Code
P1520
SUBARU
P — Powertrain
Radiator Fan Relay 1 Circuit High Input
Views:
UK: 28
EN: 51
RU: 36
AI status
Completed
Completed
100%
Causes
- Short to battery voltage on the radiator fan relay 1 input/wiring
- Stuck or welded relay contacts (relay stuck ON)
- Corroded, damaged, or loose connector at relay, fan, or ECU
- Faulty radiator cooling fan motor drawing abnormal current or feeding back voltage
- Faulty fan relay or fan control module
- Faulty ECU (input driver or sensing circuit)
Symptoms
- Cooling fan may run continuously or run when not commanded
- Engine overheating or elevated coolant temperature if fan does not operate correctly
- Cooling fan fuse may blow or show heat damage
- Poor fuel economy or increased noise from fan running unexpectedly
- Possible stored limp-home or related engine cooling codes
What to check
- Retrieve freeze frame and live data with a scan tool; check fan command vs actual fan state
- Visual inspection of fan relay, fuse, harness, and connectors for corrosion, heat damage, or loose pins
- Check for blown fuses related to cooling fan circuits
- Swap relay with a known-good identical relay (if available) to test relay function
- Back-probe the relay coil/control pins and ECU harness to measure voltage with ignition ON and while commanding the fan ON
- Measure continuity and resistance between relay connector, fan motor, and ECU connector; check for shorts to battery or ground
Signal parameters
- Typical inactive circuit voltage: near 0–1 V (low) at ECU input when relay is not energized (may vary by vehicle)
- Typical active circuit voltage: near battery voltage (~12–14 V) at the relay feed when relay is powered
- ECU input threshold often treats voltages above ~5 V as a 'high' input (manufacturer-specific)
- Expect continuity from relay feed to fused battery feed and near-zero ohms from relay output to fan motor when connected (fan motor presents several ohms of resistance while stationary)
Diagnostic algorithm
- Record stored codes and live data (fan command, coolant temp). Confirm P1520 is current or historic.
- Perform a visual inspection of relay, fuse, fan, and wiring. Repair any obvious damage or corrosion.
- With ignition ON (engine off), back-probe the radiator fan relay control/feedback pin. Note voltage at rest and while commanding fan ON via scan tool or A/C request.
- Remove relay; check whether the code clears or voltage on the ECU input changes. If removal removes the high input, suspect relay or wiring upstream.
- Swap relay with an identical known-good relay. If code or behavior changes, replace the relay.
- If swapping relay does not help, disconnect the fan motor connector and inspect for shorts to battery. Measure continuity to battery (+) and ground – look for unintended connections.
- Trace and test wiring between relay and ECU: check for short to battery, chafing, and poor grounds. Repair damaged wiring and connectors.
- If wiring and relay are good but the ECU still senses a high input, test the ECU input circuit per manufacturer procedures or consider ECU replacement after ruling out wiring and components.
- After repairs, clear codes and perform a road/functional test to confirm the fan operates correctly and the code does not return.
Likely causes
- Short to battery on relay wiring or pin (most common)
- Failed or stuck relay (relay contacts welded closed)
- Corroded/loose connector at relay or fan motor
- Faulty fan motor or fan control module
- Faulty ECU input/sensor circuit (least common)
Fault status
Status
Radiator Fan Relay 1 Circuit High Input — ECU detects higher-than-expected voltage on the radiator fan relay 1 circuit. Possible short to battery, stuck relay, or wiring/connectors fault.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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