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Error code reference

Mitsubishi Air Conditioner Error Code OL

Stall prevention (overcurrent) warning

Mitsubishi codesMitsubishi air conditioner codes

Stall prevention (overcurrent) warning - output current exceeded stall prevention level, frequency adjustment in progress.

First step from the service manual

Check Pr.0 Torque boost, Pr.7/Pr.8 accel/decel times, load weight, and Pr.22 stall prevention level.

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

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What OL means on a Mitsubishi air conditioner

The OL error code on a Mitsubishi air conditioner indicates a stall prevention (overcurrent) warning. This condition occurs when the output current from the inverter drive rises above the configured stall prevention level, typically set via parameter Pr.22. Rather than tripping a hard fault, the inverter responds by actively adjusting the output frequency downward to reduce motor load current and prevent a compressor stall. This is a protective, self-correcting response, not a full shutdown, though persistent triggering indicates an underlying problem that needs attention.

The stall prevention function monitors inverter output current in real time. When current exceeds the Pr.22 threshold during acceleration or steady-state operation, the drive reduces frequency to bring current back within acceptable limits. If Pr.7 or Pr.8 acceleration and deceleration ramp times are set too aggressively, the drive demands more current than the motor and compressor can safely handle during speed transitions, triggering this warning repeatedly. Similarly, if Pr.0 torque boost is set too high, excess voltage applied at low frequencies can cause overcurrent conditions.

Load conditions are a major contributing factor. A mechanically overloaded compressor, refrigerant overcharge, blocked airflow, or a failing compressor bearing can all increase the mechanical load on the motor, pushing output current above the stall prevention threshold. Technicians should evaluate Pr.22 to confirm the stall level is appropriate for the unit, review the accel/decel time parameters, assess the actual operating load, and inspect the refrigerant system and airflow paths for restrictions before adjusting any drive parameters.

Source: manufacturer service documentation.

Common causes of OL

  1. 01

    Acceleration or deceleration times set too short

    Wiring / connection

    If Pr.7 (acceleration time) or Pr.8 (deceleration time) are configured too aggressively, the inverter demands a surge of current during speed changes that exceeds the stall prevention threshold. Increasing these ramp times reduces the current spike during transitions.

  2. 02

    Excessive mechanical load on the compressor

    Part replacement

    A failing compressor bearing, refrigerant overcharge, or liquid refrigerant slugging increases the mechanical resistance the motor must overcome, raising output current above the Pr.22 stall prevention level. Check system pressures and listen for abnormal compressor noise.

  3. 03

    Torque boost parameter set too high

    Wiring / connection

    Pr.0 controls voltage boost applied at low frequencies to improve starting torque. If this value is set too high, it injects excess current into the motor windings at low speeds, which can trigger overcurrent detection. Adjusting Pr.0 to the appropriate level for the load often resolves the warning.

  4. 04

    Stall prevention level (Pr.22) set too low

    Wiring / connection

    If Pr.22 is configured below the normal operating current range for the unit, the inverter will trigger stall prevention even during legitimate load conditions. Verify that Pr.22 is set to the correct percentage for the specific unit model and load.

  5. 05

    Restricted airflow causing elevated load

    Wiring / connection

    Blocked condenser coils, a clogged air filter, or a failed fan motor can restrict heat exchange, causing the refrigerant system to operate at elevated pressures and increasing compressor motor current. Inspect and clean airflow paths before adjusting any drive parameters.

See the test procedure for each cause

Frequently asked questions about OL

What does OL mean on a Mitsubishi air_conditioner?
The OL code stands for stall prevention overcurrent warning. It means the inverter drive detected that output current exceeded the stall prevention level configured in parameter Pr.22. In response, the drive automatically reduces output frequency to lower the current and prevent compressor stall. This is a protective function, but recurring OL warnings point to a parameter mismatch or a mechanical issue that needs to be diagnosed.
Can I fix the OL error myself on a Mitsubishi air conditioner?
Some causes, such as restricted airflow from a dirty filter or blocked condenser, can be addressed by a homeowner. However, adjusting inverter parameters like Pr.0, Pr.7, Pr.8, and Pr.22 requires technical knowledge of drive programming and should be performed by a qualified technician. Incorrect parameter changes can damage the compressor or inverter drive. If the unit is running but showing degraded performance, turn it off and call a technician.
Is it safe to keep running my Mitsubishi air conditioner when OL is displayed?
The OL warning indicates the system is actively throttling output frequency to protect the compressor, so the unit may still operate at reduced capacity. However, continued operation with an unresolved root cause can accelerate wear on the compressor and inverter components. It is best to have the system inspected promptly rather than continuing to run it under persistent stall prevention conditions.
Will resetting the unit clear the OL error on a Mitsubishi air conditioner?
Because OL is a warning rather than a hard fault trip, the frequency adjustment is handled automatically by the inverter and the unit may continue operating. A power cycle may temporarily clear the displayed warning, but if the underlying cause such as an overloaded compressor, aggressive ramp settings, or incorrect Pr.22 level is not corrected, the warning will return. Resetting without diagnosis does not resolve the problem.
How much does it cost to repair the OL error on a Mitsubishi air conditioner?
Repair costs depend heavily on the root cause. If the fix is a parameter adjustment to Pr.0, Pr.7, Pr.8, or Pr.22, you may only pay for a service call, typically in the range of $75 to $200 depending on your area. If the compressor requires replacement due to bearing failure or internal damage, costs can range from $600 to $2,000 or more including parts and labor. Refrigerant-related issues such as overcharge correction typically fall in the $100 to $400 range.

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