668 airbag control unit fault

What does error code 668 mean in the airbag control unit?

Error code 668 indicates a critical issue with the vehicle’s onboard voltage at the airbag control unit, specifically at terminal 30 (permanent positive). The system detects that the voltage has occasionally fallen below the minimum threshold. Often, the root cause lies not in the control unit itself but in the vehicle’s external power supply.

Technically, the airbag control unit continuously monitors the voltage at terminal 30. If this voltage drops—such as during engine start—to a critical level like 11 volts, error code 668 is logged in the fault memory. The control unit assumes that safe airbag operation can no longer be guaranteed and preemptively shuts down the entire system, indicated by the illuminated SRS warning light.

Although the fault is classified as intermittent, it may reappear when driving begins (often above 10 km/h) if the power supply remains unstable. This directly affects the fundamental operational capability of the safety restraint system.

How do you recognize error code 668?

The primary symptom of error code 668 is the permanently illuminated airbag warning light (SRS symbol) on the instrument cluster. In many cases, communication with the airbag control unit via an OBD diagnostic tool is also impossible, as the module enters a protective mode. With this error active, the airbag system is completely inoperative.

Another clear sign is that the airbag warning light illuminates immediately after turning on the ignition and does not extinguish even after starting the engine, indicating a persistent, safety-critical fault.

In some instances, additional symptoms may appear, such as the absence of the vehicle identification number in the control unit data or stored crash data that can only be erased after repair. A mandatory technical inspection (TÜV) cannot be passed with the airbag light illuminated.

What causes error code 668?

The main cause of error code 668 is an insufficient or unstable power supply. Most often, this stems from an aging battery whose voltage sags during startup. Other common reasons include corroded fuses, loose ground connections, or faulty relays along the power supply path to the control unit.

A frequently overlooked issue involves electrical connections—loose contacts in relays (costing as little as €5), a loose ground connection on the vehicle body, or oxidized blade fuses directly at the battery terminal. Particularly critical are corroded connector pins on the control unit itself. Specific replacement pins (e.g., VAG part number N 907 326 03) can address corrosion-related damage.

Rarely, the problem originates directly within the control unit. Incorrect coding (for example, in control unit J234), an implausible signal at terminal 30, or internal defects caused by moisture, corrosion, or stored crash data can trigger the error. Incidentally, this error code is not limited to airbag systems; for ABS/ESP control units—such as those found in the Audi A3 8P—the related code 00668 points to an identical voltage issue.

Which vehicles experience error code 668?

Error code 668 is especially prevalent in vehicles from the Volkswagen Group. The first-generation Volkswagen Touareg (7L) and its facelift models are particularly affected. This error code is also diagnosed in certain luxury vehicles, such as the Bentley Corniche Mk4.

At Airbag24, we specialize in repairing airbag control units displaying error code 668 for the following models:

Volkswagen Touareg 7L (model years 2002–2006)

Volkswagen Touareg 7L Facelift (model years 2006–2010)

Bentley Corniche Mk4

How is error code 668 professionally repaired?

Professional repair begins with a targeted diagnosis of the external power supply before removing the control unit. At Airbag24, we inspect each submitted unit, resolve internal voltage‑related issues, replace corroded components such as pins, and erase any stored crash data. Repairs come with 2 years' statutory liability for defects.

First, a thorough vehicle diagnosis is required. Using an appropriate tester (e.g., Gutmann, Bosch, or VCDS), the fault memory is read out. Then, a multimeter measures the voltage directly at the control unit’s connector. Relays, ground connections, and fuses are also checked for corrosion and secure fit. Only after ruling out these external causes can a defect within the control unit be considered.

Following submission to Airbag24, repairs proceed at the component level. The control unit is opened and analyzed. Defective or corroded pins are expertly replaced using specialized tools such as pullers and crimping pliers. If crash data is stored, it is erased, and the original vehicle‑specific coding is restored. After successful repair and testing, the unit is returned to the customer.

WARNING: Using airbag simulators, resistors, or jumpers to disable the warning light is life‑threatening and illegal. The vehicle’s operating permit will be revoked, and no protection will be provided in the event of an accident. Repairs to the airbag system must be performed only by authorized service centers.

Is it Worth Repairing the Control Unit?

Repairing the airbag control unit is almost always the most cost‑effective solution when fault code 668 is present. Compared to a new part, which often costs over 800 euros, repair is significantly cheaper. Moreover, time‑consuming re‑coding is unnecessary, as the original vehicle‑specific unit can be retained and put back into service immediately.

The costs associated with addressing external causes are usually minimal—a relay costs around 5 euros, and new connector pins cost just a few cents. A full repair of the control unit is considerably more affordable than purchasing a new one. In particular, the modules used in the Touareg 7L are known to be highly repairable. Thus, repair is not only economical but also environmentally sustainable.