B1815 — Wiper Rear Motor Down Relay Coil Circuit Open

OBD2 code B1815 flags a problem with the ignition system (wiper rear motor down relay coil circuit open). Short, gentle driving to a shop is usually possible. Common culprits: failed ignition coil or coil pack, worn spark plugs or wrong plug gap, faulty ignition control circuit in the ECM.

body electrical

Is it safe to drive?

Drive with caution Drive with caution and avoid load. Ignition faults cause misfires that quickly overheat and destroy the catalytic converter; if the Check Engine Light flashes, stop and fix it first.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm B1815 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many B0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the ignition system for damage, corrosion, chafing or poor contact.
Subsystem check Do a continuity test end-to-end on the wiper rear motor down relay coil circuit and wiggle the harness while measuring — intermittent opens show up as fluctuating readings.
Subsystem check Swap the coil to another cylinder and see if the fault code follows it.
Subsystem check Inspect spark plugs for wear, oil or carbon tracks and check plug gaps.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm B1815 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for B1815 (parts and labor, USD, independent shop rates): Replace spark plugs: $90–$250; Repair coil wiring or connector: $80–$250; Replace an ignition coil: $150–$380. Overall, fixing B1815 usually costs between $80 and $380. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

Will B1815 clear itself?

Sometimes B1815 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

What are the symptoms of B1815?

Drivers most often notice: loss of power and jerking, misfires under load or at idle, hard starting, especially in damp weather, increased fuel consumption.

How much does it cost to fix B1815?

Replace spark plugs — $90–$250; Repair coil wiring or connector — $80–$250; Replace an ignition coil — $150–$380. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

What does OBD2 code B1815 mean?

What is the most common cause of B1815?

The most common causes are: failed ignition coil or coil pack, worn spark plugs or wrong plug gap, faulty ignition control circuit in the ECM.

When your OBD2 scanner displays code B1815, the vehicle's body control module has detected a problem in the rear wiper motor circuit. Specifically, the module is reporting an "open" condition in the wiper rear motor down relay coil circuit. This means the electrical path that should carry current to operate the rear wiper motor in the downward direction has been interrupted somewhere between the control module and the motor assembly. The module continuously monitors this circuit, and B1815 triggers only after the fault persists through multiple ignition cycles under specific operating conditions, which is why you may have noticed the rear wiper acting intermittently before the warning light appeared.

Understanding why this code sets requires knowing how the rear wiper circuit operates. When you activate the rear wiper, the body control module energizes the down relay coil, which closes contacts and allows battery voltage to reach the wiper motor, spinning it in the direction that wipes the glass. When the wipers reach the park position, the module de-energizes the coil. For B1815 to set, the module commands the coil to activate but never receives confirmation that current is actually flowing through that circuit. This open condition prevents the wiper from operating at all, leaving the rear glass uncleared during wet or snowy conditions.

Symptoms of B1815

The most obvious sign of trouble is a completely inoperative rear wiper system. Beyond the wiper failure itself, you may notice the rear wiper does not respond when activated, or it may move sluggishly if the condition is intermittent. The Check Engine light or body control module warning lamp typically illuminates on the instrument cluster once the fault is confirmed. If the open circuit is caused by corrosion or loose connections, you might experience occasional operation where the wiper works for a short time before failing again. In some cases, other wiper functions on the same switch may continue working normally, isolating the problem to the down relay circuit specifically.

Common causes of B1815

The most frequent culprit behind B1815 is a blown fuse protecting the rear wiper circuit. When this fuse opens, the entire relay coil circuit loses power. A damaged or corroded connector at the rear wiper motor or relay housing can also create an open condition by preventing proper electrical contact. Wiring chafing where the harness passes through the body, especially near the rear hatch, frequently causes broken wires that break the circuit. The relay coil itself can fail internally, creating an open winding that no longer conducts current. Corrosion on the relay contacts or in the connector pins increases resistance and may mimic an open condition to the module. Finally, a failed body control module that does not properly ground the relay circuit will also trigger this code, though this is less common than wiring issues.

How to diagnose B1815

Begin by connecting a quality OBD2 scanner to verify B1815 is stored and record the freeze frame data showing vehicle conditions when the fault occurred. Review any additional trouble codes, as related faults in the wiper system or power distribution network often point to the root cause. Check manufacturer technical service bulletins for your specific make and model—many body electrical issues have documented factory fixes or software updates that may apply to your situation.

Visually inspect all connectors in the rear wiper circuit, focusing on the motor connector and any junction points between the body control module and motor. Look for bent pins, moisture intrusion, green corrosion deposits, or white oxidation on metal contacts. Use a multimeter to perform a continuity test on the wiper rear motor down relay coil circuit, measuring from the control module output pin through the harness to the motor ground. Apply gentle pressure and wiggle the harness during testing to reveal intermittent opens that only fail under certain conditions.

Test the relay coil resistance directly at the relay or motor connector—most relay coils measure between 40 and 120 ohms depending on the vehicle. Any reading showing infinite resistance indicates an open coil that requires replacement. Verify battery voltage is present at the circuit fuse and confirm the ground path is solid before concluding the module itself has failed.

Repair options with costs

The most common and cost-effective repair is replacing the rear wiper motor assembly when internal coil failure is confirmed—many motors come with the relay integrated, so this single replacement addresses both potential failure points. If the motor tests fine, inspect and replace any damaged connectors or repair broken wires in the harness, which typically requires connector pins, weatherproofing seals, and electrical tape or heat-shrink tubing. When the relay is a separate component mounted on the motor or in a junction box, individual relay replacement is possible on some vehicles, though many manufacturers seal the relay within the motor housing.

Parts and labor estimates for B1815 repairs at independent repair shops:

Rear wiper motor replacement (parts and labor): $120–$280
Connector repair or harness fix (parts and labor): $80–$250
Individual relay replacement where serviceable (parts and labor): $60–$180

Total cost to fix B1815

Most B1815 repairs fall in the $80 to $280 range depending on the actual failure point. Diagnosing the exact cause before purchasing parts prevents spending on unnecessary components—a $250 wiper motor when the problem was simply a corroded connector is an avoidable expense. Labor rates vary by region, with metropolitan areas typically charging more than rural shops. Always request the old parts back to verify what was actually replaced, and obtain a written diagnosis before authorizing any repairs.

Can you drive with B1815?

Drive with caution when B1815 is active. The rear wiper provides essential visibility through the rear window during rain, snow, and mud—operating without it significantly reduces rearward visibility and creates a safety hazard, particularly when changing lanes or reversing. This code does not typically cause drivetrain damage, but the underlying electrical fault should be diagnosed and repaired promptly to avoid leaving you stranded if the wiper fails completely during inclement weather.