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Error code reference

Mitsubishi Air Conditioner Error Code SL

Speed limit indication

Mitsubishi codesMitsubishi air conditioner codes

Speed limit indication - speed limit level exceeded during torque control.

First step from the service manual

Check that torque command is not larger than required and speed limit level is not too low.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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What SL means on a Mitsubishi air conditioner

The SL error code on a Mitsubishi air conditioner indicates a speed limit condition during torque control operation. This fault occurs when the inverter-driven compressor motor attempts to exceed the configured speed limit threshold while the control system is actively managing motor torque. In torque control mode, the controller regulates compressor output by commanding a specific torque level, and the SL code triggers when the resulting motor speed breaches the upper speed limit boundary set within the drive parameters.

This condition typically points to one of two root causes: the torque command value being set too high for the operating conditions, or the speed limit level being configured too conservatively relative to normal operating demands. When torque demand is excessive, the motor accelerates beyond its permitted speed ceiling to meet that demand. Conversely, if the speed limit parameter is set too low, even normal torque commands can push the motor past the threshold. Both scenarios result in the control system flagging the SL fault to protect the compressor and drive components from operating outside safe boundaries.

Diagnosing this fault requires reviewing the torque command values being sent to the inverter board and comparing them against the speed limit configuration. Technicians should verify that system refrigerant charge is correct, as low refrigerant can cause the compressor to work harder and elevate torque demands. Additionally, any mechanical restriction in the compressor or excessive load conditions on the refrigeration circuit can force torque commands upward, triggering this protection response.

Source: manufacturer service documentation.

Common causes of SL

  1. 01

    Torque command set too high

    Wiring / connection

    If the torque command issued to the inverter is larger than the load actually requires, the motor will accelerate beyond the speed limit to satisfy that command. Review the torque command parameters in the controller and reduce them to match actual operating requirements.

  2. 02

    Speed limit level configured too low

    Wiring / connection

    A speed limit parameter that is set too conservatively will be exceeded even under normal torque demands. Check the speed limit setting in the drive or control board configuration and adjust it to an appropriate value for the system.

  3. 03

    Low refrigerant charge

    Wiring / connection

    Insufficient refrigerant causes the compressor to work harder to maintain system pressures, which elevates torque demand and can push motor speed past the limit. Verify system charge with manifold gauges and recharge if pressures are below specification.

  4. 04

    Excessive system load or restricted refrigerant circuit

    Part replacement

    Blockages such as a restricted filter drier, partially closed service valve, or fouled heat exchanger coil increase the load on the compressor, forcing higher torque output. Inspect the refrigerant circuit for restrictions and clean or replace components as needed.

  5. 05

    Faulty inverter control board

    Part replacement

    A malfunctioning inverter board may send incorrect torque commands or fail to properly enforce speed limit parameters, causing nuisance SL faults even under normal conditions. If all parameters and refrigerant conditions check out, the inverter board should be tested and replaced if defective.

See the test procedure for each cause

Frequently asked questions about SL

What does SL mean on a Mitsubishi air_conditioner?
The SL code stands for speed limit indication and means the compressor motor exceeded the configured speed limit while the system was operating in torque control mode. The control system detected that the motor speed went beyond the permitted upper boundary during torque-based compressor management. This is a protective fault designed to prevent damage to the compressor and inverter drive components.
Can I fix the SL error on my Mitsubishi air conditioner myself?
Some aspects of diagnosing this fault, such as checking for dirty coils or reviewing accessible settings, can be done by a knowledgeable DIYer. However, adjusting torque command parameters, checking inverter drive configurations, and verifying refrigerant charge all require specialized tools and technical knowledge. A certified HVAC technician is recommended for a complete diagnosis and repair.
Is it safe to keep running my Mitsubishi air conditioner when the SL code is active?
Continuing to operate the unit while the SL fault is active is not advisable. The fault indicates the compressor motor is being pushed beyond its speed limit, which can cause accelerated wear on the compressor, inverter board, and related components. Shutting the system down until the fault is resolved helps prevent more serious and costly damage.
How much does it cost to repair the SL error on a Mitsubishi air conditioner?
Repair costs vary widely depending on the underlying cause. If the fix involves only parameter adjustments or a refrigerant recharge, costs typically range from around 100 to 300 dollars including labor. If the inverter control board requires replacement, costs can range from 400 to 900 dollars or more depending on the specific unit model and labor rates in your area.
Will resetting my Mitsubishi air conditioner clear the SL error code?
A power reset may temporarily clear the SL fault and allow the unit to restart, but if the underlying cause is not corrected the error will return. Simply resetting the system without addressing the torque command configuration, speed limit settings, or refrigerant charge conditions will not resolve the problem. A thorough diagnostic inspection is needed to prevent the fault from recurring.

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