C0021-Brake Booster Performance

C0021 code - When the Vacuum Pressure Sensor indicates a failure in the vacuum booster. After the cold start phase (3 min after engine is running) and if the engine is running above 600 RPM there should be at least -200 mbar (-2.90 psi) of differential pressure in the vacuum-chamber for boosting the pedal force. If the pressure remains below this value for more than two minutes. Timer resets if the pressure remains above 220 mbar (3.19 psi) for 10 seconds.


Possible causes C0021 code

  • Pinched, cut or open vacuum supply line.
  • Brake Booster.
  • Vacuum pressure sensor.

Diagnose C0021-Brake Booster Performance

Step 1. Verify the DTC is active
Turn the ignition on, with the scan tool, read and record the DTC. With the scan tool, read and record Environmental Data to help identify the conditions in which the DTC was set. With the scan tool, erase the DTC, start the engine, press and release the brake pedal. Wait five minutes or until red brake lamp illuminates, with the scan tool, read and record the DTC.

Is the DTC active at this time ?

Yes, go to step 2.

No, check the vacuum supply line to brake booster for a pinched, cut, loose or cracked vacuum hose which could cause an intermittent condition. Perform the ABS Intermittent condition.

Step 2. Check Vacuum supply line
Turn the ignition off, check the vacuum supply line to brake booster for a pinched, cut, loose or open vacuum hose.

Were any problems found ?

Yes, repair as neccesary.

No, go to step 3.

Step 3. Check for Vacuum pressure signal change
Turn the ignition on, remove the vacuum pressure sensor from the booster. Block off the vacuum feed port, connect a vacuum pump to the vacuum pressure sensor. Apply vacuum to the vacuum pressure sensor. Using the scan tool, and a pressure gauge watch for the signal change, using the scan tool, watch for signal change.

Is the signal changing and does it compare to the pressure gauge reading ?

Yes, go to step 4.

No, replace the vacuum pressure sensor.

Step 4. Check brake booster check valve
Remove the brake booster check valve, apply vacuum to the booster side of the check valve.

Is the brake booster check valve holding vacuum ?

Yes, replace the brake booster.

No, replace brake booster check valve.

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