Fault 6 / Fault Code 21

6: Front left speed sensor Fault code - 21 -

Possible Causes, Elimination, Remarks

No signal from speed sensor reaches control unit, or signal is incorrect / unrealistic.

Warning: this fault is also indicated if the ABS/PSD control unit for the 928 is installed.
If assembly work has already taken place, check that the correct control unit has been installed.




Distinguishing feature: part number.
928 control unit with PSD = 928 618
911 Carrera 4 control unit = 964 618




1. Pull of the control unit plug. Measure the internal resistance / check continuity between PIN 51 and PIN 48 on plug.
Desired value 60...1600 ohm.
If the desired value is not attained, check the wires and plug connections in the circuit from the front left speed sensor. Plug connections: 1 x on left spring strut, 1 x on left spring strut mount in trunk (for distinguishing feature.

If the desired value (60...1600 ohm) is not reached although the wires / plug connections are in working order, renew the speed sensor.






2. Checking the speed sensor signal.

For this check, use ABS 2 - LED tester, or an oscilloscope.

Connect the ABS 2 - LED tester to the control unit plug with the 55-pin adapter line.

If an oscilloscope is used, separate the speed sensor plug connection at the spring strut or spring strut mount in the trunk.

Connect the oscilloscope to the plug at the speed sensor side. Depending on the tester, select special input or secondary image.

Check by turning the left front wheel by hand. ABS tester in program switch position 6 / rotary switch for wheel selection to front left wheel. Turn the wheel until the LED (No.2) lights up without flashing. If an oscilloscope is used, approx. 1 revolution per second.

Desired values / desired value display
ABS 2 - LED tester = > 2.0
Oscilloscope = sine wave > 2 Volt (measured value: peak to peak)

Possible causes of deviations

Air gap at speed sensor is too large / too small (check installation).

Pulse wheel is damaged or corroded.

Damage to wheel bearing (wheel bearings are not adjustable).