Variable Refrigerant Flow VRF Critical Interlock

Mitsubishi City Multi Error 5100: Inverter Compressor Start Failure

Published: 2026-04-14

🚨 Critical Diagnostic Alert

Error 5100 on Mitsubishi Electric City Multi VRF systems indicates that the inverter compressor failed to start after three consecutive attempts. The outdoor unit controller initiates the startup sequence: pre-charge the DC bus, energize the compressor windings in the correct sequence, and ramp the compressor from 0 to minimum operating speed (typically 20-30 Hz). If the compressor fails to reach minimum speed within the programmed time window, the controller logs a startup failure and retries.

Three consecutive startup failures trigger the 5100 lockout. The most common root cause — accounting for approximately 50% of field cases — is liquid refrigerant in the compressor sump due to a failed crankcase heater. Always check the crankcase heater first. Replacing the inverter or compressor without verifying the crankcase heater is the most expensive misdiagnosis on City Multi systems.

🔍 Symptoms Checklist

⚠️ Compressor attempts to start — audible hum for 2-3 seconds — then stops
⚠️ Error repeats 3 times before system locks out
⚠️ Outdoor unit fan starts but compressor never reaches running speed
⚠️ Inverter drive reports 'startup failure' in error log

🛠️ OEM Replacement Parts

Part Name	OEM Part Number	Est. Price
Inverter Power Module (IPM, 3-Phase Output)	`R65B45700`	$2,600
Compressor Start Capacitor (if PSC assist)	`R65B45710`	$90
DC Reactor (Line Filter, 3-Phase)	`R65B45720`	$520

📋 Interactive Diagnostic Procedure

Click each step to expand detailed diagnostic instructions. Follow in sequence — each step builds on the previous one.

1 Check Crankcase Heater Operation

A failed crankcase heater allows liquid refrigerant to migrate to the compressor sump during off-cycles. When the compressor attempts to start, it tries to compress liquid (which is incompressible), causing an immediate overload. Measure the crankcase heater resistance (should read 500-800Ω depending on wattage) and verify the heater is receiving continuous power during the off-cycle. The compressor sump should feel warm to the touch (110-130°F) after an 8-hour off-cycle — a cold sump confirms heater failure.

2 Verify Inverter DC Bus Pre-Charge Circuit

Before the inverter can start the compressor, the DC bus capacitors must be pre-charged through a current-limiting resistor and relay. If the pre-charge relay contacts are welded open or the resistor is open-circuit, the DC bus cannot charge and the inverter cannot start the compressor. Listen carefully during the initial power-up sequence — you should hear a distinct 'click' (pre-charge relay closing) followed by a brief 1-2 second delay before the compressor attempts to start.

3 Measure Compressor Locked Rotor Amps

If the crankcase heater and pre-charge circuit are functional, the compressor itself may have a mechanical problem preventing startup. With the inverter output disconnected, measure the compressor winding resistance (all three phase pairs should be <1Ω and within 5% of each other). A high resistance reading on one winding pair indicates an open winding. If resistance readings are normal but the compressor still fails to start, the scroll set may be mechanically seized — this requires compressor replacement.

🔗 Related Error Codes — Variable Refrigerant Flow VRF

Other diagnostic guides in the same equipment category. Common failure patterns often share root causes — cross-referencing speeds up diagnosis.

Variable Refrigerant Flow VRF Critical Interlock

⚡ Fault-Induced Energy Waste Estimator

When HVAC equipment runs with active error codes, it operates inefficiently and wastes electricity. Estimate the monthly cost of delaying repairs. All calculations run locally in your browser.

Unit Power Consumption (kW)

Extra Run Time Due to Fault (Hours/Day)

Local Electricity Rate ($/kWh)

Copied!

References & Industry Standards

ASHRAE 15 — Safety Standard for Refrigeration Systems
ASHRAE 34 — Refrigerant Designation & Safety Classification
AHRI 550/590 — Water-Chilling & Heat Pump Packages
SMACNA — HVAC Duct Construction Standards
Manufacturer Service Manuals — Carrier, Trane, York, Daikin, Lennox

Mitsubishi City Multi Error 5100: Inverter Compressor Start Failure

🔍 Symptoms Checklist

🛠️ OEM Replacement Parts

📋 Interactive Diagnostic Procedure

🔗 Related Error Codes — Variable Refrigerant Flow VRF

Daikin VRV U4 Error: Indoor-Outdoor Communication Failure

Mitsubishi City Multi Error 7100: High Pressure Protection — Outdoor Unit

LG Multi V Error CH24: High Pressure Sensor Malfunction

Toshiba SMMS U1 Error: Indoor Unit Address Duplication

Daikin VRV U2 Error: Power Supply Voltage Abnormal — Phase Loss Detection

Mitsubishi City Multi Error 7102: High Discharge Temperature Protection

LG Multi V Error CH34: Inverter Compressor Overload — Drive Fault

Toshiba SMMS U4 Error: Outdoor Unit PCB Communication Failure

⚡ Fault-Induced Energy Waste Estimator

References & Industry Standards