U3906 BMW Fault Code

Description:

Catalytic converters must be warm before they can work properly. On a cold start the PCM provides a rich mixture to keep the cold engine running.
All that extra fuel goes into the catalytic converter. But the exhaust isn't very hot, because, ah, the engine is pretty cold. GM has employed several strategies to fire up the catalytic converter quicker and reduce cold start emissions. One way is to use a secondary air pump that pumps outside air directly into the catalytic converter.

U3906 Code Symptoms :

There are currently two main types of cylinder deactivation used today, depending on the type of engine. U3906 code for the pushrod design which uses solenoids to alter the oil pressure delivered to the lifters. In their collapsed state, the lifters are unable to elevate their companion pushrods under the valve rocker arms, resulting in valves that cannot be actuated and remain closed.

Diagnosis And Repairing U3906

U3906 OBD 2 code on vehicles with electronically controlled automatic transmissions, the 3-4 shift solenoid is responsible for actuating the hydraulic circuits to activate clutches or bands that change gears inside the automatic transmission.

How to fix U3906 code ?

• Air fuel ratio (A/F) Sensor 1 Bank 1 harness is open or shorted
• Exhaust tube
• Open or short in the VCT circuit

Some systems require that the actuator has an adequate supply of brake fluid. Low brake fluid levels can mimic the symptoms of code U3906, so consult the manual to locate the brake fluid reservoir.

Bear in mind that a low fluid level can set code U3906 on some applications, so make sure that on fluid operated systems the fluid level is up to the mark before starting an electrical diagnosis.