What Does B0503 Mean?
This code indicates the Body Control Module (BCM) has detected a malfunction in the right-hand Daytime Running Light (DRL) relay circuit. The BCM monitors the relay control circuit and has identified an issue such as an open circuit, short to ground, short to power, or excessive resistance that prevents proper relay operation. This affects the vehicle's DRL system functionality on the passenger side.
Common Causes
35%
Faulty DRL relay with internal contacts burned, welded, or corroded preventing proper switching
30%
Damaged wiring harness or corroded connector between BCM and DRL relay with broken wires or water intrusion
20%
Blown fuse or fusible link in the DRL relay power supply circuit
15%
Failed Body Control Module with internal driver circuit failure for relay control
Diagnostic Steps
1
Step 1: Visually inspect the DRL relay location, check for physical damage, corrosion, or overheating signs. Remove and inspect relay pins for burning or corrosion. Check relay socket for melted terminals or debris.
2
Step 2: Using a digital multimeter, check for battery voltage at relay power supply terminal with ignition on. Verify ground circuit continuity (should read less than 0.5 ohms to chassis ground). Check relay fuse for continuity.
3
Step 3: With relay removed, use multimeter to measure resistance across relay coil terminals (typically 70-120 ohms). Apply 12V to coil terminals and verify audible click and continuity between normally-open contacts when energized.
4
Step 4: Check BCM relay control circuit by backprobing the relay socket control wire while commanding relay ON via scan tool bidirectional controls. Should read battery voltage when OFF and near 0V (ground) when commanded ON.
5
Step 5: Inspect wiring harness from BCM to relay socket for chafing, pinching, or damage. Pay special attention to areas near sharp edges, engine compartment firewall penetrations, and areas exposed to heat or moisture.
6
Step 6: After repairs, clear codes, cycle ignition, command DRL relay operation via scan tool, and verify right DRL operates properly. Road test with DRL conditions met and confirm no code returns.
Estimated Repair Cost
$75 - $450
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected a malfunction in the right-hand Daytime Running Light (DRL) relay circuit. The BCM monitors the relay control circuit and has identified an issue such as an open circuit, short to ground, short to power, or excessive resistance that prev...
The most common cause of B0503 (RH DRL Relay Circuit) is: Faulty DRL relay with internal contacts burned, welded, or corroded preventing proper switching
Typical repair costs for B0503 range from $75 to $450, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle is safe to drive as this only affects daytime running lights, not primary headlights or brake lights. Repair when convenient to maintain visibility to other drivers during daytime operation and comply with local DRL regulations where applicable.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B0503 to identify the root cause.
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Quick Info
Category
Body
System
Exterior Lighting System
Difficulty
Type
Generic (SAE)
Recommended Tools
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