, the clean probe stayed in stable contact, the percentage was read with its displayed status, an unexpected result was checked once, and the reservoir was closed without contamination.

If the result calls for service, use the vehicle manufacturer’s brake-fluid procedure and interval. For the delivered control layout, regional language pages or an updated revision, find the AS502 entry in the AUTOOL manual download center before beginning work.