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B1812 OBD-II Trouble Code: Lamp Backup Switch Input Circuit Short to Battery

diagnostic check engine light

B1812 code definition

The B1812 trouble code detects a lamp backup switch input circuit malfunction.

What the B1812 code means

The B1812 code is a generic OBD-II trouble code that reports an input error from the lamp backup switch circuit. This system is responsible for the backup lights, which are safety lights on the back of the vehicle that are turned on when the vehicle is placed in reverse. When the gear shifter is placed into the reverse position, an input signal is sent to the lamp backup switch circuit, and the backup lights are turned on. If the powertrain control module (PCM) or the body control module detect a voltage reading from the lamp backup switch input circuit that is not normal, then the B1812 code may be stored.

What causes the B1812 code?

Possible causes of the B1812 trouble code are:

What are the symptoms of the B1812 code?

When the B1812 code is detected, the backup light system likely will not work. This means that when the vehicle is in reverse, the safety backup lights will not turn on.

How does a mechanic diagnose the B1812 code?

The B1812 code will be diagnosed with the help of a standard OBD-II trouble code scanner. A certified mechanic can use the scanner to look at the freeze frame data and assess the code. The mechanic will also look for the presence of any additional trouble codes, as codes should always be diagnosed in the order in which they appear in the scanner. The technician will then reset the trouble codes, restart the car, and look to see if the B1812 code is still detected. If not, then it was likely triggered inaccurately, or is an intermittent error.

If the code remains detected, then the mechanic will place the car in the reverse shift position, and check the lights. If the vehicle has multiple backup lights, and only some of them are not turning on, then a blown bulb, or a possibly a damaged bulb assembly are to blame. If all of the lights fail, then the mechanic should check for blown fuses, and then visually inspect the wires and connectors. If all of these components are in working order, the voltage to and from the circuit will be checked, followed by an inspection of the lamp backup bulb switch.

After any component is replaced, the technician will reset the trouble codes, restart the vehicle, and see if the B1812 code is still detected. By doing this, the mechanic will know as soon as the issue has been resolved.

Common mistakes when diagnosing the B1812 code

The most frequently made errors when diagnosing the B1812 code come from a failure to properly follow the OBD-II diagnosis protocol. Mechanics should follow this protocol at all times, step by step, to help make sure that inspections are thorough, and repairs are accurate.

The B1812 code is frequently accompanied by other trouble codes, and often mechanics fail to address these codes in the order that they appear in. If there are multiple circuitry trouble codes, there’s a strong chance that a single blown fuse is the issue, and this is often overlooked. It is also common for fully operational backup light bulbs to be replaced because a proper diagnosis was not performed.

How serious is the B1812 code?

The B1812 trouble code will usually mean that the reverse lights will not work. These lights are an important safety feature, so the vehicle should be inspected when the code is detected.

What repairs can fix the B1812 code?

Repairs for the B1812 trouble code include:

  • Replacement of fuses
  • Lamp backup switch circuit replacement
  • Lamp backup switch replacement
  • Repair or replacement of wires and connectors
  • Backup light bulbs replacement
  • Bulb assemblies replacement

Additional comments for consideration regarding the B1812 code

The B1812 code is a manufacturer specific trouble code, which means that it only pertains to certain automakers. Some automakers have a different definition for the B1812 code. It is important that mechanics always consider the make of the vehicle they are working on when diagnosing the B1812 code.

Need help with a B1812 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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