B1739 — Mirror Driver Horizontal Switch Circuit Short To Battery

OBD2 code B1739 flags a problem with the body electrical system (mirror driver horizontal switch circuit short to battery). The car is usually drivable while you plan the repair. Common culprits: blown fuse or failed relay, corroded or loose connectors, damaged wiring in door jambs or wet areas.

body electrical

Is it safe to drive?

Short-term driving is OK Usually safe to drive — body codes rarely affect drivability. Exceptions: exterior lighting and wipers in bad weather, and central locking faults that can leave the car unsecured.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm B1739 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 body electrical system for damage, corrosion, chafing or poor contact.
Subsystem check Operate the affected function while watching its switch and motor PIDs in live data.
Subsystem check Inspect connectors and wiring at flex points (door jambs, tailgate hinges) — wires break there first.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm B1739 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for B1739 (parts and labor, USD, independent shop rates): Replace the switch or actuator: $120–$450; BCM repair or replacement: $300–$900; Repair wiring or connectors: $80–$250. Overall, fixing B1739 usually costs between $80 and $900. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

What are the symptoms of B1739?

Drivers most often notice: warning light on the dashboard, no impact on engine operation in most cases, the affected function (locks, windows, lighting, wipers) works intermittently or not at all.

Is it safe to drive with B1739?

Usually safe to drive — body codes rarely affect drivability. Exceptions: exterior lighting and wipers in bad weather, and central locking faults that can leave the car unsecured.

What is the most common cause of B1739?

The most common causes are: blown fuse or failed relay, corroded or loose connectors, damaged wiring in door jambs or wet areas.

How much does it cost to fix B1739?

Replace the switch or actuator — $120–$450; BCM repair or replacement — $300–$900; Repair wiring or connectors — $80–$250. 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 B1739 mean?

The engine control module sets trouble code B1739 when it identifies a problem in the driver-side mirror's horizontal adjustment switch circuit. Specifically, the module has detected a short to battery voltage — meaning the circuit is receiving power when it should not be. This code falls within the body electrical category of OBD2 diagnostics, which covers comfort and convenience systems rather than engine management functions.

When the vehicle is running, the body control module or the controlling ECU continuously monitors the mirror adjustment circuit. It expects to see specific voltage patterns when you operate the switch. If those readings stray outside the acceptable window for longer than the manufacturer-defined threshold — typically spanning multiple drive cycles — the module logs B1739 and illuminates the check engine light. The stored freeze frame data captures exactly what was happening at that moment: engine speed, load percentage, coolant temperature, and vehicle speed. This snapshot proves invaluable during diagnosis because it tells you the precise conditions under which the fault appeared.

Code B1739 essentially tells you one thing: somewhere in the mirror horizontal switch circuit, voltage is bleeding where it should not be. The culprit could be as simple as a faulty switch or as involved as a shorted wire inside the door harness. Understanding the circuit's operation helps you approach the diagnosis systematically rather than guessing blindly at parts.

Symptoms of B1739

The most obvious sign that B1739 has been stored is the dashboard warning light. Depending on your vehicle's configuration, this may appear as a generic check engine lamp or a body-system-specific indicator. Beyond the light itself, here is what you might experience:

The driver's side mirror fails to adjust horizontally when you operate the switch, or the movement feels sluggish and unreliable
The mirror adjustment works intermittently, functioning normally one moment and refusing to move the next
Other body functions on the same circuit — such as power folding mirrors or integrated turn signals — may also behave erratically
In most cases, engine performance, transmission operation, and drivability remain completely unaffected
The mirror may stop in one position and cannot be repositioned until the underlying fault is resolved

Common causes of B1739

Diagnosing B1739 effectively requires understanding the typical failure points in this circuit. The causes range from straightforward electrical issues to more complex module problems:

A blown fuse or failed relay feeding the mirror circuit — this ranks as the simplest and most frequent cause
The mirror adjustment switch itself has failed internally, causing an unwanted short to battery
The horizontal positioning motor within the mirror housing has shorted out
The body control module has developed a fault and is misinterpreting switch inputs
Wiring insulation has degraded, particularly in the door jamb boot where wires flex constantly
Moisture intrusion has corroded connectors or terminal pins in the mirror circuit
Connectors have loosened from vibration, creating intermittent high-resistance connections

How to diagnose B1739

Effective diagnosis follows a logical progression from simple checks to more involved testing. Follow these steps in order:

Connect an OBD2 scanner to the diagnostic port, retrieve B1739, and record the freeze frame data. Note the conditions — rpm, load, temperature, speed — when the code was set. This information guides your inspection by telling you exactly what was happening when the fault occurred.
Scan for any additional trouble codes. Body electrical faults often generate related codes that point toward the actual root cause. Also search for technical service bulletins specific to your make, model, and year — manufacturers frequently publish known issues and software updates for B-series codes.
Visually inspect all accessible wiring and connectors associated with the mirror circuit. Focus on the connector at the mirror itself, the harness passing through the driver door jamb, and the connection to the body control module. Look for damaged insulation, broken wires, corrosion on terminals, and any signs of moisture contamination.
Use your scanner's live data function to monitor the mirror switch and motor PIDs while operating the adjustment control. Observe whether the module sees the expected voltage changes when you press the switch. This test tells you whether the problem lies in the switch, the motor, or the wiring between them.
Pay close attention to flex points — particularly the door hinge area and any locations where wiring passes through sheet metal. Wires break at these stress points first because they endure constant movement. Gently tug on suspect wires to check for intermittent opens.
Clear B1739 after your inspection, then complete a drive cycle that reproduces the conditions captured in the freeze frame data. If the code returns, you have confirmed an active fault and can proceed with targeted component testing or replacement.

Repair options and costs for B1739

Addressing B1739 typically involves one of three repair paths, depending on what testing reveals:

Switch replacement — If the mirror adjustment switch has failed internally, replacing it resolves the short condition. This work may involve removing door panel trim or steering column covers to access the switch assembly. Parts and labor typically run $120–$450.

Body control module service — When the BCM itself has failed, repair or replacement becomes necessary. BCM issues often affect multiple functions beyond just the mirrors. Module replacement requires programming to match your vehicle's configuration. Parts and labor generally cost $300–$900.

Wiring and connector repair — Damaged harness sections, corroded connectors, or broken wires at flex points can be repaired or replaced. This work may involve splicing, connector renewal, or full harness replacement depending on the extent of damage. Parts and labor typically range from $80–$250.

Total repair cost for B1739

The actual cost to fix B1739 depends entirely on which component has failed. Simple fixes like a blown fuse or a faulty switch sit at the lower end of the spectrum, while BCM replacement represents the most expensive scenario. Based on typical independent shop labor rates, you should expect to pay somewhere between $80 and $900 total. The most practical approach: invest time in accurate diagnosis before purchasing parts. Replacing the BCM when the real problem is a corroded connector wastes money and creates unnecessary labor. Most B1739 failures trace back to the switch or a simple wiring issue rather than module failure.

Can you drive with B1739 active?

B1739 is classified as a short-duration fault with minimal impact on vehicle drivability. In most circumstances, you can continue operating the vehicle normally while the code is active. The mirrors may not adjust, but engine performance, braking, and steering remain unaffected.

However, consider your specific situation. If the short affects integrated functions like puddle lamps, turn signal mirrors, or heated mirror elements, those features will not work until repairs are made. In adverse weather conditions, non-functioning heated mirrors could reduce visibility. Additionally, if the fault somehow interferes with central locking, your vehicle security could be compromised. These scenarios represent exceptions rather than the rule — most drivers experience no operational concerns beyond the illuminated warning light and non-adjusting mirrors.