C1A01

Code

Generic C — Chassis

Hybrid System Main Relay Control Circuit Fault

Brand: Generic

Type: C — Chassis

Views: UK: 11 EN: 32 RU: 15

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Blown fuse for relay supply or control circuit
Faulty main (HV) relay (stuck open or welded contacts)
Open, short to ground, or short to battery in relay control wiring
Poor or corroded connector terminals at relay, fuse, or ECU/BCM
Weak or discharged 12V auxiliary battery or poor battery connections
Faulty ECU/BCM driver transistor or internal relay driver circuitry

Symptoms

Hybrid system will not enter ready state or fails to start
No audible click from the main HV relay when attempting to start
Hybrid/system warning lamp or ABS/EV warning lamps illuminated
Vehicle enters limp or failsafe mode with reduced performance
No HV bus voltage available, charging or EV drive disabled
Intermittent loss of hybrid functions or unexpected shutdowns

What to check

Read and record freeze-frame and live data, note any concurrent codes
Visually inspect relay, fuse, relay socket and associated wiring for damage, corrosion, or loose terminals
Check auxiliary 12V battery voltage and terminals (clean/tighten if needed)
Verify related fuses (power and control) are intact and seated
With ignition ON, listen for relay click during HV enable — note absence or intermittent operation
Backprobe relay control pin and coil pins while commanding HV relay ON/OFF from scan tool

Signal parameters

Control signal (ECU command): ~0 V when OFF, ~battery voltage (9–14 V) when ON
Relay coil resistance (typical range): ~40–200 ohms (varies by application) — coil should not be open or shorted
Coil current when energized: typically tens to a few hundred mA (depends on coil resistance)
Supply voltage to relay contact side: should be battery voltage (~12–14 V) at rest and when commanded (unless safety interlocks open)
Ground continuity from relay ground terminal to battery negative should be low ohms (

Diagnostic algorithm

Safety first: follow manufacturer procedures for HV system isolation and PPE before working near high-voltage components.
Confirm the code with an appropriate diagnostic tool and clear codes; attempt to reproduce. Note if code returns immediately or intermittently.
Visually inspect relay assembly, connectors and wiring for obvious damage, corrosion or overheating. Repair or clean as required.
Check auxiliary 12V battery condition and terminal connections; verify proper supply voltage to relay power terminal with key ON.
Verify fuses for relay power and control circuits; replace any blown fuses and investigate cause if repeat failure occurs.
With ignition ON and safety measures in place, command HV relay ON using a scan tool while backprobing: verify the ECU control pin changes from ~0V to ~battery voltage when commanded.
Measure relay coil resistance with circuit de-energized; confirm coil is not open or shorted. Bench-test the relay if suspect.
Check continuity and resistance of control wire between ECU/BCM connector and relay control terminal; wiggle harness to detect intermittent faults.
Check for shorts to battery positive or ground on the control circuit (use appropriate current/voltage tests, not a direct high-current short).
Verify ground connections for relay and ECU; repair poor grounds.
If wiring and relay are good but control signal absent or incorrect, suspect ECU/BCM driver fault — confirm with oscilloscope or substitute known-good module per manufacturer procedure before replacing ECM.
After repairs, clear codes and perform functional test (multiple cycles) to ensure code does not return.

Likely causes

Failed main relay (contacts or coil)
Loose/poor connector at relay or fuse block
Open/short in control wire between ECU and relay
Low 12V auxiliary battery or poor ground connection

Fault status

⚠️ Status

ECU detected abnormal condition on the hybrid main relay control circuit (open/short/no-response). Relay may fail to energize, disabling the HV system.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours