Skoda Plug-in Hybrid HV Battery Cooling Fan Relay Test
When the cooling fan on your Skoda iV plug-in hybrid runs non-stop during charging and flattens the 12V battery overnight, the culprit is rarely the fan itself — it’s the control logic and relay circuit that decides when that fan should spin.
TL;DR
The HV battery cooling fan in Skoda plug-in hybrids (Octavia iV, Superb iV) is controlled by the Battery Management System (BMS) and powered through the 12V electrical system. A known issue causes the fan to run excessively during charging, draining the 12V battery . The radiator fan relay and its control circuit are part of the engine compartment fuse box, with the Battery Monitoring Control Unit being the subject of a Škoda recall for this exact problem . Testing involves checking the relay coil resistance (typically 50-120Ω) and verifying 12V power supply to the fan circuit . High-voltage safety precautions are mandatory before any work on the cooling system .
Key Takeaways
- The HV battery cooling fan runs on 12V power, not the high-voltage battery
- A Škoda recall addresses excessive fan operation draining the 12V battery by replacing the Battery Monitoring Control Unit
- Relay coil resistance should measure 50-120Ω; infinite or zero means replacement
- The radiator fan relay is in the engine compartment fuse box, with hybrid-specific fusing
- Fan activation during charging is normal for battery thermal management — but continuous cycling indicates a fault
- High-voltage system must be deactivated before any cooling system work
How the HV Battery Cooling System Works
Skoda plug-in hybrids use active thermal management for the high-voltage battery. During charging — especially DC fast charging or prolonged AC charging — the battery generates heat that must be dissipated. The system uses:
- Coolant circulation through the battery pack
- A radiator/cooling fan to move air across the cooling fins
- The Battery Management System (BMS) to monitor temperatures and command fan operation
Here’s the critical detail: the cooling fan runs on 12V power, not the high-voltage battery . This is why a fault in the fan control circuit can drain your 12V battery completely while the car is plugged in.
One owner measured their 12V battery at just 4.1 volts after the fan ran continuously overnight during charging .
The Known Škoda iV Fan Issue
Škoda has acknowledged a specific problem with excessive cooling fan operation on plug-in hybrid models. The symptoms follow a clear pattern:
| Symptom | Description |
|---|---|
| Fan runs continuously during charging | Even after battery reaches optimal temperature |
| Fan cycles rapidly on/off | 12 seconds on, 20 seconds off, repeated for hours |
| 12V battery drains overnight | Fan consumes more power than the DC-DC converter can replenish |
| Car completely dead in morning | Battery voltage drops too low to unlock or start |
One Spanish Octavia iV owner calculated their fan was activating 1,200 times per charging cycle — a clear sign of faulty control logic .
The Official Fix
Škoda’s remedy is a recall to replace the Battery Monitoring Control Unit . One owner reported: “We have been asked to replace the battery monitoring control unit as part of the recall” . This unit manages the 12V battery charging and coordinates with the cooling system.
Important: If your iV is experiencing this issue, check with your Škoda dealer for outstanding recalls or technical campaigns before attempting any DIY diagnosis.
Relay Circuit Overview
The cooling fan circuit involves several components:
| Component | Location | Function |
|---|---|---|
| Radiator fan relay | Engine compartment fuse box | Switches high-current power to fan |
| Battery Monitoring Control Unit | Near 12V battery | Manages charging and fan coordination |
| BMS (Battery Management) | Integrated with HV system | Monitors battery temp, commands cooling |
| Fan control module | Near radiator | Variable speed control |
The radiator fan relay is listed in the engine compartment fuse box for hybrid vehicles, often designated with “for hybrid vehicles” in the fuse diagram . The fuse box also contains a dedicated fuse for “High-voltage battery cooling system pump” .
Safety Precautions
High-voltage safety is non-negotiable. Before any work on the cooling system or related electrical components:
- Deactivate the high-voltage system using the service disconnect
- Wait at least 20 seconds after deactivation for capacitors to discharge
- Wear appropriate PPE — insulated gloves and safety glasses
- Never open the cooling system while the engine or HV system is hot
Warning: Coolant in the HV system can cause thermal reactions in the battery if leaked . If you see coolant loss near the HV battery, do not proceed — seek professional assistance.
DIY Relay Test Procedure
Note: This test verifies the relay itself. It does not test the BMS or control logic that commands the relay.
Tools Required
- Digital multimeter with Ω and DC voltage functions
- Relay puller or small flathead screwdriver
- 12V power source (jumper wires to battery)
- Service manual for your specific model
Step 1: Locate the Relay
Consult your fuse box diagram in the engine compartment. For hybrid models, look for:
- A relay labeled “Radiator fan control unit” or similar
- Position varies by model year — Octavia iV and Superb iV may differ
Step 2: Visual Inspection
Remove the relay and inspect:
- Corrosion on terminals (green or white deposits)
- Burnt or discolored housing
- Loose or bent pins
- Burning smell — indicates internal failure
Step 3: Test the Coil
Set your multimeter to resistance (Ω) mode:
- Identify the coil terminals — typically pins 85 and 86
- Measure resistance across the coil
- Expected reading: 50-120Ω
| Reading | Interpretation |
|---|---|
| 50-120Ω | Coil intact — proceed to contact test |
| Infinite (OL) | Open coil — replace relay |
| 0-5Ω | Shorted coil — replace relay |
Step 4: Test the Contacts
With the relay removed:
- Set multimeter to continuity mode
- Test between pin 30 (power input) and pin 87 (fan output)
- Normal (de-energized): No continuity (open circuit)
- Apply 12V to coil terminals (85/86) — you should hear a click
- With coil energized: Continuity should exist between 30 and 87
If contacts are always closed (continuity without coil power), the relay is stuck and will drain your battery.
Step 5: Test Power Supply
With relay removed and ignition OFF:
- Measure voltage at pin 30 (power input) to ground
- Expected: Battery voltage (12.4-12.6V)
- If no voltage, check the fuse feeding the relay
Skoda iV Model Comparison
| Model | Fan Control Relay | BMS Recall Applicable | Fuse Location |
|---|---|---|---|
| Octavia iV (2020-2023) | Engine compartment, hybrid-specific | Yes | Engine bay fuse box |
| Superb iV (2019-2023) | Engine compartment, fuse 6 listed | Yes | Engine bay fuse box |
| Octavia iV (2024+) | Updated control module | Check VIN for campaigns | Engine bay fuse box |
Always verify fuse and relay assignments against your specific model year’s documentation.
FAQ
Why does my Skoda iV’s cooling fan run constantly during charging?
This indicates a known issue with the Battery Monitoring Control Unit that Škoda has addressed via recall. The fan should only run when battery temperature requires cooling .
Can a faulty fan relay drain my 12V battery?
Yes. If the relay sticks closed, the fan runs continuously even when the car is off, draining the 12V battery .
What is the normal resistance for a Skoda cooling fan relay coil?
50-120Ω is typical for automotive relays. Infinite resistance means an open coil; near-zero means a short .
Where is the radiator fan relay on a Skoda iV hybrid?
In the engine compartment fuse box. Hybrid models have specific fuse and relay assignments — consult your owner’s manual for exact locations .
Is it safe to drive with a faulty cooling fan relay?
If the fan doesn’t work, the HV battery can overheat. If the fan runs constantly, your 12V battery will drain. Both are safety and reliability concerns — address promptly.
Does the HV battery cooling fan use high voltage?
No. The fan runs on 12V power from the vehicle’s low-voltage system, even though it cools the HV battery .
Can I replace the fan relay myself?
Yes, if you can locate it in the fuse box. However, if the underlying issue is the BMS or control unit, relay replacement won’t fix it. Check for recalls first.
Final Thoughts
The HV battery cooling fan relay is a simple component with a big impact on your Skoda iV’s reliability. A stuck relay drains your 12V battery; a failed relay lets your HV battery overheat. But before you replace the relay, check for Škoda’s recall on the Battery Monitoring Control Unit — that’s the root cause for many iV owners .
Always deactivate the high-voltage system before working near HV components. When in doubt, leave it to a qualified technician.
Which Škoda model best fits your driving needs? Share your thoughts in the comments.
References:
- BRISKODA Forum – SkipperEye (Octavia iV fan draining 12V battery, recall for Battery Monitoring Control Unit)
- BRISKODA – Octavia iV Estate Flat 12V Battery (Fan running during charging, BMS control unit)
- AreaVAG – Problema al cargar baterías Octavia híbrido (Fan cycling 12s on/20s off, 1200 activations per charge)
- PiecesEtPneus – Tester le relais de courant au multimètre (Coil resistance 50-120Ω, contact testing)
- StartMyCar – 2022 Skoda Superb Fuse Box Diagram (Radiator fan control unit for hybrid vehicles)
- RBC Autonews – Как проверить реле на работоспособность (Relay testing: coil resistance 30-200Ω range)
