B1347 — Heated Backlite Relay Circuit Failure #

Note: The description of DTC B1347 may vary depending on the vehicle manufacturer (OEM). Below are known label variations:

• Heated Rear Window Relay Circuit Failure (Ford, Lincoln, Mercury)
• Rear Defogger Relay Circuit Malfunction (Honda, Acura)
• Rear Window Defogger Relay Circuit Failure (Toyota, Lexus)
• Heated Backlite Relay Circuit Error (GM, Chevrolet)

Summary of B1347 Code #

The B1347 diagnostic trouble code (DTC) signals a problem within the heated backlite relay circuit—the electrical path controlling your vehicle’s rear window defogger or defroster. This system uses a relay, which is an electronic switch, to deliver power to the heating grid embedded in the rear glass. When B1347 is set, the vehicle’s onboard computer (ECU or BCM) has detected a fault that interrupts or disrupts this circuit. This could mean that your rear window defrost feature won’t operate when needed, limiting visibility in adverse weather. While B-codes typically relate to body systems and are less critical than engine or brake issues, neglecting this code can cause inconvenience and, in some climates, safety concerns.

Common Symptoms for Code B1347 #

If your vehicle has logged a B1347 code, you’ll likely notice issues with the rear window defogger function. This can manifest in several ways, depending on the exact nature of the fault. The symptoms often include:

• Rear window defogger does not activate when switched on
• Rear window remains foggy, icy, or covered in condensation
• Indicator light for rear defrost may fail to illuminate
• In some cases, intermittent operation of the rear defogger
• Fuse for the rear window defogger may blow repeatedly

Probable Causes for B1347 (Most Common First) #

The B1347 code can be triggered by a range of issues, most commonly related to faults in the relay or the wiring. Here’s an ordered list, from most to least frequent:

1. Faulty heated backlite relay (relay contacts worn, stuck, or corroded)
2. Open or shorted wiring in the backlite relay circuit (damaged, cut, or corroded wires between the relay, switch, and defogger grid)
3. Defective defogger switch (malfunctioning dashboard control)
4. Blown fuse in the rear defogger/heated backlite circuit
5. Faulty rear window defogger grid (open circuit in the embedded heating element)
6. Failed Body Control Module (BCM) or ECU (rare, but possible if all else checks out)

Step-by-Step Diagnosis for Code B1347 #

A systematic approach ensures you don’t overlook simple issues before moving on to more complex diagnostics. Here’s a proven step-by-step method:

• Scan for additional codes: Confirm no related codes (B, U, or C-series) are present that might point to broader electrical issues.
• Visual inspection of the relay: Locate the heated backlite relay (usually in the fuse/relay box). Check for signs of corrosion, overheating, or damage.
• Test the relay: Use a multimeter to check relay operation. Swap with a known good relay (if available) to rule out relay failure.
• Check fuses: Inspect the rear defogger fuse and related circuits for continuity and correct amperage.
• Inspect wiring and connectors: Examine all wiring and connectors between the relay, switch, and defogger grid for signs of damage, corrosion, or loose connections.
• Test the defogger switch: Verify that the dashboard switch is sending the correct signal to the relay (use multimeter or scan tool data when available).
• Check the defogger grid: Inspect the rear window grid for visible breaks or open circuits (often a thin, barely visible line in the glass).
• Assess BCM or ECU function: Only after all hardware checks have passed, consider BCM or ECU faults. Advanced scan tools may be needed for this step.

Possible Repairs for Code B1347 #

Repairing a B1347 code depends on the root cause identified during diagnosis. Each fix targets a specific failure point:

• Replace faulty relay: Swap in a new heated backlite relay if the original fails testing.
• Repair or replace damaged wiring: Fix any broken, corroded, or shorted wires in the circuit.
• Replace a faulty switch: Install a new rear defogger switch if the original is malfunctioning.
• Replace blown fuse: Use the correct amperage fuse and investigate underlying causes for repeated failures.
• Repair or replace rear defogger grid: Some grid breaks can be fixed with a special conductive repair kit; others require replacement of the rear window.
• Replace or reprogram BCM/ECU: Only after all other components are ruled out—BCM/ECU replacement or reprogramming may be necessary in rare cases.

Is It Safe to Drive With Code B1347? #

Yes, you can generally drive with a B1347 code—the code does not affect critical engine, braking, or steering systems. However, you may experience reduced rear visibility in foggy, rainy, or icy conditions, which can compromise safety, especially in colder climates or during early morning commutes. If your local weather often leads to rear window fog or ice, address the issue promptly to avoid hazardous driving conditions.

Vehicles Most Commonly Affected by B1347 #

While B1347 is not exclusive to one brand, it appears more frequently on certain vehicles, especially those using similar relay/control circuits for the rear window defogger. Vehicles commonly reporting this code include:

• Ford (Focus, Fusion, Escape, Explorer, F-series trucks)
• Lincoln (MKZ, Navigator, Town Car)
• Mercury (Milan, Mariner, Grand Marquis)
• Honda (Accord, Civic, CR-V)
• Toyota (Camry, Corolla, RAV4)
• General Motors — Chevrolet, GMC, Buick (Malibu, Equinox, Silverado, Enclave)
• Lexus (ES, RX, GX models)

Common Mistakes & Best Practices for B1347 #

Missteps can lead to wasted time and unnecessary expense. To avoid common pitfalls, consider the following:

• Assuming the relay is always at fault—check wiring and fuses first.
• Overlooking the rear window grid—hairline breaks can be hard to spot but will cause the system to fail.
• Replacing the switch or relay without testing—always use a multimeter or swap with a known good part for confirmation.
• Ignoring repeated blown fuses—this often signals an underlying short or excessive load in the circuit.
• Neglecting to check for TSBs (Technical Service Bulletins) from the OEM for known issues or recall campaigns.

Express FAQ for B1347 #

Q: Can I reset the B1347 code myself?
A: Yes, you can clear the code using an OBD-II scan tool, but if the underlying problem persists, the code will return once the system detects the fault again. Always diagnose and repair the root cause first.

Q: Is the repair expensive?
A: Most repairs for B1347 are affordable—replacing a relay, fuse, or repairing wiring is relatively inexpensive. If the rear window grid or BCM needs replacement, costs can rise significantly.