B1738 — Mirror Driver Horizontal Switch Circuit Open

The diagnostic trouble code (DTC) B1738 indicates an issue with the driver’s side mirror horizontal adjustment switch circuit. Essentially, the vehicle’s electronic control unit (ECU) detects an open or malfunctioning circuit within the switch responsible for adjusting the mirror horizontally. Note that specific labels or code descriptions may vary between different vehicle manufacturers, but the underlying issue remains similar.

System affected: Mirror Adjustment / Body Control Module
Severity: Moderate; can impact driver visibility
Main symptoms: Non-responsive mirror adjustment switch, warning lights
Driveability: Limited — the issue does not typically prevent driving but affects mirror operation

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Manufacturer variations for trouble code B1738

Depending on the vehicle manufacturer, the fault code for issues related to the driver’s side mirror horizontal switch circuit might be labeled differently. It is important to consult the specific OEM (Original Equipment Manufacturer) documentation for exact code definitions. Known variations include:

Ford: B1738 — « Mirror Driver Horizontal Switch Circuit Open »
Mercedes-Benz: B1738 — « Left Side Mirror Adjustment Circuit Fault »
Volkswagen / Audi: B1738 — « Mirror Adjustment Control Circuit Open »
BMW: B1738 — « Mirror Adjustment Switch Circuit Malfunction »
Toyota: Similar code structure, sometimes hidden within broader mirror or body control modules

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What does trouble code B1738 mean?

This OBD-II code signifies that the vehicle’s Electronic Control Unit (ECU), which manages various electronic functions, has detected an open circuit or malfunctioning condition within the driver’s side mirror horizontal adjustment switch circuit. Essentially, the ECU monitors the electrical continuity of the switch wiring. When it detects that the circuit is broken or not completing correctly—meaning there’s an open somewhere along the wiring path—it triggers this fault code.

Typically, the appearance of this code occurs when the driver attempts to adjust the mirror, but the movement does not occur, or when the vehicle’s diagnostic system detects irregularities during system checks. It may happen after an incident where the mirror was hit or due to age-related wiring wear and tear.

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Severity and risks of trouble code B1738

The severity of B1738 is generally considered moderate. While it does not threaten immediate safety or the ability to drive, it impacts a key aspect of vehicle safety and driver convenience—mirror adjustability. If left unaddressed, it could lead to poor visibility, increasing the risk of accidents.

Potential risks include incomplete or incorrect mirror positioning, which can impair side visibility, especially in traffic or reversing maneuvers. However, the overall driveability of the vehicle remains unaffected, and it is often safe to drive in the short term.

The latest insight is: Limited — the vehicle can typically still be driven, but the mirror functions may be compromised until repaired.

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Symptoms of trouble code B1738

When a B1738 fault occurs, several signs may become evident, though they can vary depending on the vehicle model. Common symptoms include:

Non-responsive mirror adjustment switch: Attempting to move the mirror horizontally results in no movement.
Warning lights or messages: Occasionally, the vehicle’s dashboard may display a warning related to mirror or body control systems.
Unusual electrical behaviour: Flickering or intermittent operation of mirror adjustment functions.
Physical damage or misadjustment of the mirror following an impact or accident, which can sometimes trigger the fault.

It’s important to note that these symptoms are often subtle, and the main indicator is the diagnostic trouble code stored in the vehicle’s ECU.

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Most likely causes of trouble code B1738

Diagnosing the root cause of B1738 involves exploring common issues that could trigger this circuit-related fault. Below are the most probable causes, listed from most to least likely:

Open circuit or broken wiring: Damage or disconnection in the wiring harness that connects the switch to the ECU or control module.
Faulty mirror horizontal adjustment switch: The switch itself may be defective or worn out due to age or impact.
Corrosion or poor electrical contact: Corrosion at connectors or poor connections can result in inconsistent electrical flow, mimicking an open circuit.
Faulty or damaged driver’s side mirror motor or mechanism: While primarily a mechanical issue, a defective motor with wiring issues can mimic circuit open faults.
Control module or ECU malfunction: Less common but possible if internal circuitry fails or if there’s a software glitch.

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How to diagnose trouble code B1738

Diagnosing B1738 requires a systematic approach to identify whether the problem is electrical, mechanical, or related to the switch itself. Here are key steps:

Visual Inspection: Check the wiring harness leading to the mirror switch for any visible signs of damage, corrosion, or disconnection. Inspect connectors for corrosion or looseness.
Check the fuse: Verify the relevant fuse for the mirror adjustment circuit isn’t blown, as this can cause circuit open issues.
Live Data / Parameter Check: Use a compatible scan tool to access live data from the body control module or mirror controller, verifying if the switch is being detected and functions as expected.
Continuity Tests: With an ohmmeter, test continuity across the wiring circuit when the switch is operated, rejecting open circuits or shorted wiring.
Switch Functionality: Remove the switch, if accessible, and test its operation independently or replace temporarily with a known-good unit.
Inspect Mirror Motor and Gear Mechanism: Confirm mechanical functionality of the mirror motor, ensuring that it is not mechanically jammed or damaged.

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Possible repairs for trouble code B1738

Once the cause has been identified, repairs can be tailored to address the specific issue. Most repairs tend to vary between simple fixes and more complex repairs requiring professional expertise.

Wiring repair or replacement: Restoring broken, frayed, or corroded wiring is a basic repair often performed by a technician.
Connector cleaning or replacement: Ensuring clean and secure connections can resolve contact issues, which is a straightforward task for a professional.
Replacing the mirror adjustment switch: Faulty switches should be replaced, a task recommended to be done by trained technicians.
Replacing or repairing the mirror motor or mechanism: Mechanical issues may require removing and replacing the mirror assembly or motor component.
Control module reprogramming or replacement: Less common but necessary if the ECU or body control module is faulty; often requires professional diagnosis and programming.

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Vehicles commonly associated with trouble code B1738 in Europe

The B1738 code can affect various makes and models, mainly those equipped with electronically adjustable side mirrors. In Europe, common marques include:

Volkswagen Group: VW, Audi, Skoda, Seat — especially models from the mid-2000s onward with electronic mirror controls.

Ford: Focus, Fiesta, Mondeo, Kuga, with models featuring electronic side mirrors.

Mercedes-Benz: C-Class, E-Class, S-Class, particularly those with integrated mirror control systems.

BMW: 3 Series, 5 Series, X Series with electric mirror adjustments.

Toyota: Corolla, Prius, RAV4, among others, with electronic mirrors.

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Frequent mistakes with trouble code B1738

Accurate diagnosis and repair of B1738 can be muddled by common mistakes. These missteps often lead to unnecessary parts replacement or incomplete repairs. Some frequent mistakes include:

Assuming the switch is always defective without inspecting wiring: Electrical connections can often be the culprit, and replacing the switch may be unnecessary.
Ignoring visual inspection: Overlooking obvious wiring damage or corrosion can prolong the problem.
Replacing parts without proper diagnosis: Swapping the mirror motor or control module without confirming the cause can be costly and ineffective.
Failing to verify the fuse or power supply: A blown fuse or power issue might be the root cause but is sometimes neglected.
Rushing diagnosis without live data access: Relying solely on visual checks can miss intermittent or electronic control issues.

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FAQ — trouble code B1738

Can this code disappear on its own? In most cases, no. Once the wiring or switch fault is present, the code will typically remain until repaired or replaced. Temporary issues, like loose connectors, might sometimes clear if fixed.

Can I keep driving? Yes, generally the vehicle can still be driven, as the primary concern is the non-functionality of the mirror adjustment. However, for safety reasons, it’s advisable to repair the issue promptly.

Why does the code return after clearing? This suggests the underlying problem persists, such as damaged wiring, a faulty switch, or an electrical connection issue. Without addressing the root cause, the fault will likely recur.