What’s the difference between DOT 3 and DOT 4 power steering fluid?

DOT 3 and DOT 4 are brake fluids, not power steering fluids. Their main differences are boiling point and formulation—DOT 4 has a higher boiling point and different additives than DOT 3—but neither should be used in a power steering system unless your owner’s manual explicitly specifies it. Most vehicles require automatic transmission fluid (ATF), a dedicated power steering fluid (PSF), or a central hydraulic fluid (CHF); using brake fluid in power steering can damage seals and the pump.

What DOT 3 and DOT 4 actually are

DOT 3 and DOT 4 are glycol-ether–based brake fluids defined by U.S. Department of Transportation specifications for hydraulic brake systems. They’re engineered to withstand high temperatures generated by braking, manage moisture absorption, and protect brake system components. Power steering systems, by contrast, are lubricated hydraulic circuits that typically rely on petroleum- or synthetic-based fluids designed for lubrication, anti-wear protection, and compatibility with their seals.

Key differences between DOT 3 vs. DOT 4 (as brake fluids)

While neither is a proper substitute for power steering fluid, understanding how they differ as brake fluids helps clarify the confusion.

• Boiling point: DOT 4 has higher dry and wet boiling points than DOT 3, making it more resistant to fade under sustained high heat in braking systems.
• Formulation: DOT 4 typically contains borate esters and enhanced inhibitors for high-temperature stability; DOT 3 is primarily glycol ethers.
• Hygroscopicity: Both absorb water from the air; DOT 4 often absorbs moisture slightly faster, which can reduce its boiling point over time.
• Corrosion protection: Both include corrosion inhibitors, with DOT 4 tuned for more demanding brake environments.
• Compatibility: DOT 3 and DOT 4 can generally mix in brake systems, but the resulting performance defaults to the lower spec; this compatibility does not extend to power steering systems.
• Use cases: DOT 3 is common in older and lighter-duty vehicles; DOT 4 (including DOT 4 LV for modern ABS/ESC) is common in newer cars requiring higher performance brake fluid.

These distinctions matter for brakes, not steering. The chemistry that makes DOT 3/4 effective in brakes can be harmful in power steering applications that require different lubrication and seal compatibility.

Why brake fluid should not go in a power steering system

Power steering systems typically use fluids based on petroleum or synthetic oils with friction modifiers and anti-wear additives. Brake fluid is a polar, glycol-based solvent. Mixing them can swell, soften, or crack steering-system elastomers (commonly nitrile/HNBR), strip lubricity, and trigger pump failure or leaks. Even a small amount of brake fluid can jeopardize seals, hoses, and the pump, leading to costly repairs.

What fluid does your power steering system use?

Always verify the exact specification in the owner’s manual or under-hood labeling. Many late-model vehicles use electric power steering and have no hydraulic fluid at all. Among hydraulic systems, the required fluid varies by manufacturer and model.

Below are common categories you may encounter; check your documentation for the precise spec before topping up or flushing.

• ATF-based fluids: Many domestic and Japanese vehicles specify ATF (e.g., Dexron II/III, Dexron VI, or Mercon V) for the power steering system.
• Dedicated Power Steering Fluid (PSF): Some manufacturers require their own PSF blend (e.g., Honda/Acura PSF, certain European OEM-labeled fluids).
• Central Hydraulic Fluid (CHF): European systems and some electro-hydraulic units often require mineral or synthetic CHF (e.g., Pentosin CHF 11S or CHF 202); these are not interchangeable with ATF unless explicitly stated.
• Special cases and EPS: Vehicles with electric power steering (EPS) have no hydraulic fluid; some electro-hydraulic assist units still specify CHF or PSF.

Using the exact specified fluid preserves steering feel, pump longevity, and seal integrity. When in doubt, consult the service manual or dealer parts catalog by VIN.

If you added DOT 3 or DOT 4 to the power steering reservoir: What to do

If brake fluid went into the steering reservoir—even by mistake—act quickly to minimize damage. The steps below outline a practical response.

1. Do not start or run the engine. Circulating the fluid spreads the contaminant through the pump, gear/rack, and hoses.
2. Extract the reservoir contents immediately using a fluid extractor or syringe/turkey baster, and dispose of it properly.
3. Refill the reservoir with the correct fluid and repeat the extract/refill several times to dilute contamination.
4. If the engine has already been run, perform a full system flush: disconnect the return line to a catch bottle, cap the reservoir port if required, add correct fluid, and with the front wheels raised, briefly run the engine while turning lock-to-lock until clean fluid returns. Reconnect and top off.
5. Inspect for leaks, squeal, groan, or heavy steering over the next days/weeks; damaged seals or pump wear may appear after contamination.
6. If you’re unsure about the procedure or contamination was significant, have a professional shop flush and evaluate the system.

Rapid dilution and flushing can reduce harm, but brake fluid may already have affected seals; ongoing monitoring is essential.

Frequently confused: DOT 5

Some readers encounter DOT 5 and assume it’s an “upgrade.” It isn’t applicable to power steering, and it differs markedly from DOT 3/4.

• DOT 5 is silicone-based, non-hygroscopic, and not compatible with DOT 3/4 systems or most ABS calibrations unless specified by the manufacturer.
• It should never be mixed with glycol-based brake fluids or used in power steering systems.
• It offers unique benefits in specific classic or specialty brake systems but is irrelevant to steering fluid requirements.

In short, DOT 5 is a distinct brake fluid type, not a power steering fluid—and mixing it with other chemistries causes problems.

Summary

DOT 3 and DOT 4 are brake fluids with different boiling points and formulations; neither is a power steering fluid. Power steering systems typically require ATF, dedicated PSF, or CHF specified by the vehicle manufacturer. Using brake fluid in power steering can damage seals and the pump. Always follow the owner’s manual, and if brake fluid was added by mistake, extract and flush the system promptly with the correct fluid and monitor for issues.

What is the difference between DOT 3 and DOT 4 power steering fluid?

DOT 3 and DOT 4 are types of brake fluid, not power steering fluid. While they can be used interchangeably in some brake systems, they are not suitable for power steering systems. Power steering systems require a specific type of fluid designed for their hydraulic systems, and using brake fluid can cause damage. 
Key Differences between DOT 3 and DOT 4 Brake Fluid: 

• Boiling Point: DOT 4 has a higher wet and dry boiling point than DOT 3, making it more resistant to boiling under high temperatures and demanding driving conditions. 
• Moisture Absorption: DOT 4 tends to absorb moisture more quickly than DOT 3. 
• Compatibility: DOT 3 and DOT 4 are generally compatible, meaning you can use DOT 4 in a system designed for DOT 3, but not the other way around. 

Power Steering Fluid: 

• Power steering systems rely on a dedicated fluid designed to lubricate and transfer pressure within the system.
• Brake fluid is not designed for this purpose and can cause damage to power steering components. 

Recommendations:

• Always use the type of brake fluid recommended by your vehicle’s manufacturer. 
• Never use brake fluid in a power steering system, and vice versa. 
• If you are unsure about the correct fluid for your vehicle, consult your owner’s manual or a qualified mechanic. 

Can you mix dot 3 and dot 4 together?

Yes, you can mix DOT 3 and DOT 4 brake fluids because they are both glycol-based and compatible, but it will lower the boiling point of the mixture. While not catastrophic, mixing them is not ideal, as it dilutes the higher performance characteristics of DOT 4 and can lead to reduced braking performance. It’s best to stick with the type of fluid your vehicle’s manufacturer recommends, but if you must mix them in an emergency, DOT 4 can be added to DOT 3, but DOT 3 should not be added to a DOT 4 system without a full system flush. 
Why mixing is not recommended:

• Lowered Boiling Point: DOT 4 has a higher boiling point than DOT 3. Mixing them will reduce the overall boiling point, which can lead to brake fade under heavy braking conditions, especially in high-performance situations or with prolonged use of the brakes. 
• Reduced Performance: The higher-performance additives and characteristics of DOT 4 are diluted when mixed with the lower-spec DOT 3 fluid. 
• Potential for Degradation: Mixing fluids can accelerate their degradation over time, potentially affecting the longevity of your brake system components. 

What to do if you’ve mixed them:

• Check your owner’s manual: The safest and most recommended action is to always consult your vehicle’s owner’s manual to confirm the required brake fluid type. 
• Upgrade if possible: If you’ve accidentally added DOT 3 to a system that requires DOT 4, you can upgrade by replacing the fluid with 100% DOT 4 or even DOT 5.1 for a performance boost. 
• Flush the system: If you’ve added DOT 3 to a DOT 4 system, the best practice is to flush the entire system and refill it with the correct, higher-specification fluid. 

Key takeaway:
While you can physically mix DOT 3 and DOT 4, it’s a practice that should be avoided for optimal braking performance and safety. Always use the brake fluid recommended by your vehicle’s manufacturer and avoid mixing fluids unless absolutely necessary.

What happens if I use DOT 3 instead of DOT 4?

Using DOT 3 brake fluid in a system designed for DOT 4 can lead to reduced braking performance, potentially causing brake fade or even failure under demanding conditions. DOT 4 has a higher boiling point, meaning it can handle more heat before vaporizing, which is crucial for maintaining braking power. While DOT 3 and DOT 4 can be mixed, it’s generally not recommended to use DOT 3 in a system designed for DOT 4, especially in vehicles with ABS or that experience high braking demands. 
Here’s a more detailed explanation:

• Boiling Point Difference: DOT 4 brake fluid has a higher boiling point than DOT 3, meaning it can withstand higher temperatures before turning into vapor. 
• Moisture Absorption: DOT 4 is also more resistant to absorbing moisture from the air, which can lower the boiling point of the fluid. 
• Impact on Braking: In high-performance or demanding driving situations, DOT 3 fluid might boil, leading to a spongy brake pedal, reduced braking force, and potentially brake failure. 
• Mixing: While DOT 3 and DOT 4 are generally compatible and can be mixed, it’s not recommended to switch to DOT 3 if your vehicle requires DOT 4, especially in systems with ABS or where high braking temperatures are common. 
• Consequences of using DOT 3 in a DOT 4 system:
  • Reduced braking performance: The brake pedal may feel spongy or unresponsive, especially after repeated braking. 
  • Potential brake fade: Braking power may decrease under heavy or prolonged use. 
  • Increased risk of brake failure: In extreme cases, the fluid could boil, causing complete brake failure. 
• Recommended Action: If you accidentally use DOT 3 in a DOT 4 system, it’s best to flush the entire brake system and refill it with the correct DOT 4 fluid, according to a Quora user. 

This video explains the differences between DOT 3 and DOT 4 brake fluids and when it is acceptable to use one instead of the other: 59sCars SimplifiedYouTube · Aug 25, 2018

Can you use DOT 4 for power steering?

No, you cannot use DOT 4 brake fluid in a power steering system because the two fluids have different chemical compositions and will cause damage to power steering seals and hoses, potentially leading to system failure. Brake fluid is corrosive to the types of rubber seals used in power steering systems and lacks the necessary lubricating properties. You should always use the type of fluid approved by your vehicle’s manufacturer, which is typically automatic transmission fluid (ATF) or a specific central hydraulic fluid (CHF). 
This video explains why brake fluid and power steering fluid are not interchangeable: 37sAuto V Fix YouTube · Sep 19, 2022
Why brake fluid is not suitable for power steering:

• Incompatible seals: Brake fluid is alcohol-based and chemically degrades the rubber seals and hoses in a power steering system, causing them to swell and fail. 
• Poor lubrication: Brake fluid does not provide the necessary lubrication for the power steering pump and rack, which are exposed to significant metal-on-metal contact. 
• Potential for system failure: Damage from incompatible fluids can lead to leaks, reduced steering performance, and costly repairs to the entire power steering system. 

What to do if you accidentally put brake fluid in the power steering system:

1. 1. Do not use the vehicle: Stop using the vehicle immediately to prevent further damage. 
2. 2. Act quickly: The longer brake fluid remains in the system, the more extensive the damage can be. 
3. 3. Drain and flush the system: Tow the vehicle to a service facility to have the entire power steering system drained and flushed. 
4. 4. Refill with the correct fluid: The system should then be refilled with the manufacturer-approved power steering fluid. 

This video explains how to check and add power steering fluid: 58sYourMechanicYouTube · Jun 1, 2016