Hybrid Electric Vehicle (HEV) Problems & Solutions

No.	HEV System Problem		Common Symptoms		Possible Causes			Solution / Repair
1	Hybrid Battery Low State of Charge		Reduced EV assistance, engine runs frequently		Low battery charge, battery degradation			Check SOC, charging, and battery condition
2	Hybrid Battery Not Charging		Battery level remains low		Regenerative braking, inverter, or battery fault			Diagnose charging and energy-flow system
3	Hybrid Battery Overheating		Warning light, reduced power		Blocked cooling duct, fan, or temperature problem			Clean cooling system and diagnose fan/sensors
4	Hybrid Battery Cooling Fan Fault		Battery temperature rises		Fan motor, fuse, wiring, or controller fault			Test fan circuit and replace faulty component
5	Hybrid Battery Cooling Duct Blocked		Battery overheating/reduced performance		Dust or obstruction			Clean ducts and air intake
6	Hybrid Battery Cell Imbalance		Reduced range, battery warning		Uneven cell aging/voltage			Perform battery diagnostic and repair/replace as specified
7	Hybrid Battery Cell Weakness		Rapid SOC changes		Weak/aged cells			Test individual cell/module performance
8	Hybrid Battery Internal Resistance High		Poor acceleration, frequent engine operation		Battery aging			Perform battery health test
9	Hybrid Battery Temperature Sensor Fault		Battery warning		Sensor or wiring fault			Test sensor and circuit
10	Battery Voltage Sensor Fault		HV battery DTC		Sensor or BMS wiring problem			Check sensing circuit
11	Battery Management System (BMS) Fault		Hybrid warning/no hybrid operation		BMS hardware/software/communication fault			Scan BMS and verify power, ground, and communication
12	BMS Communication Fault		Multiple hybrid DTCs		CAN wiring or module fault			Check CAN network and BMS
13	High-Voltage Interlock Fault		Vehicle won’t enter READY		Open interlock circuit/connector			Inspect interlock circuit using approved HV procedures
14	Hybrid Battery Contactor Fault		No READY mode		Contactor or control circuit fault			Diagnose contactor control circuit safely
15	Pre-Charge Circuit Fault		HV system fails initialization		Pre-charge resistor, relay or wiring fault			Test pre-charge system according to manufacturer procedure
16	Inverter Fault		Hybrid warning/reduced power		Internal inverter or control fault			Scan inverter and verify HV supply/cooling
17	Inverter Overheating		Power reduction		Cooling-system failure or excessive load			Check inverter cooling system
18	Inverter Cooling Pump Fault		Inverter temperature rises		Pump, fuse or wiring fault			Test pump and control circuit
19	Inverter Communication Fault		Hybrid system warning		CAN/network fault			Check communication and module power
20	DC-DC Converter Fault		12-V battery not charging properly		Converter or wiring fault			Test DC-DC output and control signals
21	12-V Battery Weak		Difficult startup/electrical warnings		Aging or discharged battery			Test and replace battery if necessary
22	12-V Battery Sensor Fault		Battery-management warning		Sensor/wiring fault			Check battery current/voltage sensor
23	Hybrid Motor Not Operating		Reduced/no electric drive		Motor, inverter,r or control fault			Scan system and test motor circuit
24	Electric Motor Overheating		Reduced power		Cooling failure or excessive load			Check motor temperature and cooling
25	Motor Position Sensor/Resolver Fault		Poor acceleration/no drive		Resolver or wiring fault			Diagnose position-feedback circuit
26	Motor Phase Fault		Vibration/reduced power		Phase wiring or inverter fault			Inspect HV motor connections safely
27	Motor Temperature Sensor Fault		Motor warning/limited power		Sensor or wiring failure			Test temperature sensor circuit
28	Hybrid Generator Fault		Battery charging problem		Generator, inverter or wiring fault			Diagnose generator and power electronics
29	Regenerative Braking Fault		Reduced energy recovery		ABS, inverter, ER, or motor-control fault			Scan braking and hybrid systems
30	Regenerative Braking Weak		Battery receives little energy during braking		Battery SOC, temperature or system fault			Check battery conditions and regenerative control
31	Engine Auto Start/Stop Fault		Engine doesn’t start/stop correctly		HCU, battery, sensor,r or engine fault			Scan hybrid and engine control systems
32	Hybrid Control Unit Fault		Multiple hybrid warnings		HCU internal or software fault			Check HCU power, ground, and communication
33	Hybrid Control CAN Communication Fault		Multiple ECU communication DTCs		CAN wiring/connector/module fault			Test CAN network
34	Engine-Motor Coordination Fault		Harsh transitions/reduced performance		HCU, sensors, or software issue			Diagnose power-management system
35	Hybrid Transmission Control Fault		Poor shifting/driveability		TCM, actuator, or communication fault			Scan transmission/hybrid control modules
36	Power-Split Device Fault		Abnormal noise/reduced performance		Mechanical wear or control problem			Inspect transmission/power-split assembly
37	Hybrid Transmission Oil Temperature Fault		Transmission warning		Temperature sensor or overheating			Check oil level, cooling, leaks, a nd sensor
38	Hybrid Cooling System Fault		Multiple temperature warnings		Low coolant, pump or valve fault			Check coolant level, leaks, pump and valves
39	Electric Coolant Pump Fault		Battery/inverter/motor overheating		Pump motor or control fault			Test pump power and operation
40	Coolant Valve/Actuator Fault		Incorrect thermal management		Valve or actuator failure			Test actuator and coolant flow
41	Hybrid A/C Compressor Fault		A/C unavailable		Electric compressor, inverter or control fault			Diagnose compressor and HV circuit safely
42	Hybrid A/C Insulation Fault		HV isolation warning		Compressor insulation breakdown			Perform approved insulation diagnosis
43	Hybrid Vehicle Won’t Enter READY Mode		Vehicle doesn’t become drive-ready		12-V, HV, interlock, BMS, or ECU fault			Scan all relevant modules and follow HV safety procedure
44	READY Light Goes OFF Unexpectedly		Vehicle suddenly loses hybrid drive		HV battery, inverter, sensor, or communication fault			Retrieve DTCs and diagnose root cause
45	Hybrid Warning Light ON		Dashboard warning		Fault stored in hybrid ECU			Perform complete diagnostic scan
46	Reduced Power / Limp Mode		Vehicle accelerates poorly		HV battery, inverter, motor, or cooling fault			Check DTCs, temperatures and battery condition
47	Poor Fuel Economy		Fuel consumption increases		Battery degradation, engine issue, tire/brake drag, or control problem			Diagnose both engine and hybrid systems
48	Hybrid System Excessive Noise		Whining/buzzing during operation		Motor, inverter, transmission, or cooling component			Identify noise source and inspect component
49	Hybrid Battery State-of-Charge Fluctuates Rapidly		Battery gauge moves up/down quickly		Weak battery cells or inaccurate SOC estimation			Test battery health and BMS data
50	Hybrid System Intermittent Failure		Warning appears and disappears		Loose connector, wiring, temperature, or communication fault			Check DTC history, connectors, grounds, and network
No.		HEV System Problem		Common Symptoms		Possible Causes	Solution / Repair
51		Engine Does Not Start in Hybrid Mode		READY appears but engine fails when required		Engine ECU, fuel, or hybrid control fault	Scan engine and hybrid ECUs; diagnose starting system
52		Engine Starts Roughly During Hybrid Transition		Jerking during engine engagement		HCU synchronization, ignition, or fuel issue	Check HCU data and engine performance
53		Frequent Engine Starting		Engine runs more than normal		Weak HV battery, low temperature, or high load	Test battery health and operating conditions
54		Engine Does Not Stop at Idle		Stop/start unavailable		Battery SOC low or system inhibition	Check hybrid operating conditions and DTCs
55		Delayed EV Mode Activation		Electric drive unavailable initially		Low battery charge or temperature	Check HV battery SOC and thermal condition
56		EV Mode Cancels Quickly		Engine starts unexpectedly		High accelerator demand or battery limitation	Verify battery performance and hybrid control logic
57		Hybrid Battery Fan Excessive Noise		Loud cooling fan from rear		Dirty fan or continuous cooling demand	Clean fan and inspect temperature sensors
58		Battery Cooling Air Intake Blocked		Battery overheats		Luggage, dust, or obstruction	Clean and keep intake unobstructed
59		Battery Cooling Filter Dirty		Reduced cooling efficiency		Dust accumulation	Clean or replace cooling filter if equipped
60		Battery Cooling Fan Runs Continuously		Fan remains ON after driving		High battery temperature or control fault	Check thermal data and cooling controller
61		Battery Cooling Fan Intermittent		Cooling operates occasionally		Loose connector or motor wear	Inspect wiring and replace faulty fan
62		Battery Case Moisture Intrusion		Hybrid warning		Water entering battery housing	Inspect seals and repair according to manufacturer procedure
63		HV Battery Ventilation Fault		Battery temperature warning		Vent duct disconnected or damaged	Repair ventilation path and inspect housing
64		Battery Module Connector Loose		Intermittent hybrid fault		Poor internal connection	Inspect and repair using approved battery procedure
65		Battery Current Sensor Calibration Error		Incorrect SOC estimation		Sensor offset or programming issue	Perform calibration with approved diagnostic tool
66		Battery Voltage Difference Excessive		Reduced power		Uneven module voltage	Diagnose battery modules and replace as required
67		Battery Capacity Degradation		Reduced EV assistance		Natural battery aging	Perform battery capacity test
68		Battery Charge Acceptance Low		Slow regenerative charging		Battery aging or temperature	Test battery health and thermal system
69		Battery Self-Discharge High		Battery loses charge quickly		Internal leakage or module fault	Diagnose battery pack and BMS
70		Battery Isolation Resistance Low		Hybrid warning		Moisture or insulation damage	Perform insulation resistance test safely
71		High-Voltage Cable Damage		Intermittent drive fault		Physical damage or corrosion	Inspect and replace damaged HV cable
72		High-Voltage Connector Corrosion		Communication or power interruption		Moisture and oxidation	Clean/replace connector following HV procedure
73		Service Plug Improper Installation		Vehicle will not enter READY		Plug not fully locked	Install and verify correctly
74		HV Fuse Open Circuit		Complete hybrid shutdown		Excess current or component fault	Find root cause before replacing fuse
75		HV Relay Control Fault		HV battery not connected		Relay driver or wiring issue	Diagnose relay command circuit
76		Pre-Charge Relay Stuck Open		READY mode unavailable		Relay failure	Test relay and replace if defective
77		Pre-Charge Resistor Overheated		Hybrid warning		Excessive current or relay fault	Inspect pre-charge circuit
78		Inverter Power Module Fault		Reduced propulsion		Internal semiconductor failure	Diagnose inverter and replace if required
79		Inverter Gate Driver Fault		Motor does not respond		Control electronics fault	Test inverter control module
80		Inverter DC Bus Fault		Hybrid system shuts down		Voltage imbalance or capacitor issue	Diagnose inverter safely
81		Inverter Coolant Leakage		Temperature warning		Hose, seal, or radiator leak	Repair leak and refill specified coolant
82		Inverter Coolant Low Level		Reduced hybrid performance		Coolant leakage	Inspect cooling system and refill after repair
83		Inverter Cooling Radiator Blocked		Inverter overheats		Dirt or debris	Clean radiator fins
84		Inverter Cooling Valve Stuck		Poor coolant circulation		Valve actuator fault	Test valve operation and replace if necessary
85		DC-DC Converter Low Output		12-V battery warning		Converter efficiency fault	Measure output voltage and diagnose converter
86		DC-DC Converter Overvoltage		Electrical component damage risk		Internal regulator fault	Test converter and replace if defective
87		DC-DC Converter Communication Fault		Hybrid DTCs		CAN network or converter fault	Check communication and wiring
88		12-V Battery Repeatedly Discharges		Vehicle won’t start		Weak battery or converter charging problem	Test 12-V battery and DC-DC charging
89		Hybrid Generator Overheating		Reduced charging		Cooling or generator fault	Check generator temperature and cooling
90		Generator Resolver Fault		Engine charging irregular		Position sensor failure	Diagnose resolver and wiring
91		Generator Phase Circuit Fault		Poor charging performance		Phase wiring or inverter fault	Inspect generator connections safely
92		Power-Split Gear Noise		Whining during driving		Mechanical wear	Inspect transmission/power-split assembly
93		Planetary Gear Excessive Wear		Noise and vibration		Internal gear damage	Overhaul or replace transmission assembly
94		Hybrid Transmission Bearing Fault		Grinding noise		Bearing wear	Inspect and replace bearing
95		Transmission Oil Pump Electrical Fault		Transmission overheating		Electric pump or wiring issue	Test pump circuit and operation
96		Transmission Oil Cooling Fault		High transmission temperature		Cooler blockage or pump problem	Inspect cooling system
97		Hybrid Transmission ECU Fault		Gear selection problems		TCM internal fault	Scan TCM and verify power/network
98		Shift Actuator Position Fault		Incorrect gear engagement		Position sensor or actuator issue	Calibrate or replace actuator
99		Transmission CAN Communication Fault		Hybrid/transmission warning		Network wiring fault	Diagnose CAN communication
100		Hybrid System Master Warning with No Drive		Vehicle cannot move		Multiple hybrid control faults	Perform complete system scan and follow manufacturer diagnostic procedure

Hybrid Electric Vehicle (HEV) Problems & Solutions Diagnostic Focus for Problems 51–100

Diagnostic Step	Purpose
1. Scan All ECUs	Retrieve Hybrid, Engine, BMS, Inverter, and Transmission DTCs
2. Check 12-V Battery	Verify stable low-voltage power supply.
3. Inspect HV Safety Circuit	Check service plug, interlock, and HV fuse.
4. Test HV Battery	Evaluate SOC, voltage balance, and temperature.
5. Check Cooling System	Inspect battery, inverter, and motor cooling operation.
6. Verify CAN Communication	Confirm communication between hybrid control modules.
7. Test Motor & Generator	Check resolver, phase circuits, and inverter operation.
8. Inspect Transmission	Diagnose power-split device, actuators, and TCM.
9. Repair & Calibrate	Perform manufacturer-approved repair and initialization.
10. Final Road Test	Verify READY mode, EV operation, and regenerative braking.

Safety Reminder: HEV systems contain high-voltage electrical circuits that require proper isolation procedures, insulated tools, approved PPE, and manufacturer-recommended diagnostic equipment before inspection or repair.