GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2007 🢒 STS 🢒 DTC 🢒 DTC B3121

Circuit Description

The keyless entry antenna - rear compartment is located in the rear compartment area and is used as a short range low frequency (LF) antenna. As a vehicle with a closed rear compartment lid is approached with a keyless entry transmitter and the exterior rear compartment lid release pad is pressed, the keyless entry antenna broadcasts a challenge to the transmitter in an approximate 1 meter range. If the response from the transmitter via radio frequency (RF) is valid, the latched rear compartment will be passively unlatched, allowing the rear compartment lid to be opened.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B3121 Keyless Entry Antenna 3 Performance

DTC Symptom

DTC Symptom Descriptor

02

Short to Ground

05

Short to Battery or Open

Conditions for Setting the DTC

The remote control door lock receiver (RCDLR) will set this DTC during antenna authentication if the keyless entry antenna - rear compartment is open, shorted to battery, or shorted to ground. The RCDLR can only set a DTC when it is the vehicle system voltage is between 9-16 volts.

Action Taken When the DTC Sets

The RCDLR will set and store a DTC B3121.

Conditions for Clearing the DTC
    • The RCDLR no longer detects a open, short to power, or short to ground on the antenna circuit.
    • The use of a scan tool

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Verifies that DTC B3121 is a current DTC.

    2.

  2. Verifies that the exterior rear compartment lid release pad is operating properly. A valid input from the lid release pad is required by the RCDLR to authenticate the antenna.

    3.

  3. Testing for power and ground to the keyless entry antenna.

Step

Action

Yes

No

Schematic Reference: Remote Function Schematics

Connector End View Reference: Remote Function Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Important: To achieve service by pass mode, key ON, engine OFF, a programmed transmitter must be detected within the passenger compartment of the vehicle.

  1. Install a scan tool.
  2. Push and hold the ACC button for 5 seconds to achieve the service bypass mode, with the engine OFF.
  3. Select display DTCs function on the scan tool in the remote control door lock receiver (RCDLR) menu.

Does the scan tool display DTC B3121 as a current DTC?

Go to Step 3

Go to Testing for Intermittent Conditions and Poor Connections

3

  1. With the scan tool, observe the trunk release switch parameter in the RCDLR miscellaneous inputs data list.
  2. Operate the exterior rear compartment lid release pad.

Does the trunk release switch state display INACTIVE?

Go to Trunk Release Inoperative

Go to Step 4

4

Important: 

   • The voltage signal sent by the RCDLR to the passive antenna is a AC voltage signal. The DMM must be set on AC voltage setting to record the voltage pulse to the passive antenna.
   • Use a DMM to record the voltage pulse using the MIN MAX setting to capture the AC voltage pulse to the passive antenna.

  1. Disconnect the keyless entry antenna - rear compartment at the rear compartment latch area.
  2. With the DMM measure/capture for an AC voltage signal between the keyless entry antenna signal circuit and the low reference circuit.
  3. With the scan tool, go to the RCDLR special functions menu. Select Rear Fascia Antenna.
  4. Authenticate the antenna using the scan tool.

Was an AC voltage signal captured on the DMM?

Go to Step 7

Go to Step 5

5

Test both the keyless entry antenna signal and low reference circuit for an open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 6

6

Inspect for poor connections at the RCDLR. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 8

7

Inspect for poor connections at the keyless entry antenna - rear compartment. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 10

8

Test the keyless entry antenna signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 9

9

Replace the RCDLR. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 12

--

10

Test both the keyless entry antenna signal and low reference circuit for a short to power. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 10

11

Replace the keyless entry antenna - rear compartment. Refer to Ignition Lock Key Transmitter Antenna Replacement - Rear Compartment .

Did you complete the replacement?

Go to Step 12

--

12
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the conditions for running the DTC as specified in the supporting text.

Does the DTC reset?

Go to Step 3

System OK