B3718 – Front Wiper Relay Drive Circuit High (BCM)

System: Body | Standard: ISO/SAE Controlled | Fault type: Circuit High

Definition source: SAE J2012 naming/structure guidance

B3718 means your vehicle’s body control system has detected an electrical problem in the front windshield wiper relay control circuit. In plain terms, the wipers may not work correctly—such as being stuck on, not turning on, or operating unpredictably—especially when you need them most in rain or snow. Technically, B3718 is defined as “Front Wiper Relay Drive Circuit High (BCM),” which indicates the BCM is seeing the relay drive circuit signal higher than expected. This points to a circuit/command issue (not a guaranteed bad relay or BCM) that needs circuit-level testing.

B3718 points to a “circuit high” condition in the BCM-controlled front wiper relay drive circuit. Start by checking the front wiper relay drive wiring/connectors for shorts to power, corrosion, or damaged terminals before suspecting the relay or BCM.

What Does B3718 Mean?

The meaning of B3718 is that the Body Control Module (BCM) has detected the front wiper relay drive circuit is reading higher than the BCM expects when controlling the front wipers. For most drivers, this can show up as wipers that don’t respond normally to the stalk switch, run at the wrong time, or stop working. In technical terms, “circuit high” means the BCM’s monitored voltage/state on the relay drive control line is above its expected range for the commanded operating mode. The exact circuit design (high-side vs low-side driver, integrated vs external relay, and feedback strategy) can vary by make/model, so confirmation requires wiring-diagram-based testing.

Theory of Operation

Under normal operation, the BCM receives a wiper request (typically from the multifunction switch or a networked input) and then commands front wiper operation by driving a front wiper relay control/drive circuit. When the relay is commanded, the relay coil is energized and switches power to the wiper motor circuit, allowing the motor to run at the selected speed or mode.

To set B3718, the BCM detects the relay drive circuit is “high” compared to what it expects for the current command state. Depending on design, the BCM may monitor the driver output directly, monitor current flow, or monitor a feedback/diagnostic sense of the relay control line. A “high” reading is commonly associated with an unintended voltage on the control line (such as a short to B+), a wiring/connector fault that biases the circuit high, or an internal driver/feedback issue in rare cases.

Symptoms

You will usually notice wiper behavior problems first, especially in wet weather, along with possible body-related warning messages depending on the vehicle.

• Wipers inoperative: front wipers do not run in any mode (low/high/intermittent) or only work intermittently
• Wipers stuck on: front wipers may run continuously or turn on unexpectedly
• Wrong speed/mode: intermittent behaves like low speed, low behaves like high, or mode changes feel inconsistent
• No park/incorrect park: wipers may stop mid-windshield or fail to return to the parked position reliably
• Intermittent operation: problem comes and goes over bumps, after a car wash, or with temperature/humidity changes
• Related warnings: possible BCM/body fault message, service indicator, or stored B-code without obvious dashboard symptoms on some models
• Washer interaction issues: washer command may not trigger wipe cycles correctly or may trigger unexpected wiping

Common Causes

• Cause: Front wiper relay drive control circuit shorted to battery voltage (B+) somewhere in the harness, causing a persistently high command/feedback
• Cause: High resistance, corrosion, or water intrusion at the relay, BCM, or intermediate connector pins that distorts the BCM’s relay drive circuit voltage
• Cause: Incorrect relay installed (wrong part number/internal diode/resistor design) or an internally shorted relay coil creating abnormal circuit-high behavior
• Cause: Power feed backfeeding into the relay drive circuit due to a wiring cross-connection, prior repair, or chafed insulation contacting a powered circuit
• Cause: Poor BCM ground or shared ground issue that causes the BCM’s circuit monitoring to interpret the relay drive line as higher than expected
• Cause: Fuse/relay box damage (heat, moisture, terminal spread) creating unintended voltage paths on the wiper relay control circuits
• Cause: Aftermarket accessory wiring (remote start, alarm, lighting, dash cam hardwire) tied into wiper circuits causing unintended voltage on the relay drive line
• Cause: BCM internal driver or circuit monitoring fault (rare) after all external wiring/relay checks pass

Diagnosis Steps

To diagnose the B3718 code accurately, use a capable scan tool that can read Body/BCM codes and data, a digital multimeter for voltage/ground checks, and the correct wiring diagram for your exact vehicle. Back-probing pins, inspecting the underhood fuse/relay box, and performing continuity/short-to-power tests are usually required for a “circuit high” fault.

1. Confirm B3718 is present in the BCM and record freeze-frame/body data (wiper switch state, vehicle voltage, and when the fault set). Check for related Body or network codes that might change the diagnostic path.
2. Verify the customer concern: operate front wipers in all modes (mist, intermittent, low, high) and note abnormal behavior (stuck on, inoperative, wrong speeds, or intermittent operation).
3. Perform a quick battery/charging system sanity check (low or unstable system voltage can skew module circuit monitoring). If voltage is unstable, address that first before deep circuit testing.
4. Inspect the underhood fuse/relay box and the front wiper relay area for water intrusion, heat damage, loose terminals, bent pins, or signs of previous relay “swaps.” Reseat the relay and inspect the cavity for spread terminals.
5. Inspect the wiring harness routing from the fuse/relay box toward the BCM and along the cowl/firewall area for rub-through, pinched sections, or contact with sharp brackets. Focus on areas where moisture and vibration are common.
6. With the key on (engine off), use the scan tool to command the front wiper relay ON/OFF if bi-directional controls are available. Observe whether the relay drive circuit behaves as expected and whether B3718 resets immediately.
7. Using the wiring diagram, identify the BCM’s front wiper relay drive circuit and test for voltage on the control line with the relay installed and then removed. A “circuit high” condition commonly appears as unexpected battery voltage present on the control line when it should not be.
8. If unexpected voltage is present, isolate the source: unplug the BCM connector (as appropriate per service info) and recheck the harness side for voltage. If voltage remains with the BCM unplugged, suspect a short-to-power/backfeed in the wiring or fuse/relay box.
9. Perform continuity and short-to-power checks on the relay drive circuit (power off, battery disconnected as required). Check for continuity to B+ and for cross-shorts to adjacent circuits in the same connector or relay box.
10. Test the relay itself: verify correct part number and inspect for internal shorts. If possible, swap with a known-good identical relay from a non-critical circuit only when the relay type matches exactly, then retest (do not assume the relay is the cause without circuit checks).
11. Inspect grounds related to the BCM and fuse/relay box. A poor ground can cause false “high” readings due to reference shifts. Voltage drop test grounds under load where possible.
12. After repairs, clear codes, run the wipers through multiple cycles and modes, and recheck for pending/stored B3718. Confirm the fix by duplicating the original conditions that triggered the code.

Professional tip: For B3718 “Front Wiper Relay Drive Circuit High,” don’t replace the BCM first. In most cases the fastest win is finding an unintended B+ backfeed at the relay control line caused by moisture in the fuse/relay box, harness chafing, or an incorrect relay type.

Possible Fixes

• Repair chafed wiring or insulation damage causing a short-to-power or backfeed on the front wiper relay drive circuit
• Clean, dry, and repair terminals/connectors at the wiper relay, fuse/relay box, or BCM; address water intrusion and terminal tension issues
• Replace the front wiper relay with the correct OEM-specified relay type if testing indicates an internal fault or incorrect relay design is installed
• Repair or replace damaged fuse/relay box components (melted cavities, corroded bus bars, loose terminals) that can feed voltage into the control circuit
• Restore proper BCM/fuse box grounds (clean ground points, repair ground wiring, correct voltage drop issues)
• As a last step after external circuit verification, perform BCM software update or replace/reprogram the BCM if it fails driver output tests and all wiring/relay checks pass

Can I Still Drive With B3718?

In most cases you can still drive with a B3718 code, but you should treat it as a visibility and safety concern, not just an inconvenience. Because B3718 means the BCM is detecting a “Front Wiper Relay Drive Circuit High” condition, the front wipers may behave unpredictably (not turning on, staying on, working only on certain speeds, or not parking correctly). If weather conditions require wipers for safe visibility, do not continue driving until the cause is diagnosed. If the wipers appear stuck on or the system is acting erratically, disconnecting or pulling the wiper-related fuse is sometimes used as a temporary measure, but only follow a vehicle-specific service procedure.

How Serious Is This Code?

B3718 is usually moderate in severity: it typically won’t cause engine damage or leave you stranded, but it can quickly become a safety issue if you lose windshield clearing capability in rain, snow, road spray, or at night. When the fault is only an electrical “circuit high” detection with normal wiper operation, it may be mostly an inconvenience (warning message, stored code, occasional malfunction). It becomes more serious when the wipers won’t run, won’t shut off, won’t park, or operate at the wrong time—any of which can reduce forward visibility and increase accident risk. Diagnose promptly, especially before long trips or wet-season driving.

Repair Costs

Last updated: March 22, 2026