Burner is locked out with fault code F7 on Vitodens 100

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Symptoms:

The burner is locked out, and fault code "F7" is displayed on the boiler control.

Boiler water pressure is 12 psig or greater, but the boiler controller is reading pressure lower than the permissible pressure, resulting in an F7 fault.

Cause:

Faulty pressure sensor, which could be caused by damage to the sensor harness or a bad connection at the harness.

Resolution:

Measure the voltage on X9 terminals 1 and 2 to see what the sensor is measuring for boiler pressure – review section "Testing the water pressure sensor" below for details. If the pressure reading is lower than the actual pressure, then replace the pressure sensor.

If the sensor harness is damaged, then replace the harness.

If the problem persists after replacement, then contact Viessmann for a sensor bypass kit.

Testing the water pressure sensor

The following image displays the X9 plug on the boiler control, as viewed from the front.

Measuring between terminals 1 and 2 will reveal the return voltage from the sensor transducer to the control – this voltage depends on the boiler pressure (see graph below).

Measuring between terminals 1 and 3 will reveal the voltage going to the sensor transducer from the control – normally this should be 5 VDC.

Pressure sensor graph