Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSJEEP2009WRANGLER SAHARA, AUTOMATICREPAIR AND DIAGNOSIS (SINGLE PAGE)BRAKESANTI-LOCK BRAKESANTI-LOCK BRAKE SYSTEM - ELECTRICAL DIAGNOSTICSDTC TROUBLESHOOTINGC121D-BRAKE PRESSURE SENSOR CIRCUITDIAGNOSTIC TEST
2009 Jeep Wrangler Sahara, Automatic
Diagnostic Test
2009 Jeep Wrangler Sahara, AutomaticSECTION Diagnostic Test
- CHECK FOR A DTC C121D-BRAKE PRESSURE SENSOR CIRCUIT NOTE: This DTC must be active for the results of this test to be valid.
- Turn the ignition on.
- With the scan tool, read and record DTCs.
- With the scan tool, read and record Freeze Frame information.
- With the scan tool, erase DTCs.
- Start the engine.
- Slowly depress and release the brake pedal.
- With the scan tool, read and record DTCs.
Does the scan tool display: C121D-BRAKE PRESSURE SENSOR CIRCUIT?
Yes
- Go to step 2
No
- Perform ABS INTERMITTENT CONDITION diagnostic procedure. See DTC-Based Diagnostics/MODULE, Antilock Brake (ABS) - Standard Procedure .
- HCU/ECU ISOLATION
- Turn the ignition off.
- Separate the ECU from the HCU.
- With the scan tool, monitor brake pressure.
Does the scan tool display brake pressure above 218 bar (3162 psi)?
Yes
- Replace the HCU in accordance with the Service Information.
- Perform ABS VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Antilock Brake (ABS) - Standard Procedure .
No
- Replace the ECU in accordance with the Service Information.
- Perform ABS VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Antilock Brake (ABS) - Standard Procedure .
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.