Possible causes are switch, wiring or pcm. Most likely a switch.
From dealerconnect:
TEST ACTION APPLICABILITY
1 NOTE: Verify battery voltage is greater than 10 volts.
Record Freeze Frame Data that was set by the related DTC before continuing.
With the DRBIIIt, read DTCs and record the related Freeze Frame data.
Is the Good Trip Counter displayed and equal to zero?
All
Yes ! Go To 2
No ! Refer to the INTERMITTENT CONDITION symptom in the
Diagnostic category.
Perform POWERTRAIN VERIFICATION TEST VER - 5 - NGC.
2 Turn the ignition off.
Remove the Brake Lamp Switch and disconnect the harness connector.
Measure the resistance between the (Z1) Ground circuit terminal and the (K29)
Brake Switch Signal terminal at the Brake Lamp Switch.
Apply and release the brake pedal plunger while monitoring the ohmmeter.
Does the resistance change from below 5.0 ohms to an open circuit?
All
Yes ! Go To 3
No ! Replace the Brake Lamp Switch.
Perform POWERTRAIN VERIFICATION TEST VER - 5 - NGC.
3 Turn the ignition off.
Disconnect the PCM harness connectors.
Measure the resistance between ground and the (K29) Brake Switch Signal circuit in
the Brake Lamp Switch harness connector.
Is the resistance below 100 ohms?
All
Yes ! Repair the short to ground in the (K29) Brake Switch Signal
circuit.
Perform POWERTRAIN VERIFICATION TEST VER - 5 - NGC.
No ! Go To 4
4 NOTE: Before continuing, check the PCM harness connector terminals for
corrosion, damage, or terminal push out. Repair as necessary.
Using the schematics as a guide, inspect the wire harness and connectors. Pay
particular attention to all Power and Ground circuits.
If there are no possible causes remaining, view repair.
All
Repair
Replace and program the Powertrain Control Module in accordance
with the Service Information.
Perform POWERTRAIN VERIFICATION TEST VER - 5 - NGC.
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