Power steering fluid is a hydraulic fluid used in the steering system of a vehicle. The fluid runs through the power steering pump and hoses, providing an assist to the steering system and making the vehicle easier to steer. In addition to providing power to the system, power steering fluid works as a lubricant and a sealant to prevent corrosion and leaks in the system.

Although some fluids are water based, most are made from mineral oil. Mineral oil-based fluids are more common because they provide better lubrication than water-based fluids and can withstand the high temperatures and heat that the engine in a running vehicle creates. Other oils, such as rapeseed, canola, and silicone oils, are sometimes used for hydraulic fluids .

If a power steering system is leaking and becomes low on fluid, the difference will be noticeable when turning the steering wheel. The steering wheel may be more difficult to turn, and there may be a groaning noise. Spots of power steering fluid collecting underneath the car can also be a sign of a power steering fluid leak. After filling the power steering fluid reservoir, fluid may appear underneath the car, but this does not necessarily mean there is a leak. The fluid on the ground could just be from overfilling the reservoir.

If the vehicle is low on fluid, adding power steering fluid will restore ease in steering and eliminate the groaning noise when turning the steering wheel. If adding fluid does not resolve the problem, or if it only solves the problem for a short period of time, there might be a more serious problem with the power steering system. The power steering pump, valve, pulley, or one of the hoses might need to be replaced. Hoses often crack or become loose, causing a fluid leak.

The power steering fluid level should be checked at every oil change, and the entire power steering system should be flushed out, removing all of the fluid every 60,000 miles. After flushing the system completely, new power steering fluid should be put into the reservoir. Old fluid should never be re-used, and should be disposed of properly.

Although brake fluid, transmission fluid, and power steering fluid are all hydraulic fluids, they are not interchangeable. Putting the wrong fluid into the wrong system can cause serious damage. The fluids that are recommended for the vehicle should always be used.