C0021 on 2019-2020 GMC Sierra 1500: Brake Booster Performance Fault Causes and Fixes

What's Unique About the 2019-2020 Gmc SIERRA 1500

For this specific generation of GMC Sierra and its platform-mate, the Chevrolet Silverado, code C0021 is overwhelmingly linked to software problems rather than mechanical failure. General Motors has issued multiple Technical Service Bulletins (TSBs) and recall campaign N192268090 related to software errors in the Brake System Control Module that can incorrectly trigger this code. TSB PIT5728D even addresses a scenario where the initial recall fix was flawed and caused C0021 to appear, necessitating another reprogram. Unlike many other vehicles where C0021 would immediately suggest a bad brake booster, the first diagnostic step for these trucks is always to check the vehicle's software and recall status.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Hard or stiff brake pedal.
• Increased effort needed to stop the vehicle.
• "Service Brake Assist" message on the Driver Information Center (DIC).
• Check Engine Light (SES MIL) is on.
• "Service Brake System", "Reduced Engine Power", and/or "Speed Limited" messages on the DIC.
• Hissing noises coming from the engine bay near the firewall when the brake pedal is pressed, indicating a possible vacuum leak.

Most Likely Causes

1. Brake System Control Module (BSCM) Software Requires Update 🔴 High Probability GM has documented numerous software-related issues. TSBs #PIT5730A, #20NA073, and #PIT5728D all point to software calibrations as the cause. This can be triggered by a previous recall fix (N192268090), remote start via the MyGMC app 🎬 Watch: How a simple software reset can fix this error., or other electrical anomalies during module initialization.
   How to confirm: A GM dealer or a shop with a Techline Connect subscription can check the vehicle's VIN against open recalls (like N192268090) and TSBs. The presence of code P0606 alongside C0021 is a very strong indicator that a software update is needed.
   Typical fix: Reprogram the Brake System Control Module (BSCM) with the latest updated software calibrations available on GM's Tis2Web. This repair may be covered under warranty or a specific customer satisfaction program.
   Est. part cost: $0-$300
2. Failed Power Brake Booster 🟡 Medium Probability While less common than software issues on these years, the brake booster is a mechanical part with a rubber diaphragm that can fail, causing a vacuum leak and loss of assist. Some owners have reported actual booster failures leading to a hard pedal and hissing sounds.
   How to confirm: Listen for a distinct hissing sound when the brake pedal is pressed. With the engine off, pump the brakes to deplete vacuum, then start the engine while holding the pedal; if the pedal doesn't drop slightly, the booster has likely failed. A technician can also use a vacuum gauge to test if it holds vacuum, which should be around 15–22 inHg at idle.
   Typical fix: Replace the power brake booster assembly. This is a significant repair that often requires removal of other components for access. 🎬 See this step-by-step walkthrough for replacing the brake booster.
   Est. part cost: $105-$550
3. Sticking Brake Master Cylinder Pressure Sensor ⚪ Low Probability TSB #20-NA-182 notes that on some 2019-2020 GM trucks, specifically the 4500HD/5500HD/6500HD models, a sticking pressure sensor can cause the brake booster pump to run continuously and set a C0021 code. While for HD trucks, it's a known issue in the same family.
   How to confirm: The primary symptom is the brake booster pump motor running constantly, even with the ignition off.
   Typical fix: Replace the brake master cylinder pressure sensor with the updated part, GM P/N 19407420.
   Est. part cost: $50-$100

Rare But Worth Checking

• Vacuum Leak: A cracked or disconnected vacuum hose leading to the brake booster can cause a loss of assist, mimicking a failed booster. TSB 21-NA-045 addresses issues with the power brake booster vacuum pipe routing on some GM trucks, which can lead to similar symptoms and other DTCs. This is a simple but crucial check before replacing expensive components.
• Low Battery Voltage / Post-Battery Change Glitch: These vehicles have sensitive electronic modules. A failing battery or an improper battery replacement procedure can cause voltage drops that trigger a cascade of communication codes, including C0021 and P0606. TSBs for reprogramming specifically warn that stable battery voltage is critical during the process.

Diagnosis Steps

1. Scan all modules, especially the Brake System Control Module (BSCM), for codes. Note if C0021 is present with P0606 or P25A2, as this strongly suggests a software issue.
2. Using the vehicle's VIN, check with a GM dealer or use a professional resource to find open recalls (e.g., N192268090) and applicable TSBs (#PIT5730A, #20NA073, #PIT5728D).
3. If an applicable TSB is found, the primary fix is to have a dealer or qualified shop reprogram the BSCM with updated software. Ensure a stable power supply is used during reprogramming.
4. If no TSBs apply or the reprogram does not resolve the issue, begin mechanical diagnosis.
5. Check battery health and connections. A weak battery can cause spurious module faults on this platform.
6. With the engine running, listen for a hissing sound near the brake booster and its vacuum hose as you press and release the brake pedal.
7. Inspect the vacuum hose from the engine to the booster for any cracks, loose fittings, or damage.
8. Perform a functional test: Turn the engine off and pump the brake pedal 4-5 times to deplete stored vacuum. Hold the pedal down with firm pressure and start the engine. The pedal should drop slightly as vacuum builds. If it does not, the booster or its vacuum supply is faulty.
9. If the brake booster pump runs continuously, investigate the brake master cylinder pressure sensor per TSB 20-NA-182 (primarily for HD models, but worth noting).
10. If the booster is suspected, a technician should confirm the failure with a vacuum gauge before ordering a replacement part.

Parts You'll Likely Need

• BSCM Reprogramming Service — This is the most common documented fix for the C0021 code on this specific vehicle, as per multiple manufacturer TSBs and recall N192268090.
  Trusted brands: GM Dealer Service
  OEM price range: $150-$300
  Aftermarket price range: N/A
• Power Brake Booster (OEM #84472897) — If a mechanical failure is confirmed after ruling out software, the booster itself is the next most likely cause. This part number supersedes 22872386 and 84031701.
  Trusted brands: ACDelco, GM Genuine
  OEM price range: $450-$550
  Aftermarket price range: $200-$350
• Brake Master Cylinder Pressure Sensor (OEM #19407420) — For the specific symptom of the brake booster pump running continuously, as described in TSB 20-NA-182 for HD models.
  Trusted brands: GM Genuine
  OEM price range: $50-$100
  Aftermarket price range: N/A

Related Codes That Often Appear With This One

• P0606 — This code, for 'Control Module Processor Performance,' is frequently set alongside C0021 and is explicitly mentioned in multiple TSBs (PIT5730A, 20-NA-073) as being related to the BSCM software glitch.
• P25A2 — This code means 'Brake System Control Module Requested MIL Illumination.' The BSCM cannot turn on the check engine light itself, so it requests the Engine Control Module (ECM) to do so, setting P25A2 in the process.
• C05D2, C0595, C2A3B, C0024, C053D — TSB #20-NA-073 lists these additional brake system codes as potentially appearing with C0021 due to the same underlying software issue in the BSCM.
• C05B0, C058E — TSB #PIT5638C also links these codes with C0021 and a "Service Brake Assist" message, pointing to a software calibration fix.

Technical Service Bulletins (TSBs) & Recalls

• PIT5730A: Addresses C0021 with P0606, hard brake pedal, and specifies a BSCM reprogram.
• 20-NA-073: Links C0021 with P0606 and a host of other brake system DTCs, again pointing to a software cause and reprogram fix. This TSB was updated multiple times to include more DTCs and vehicle models.
• PIT5728D: Notes that a previous recall fix (N192268090) could cause C0021 to appear, requiring another BSCM reprogram.
• 20-NA-182: Mentions C0021 can be caused by a sticking brake master cylinder pressure sensor on 2019-2020 GM HD trucks, causing the pump to run constantly.
• N192268090: A major safety recall to reprogram the BSCM because a software error could disable the ESC and ABS systems without warning. This is the root of many C0021 issues.

Platform-Specific Known Issues

• A faulty software update related to recall N192268090 caused C0021 and P0606 to set, requiring another reprogram to fix the fix. This was sometimes triggered by using the MyGMC remote start app.

Mechanic-Grade Diagnostic Values

• Brake Booster Vacuum Supply — expected: 15–22 inHg (inches of Mercury) at idle.. Failure: A reading significantly below 15 inHg, or one that drops rapidly when the engine is off and the pedal is pumped, indicates a leak in the booster or supply line.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): BSCM Programming (SPS) — This is the primary service function required to fix the software anomaly that causes C0021 and P0606. It is used to load the updated calibration files from GM's Tis2Web into the Brake System Control Module.
• GDS2 (GM Global Diagnostic System 2): Trailer Brake Type Data Parameter — When diagnosing related trailer brake faults (like C1114), a technician can use a trailer simulator tool (GM P/N EL-52641) and observe this data parameter. For a 2019-2020 Sierra, it should display "Electromagnetic" brakes, confirming the truck's system is operating correctly and the fault is likely with the trailer itself.

Wiring & Ground Locations

• EBCM/BSCM Ground — On the frame under the driver's door area.. A poor ground connection at this location can cause a variety of electrical issues with the brake system, including setting false diagnostic trouble codes. A Reddit user reported a dealer diagnosing a bad ground as the cause for a 'Service Brake System' message on a 2019 Silverado. GM has also issued bulletins for other model years regarding improperly secured ground bolts causing widespread electrical problems.
• EBCM/BSCM Connector — The Electronic Brake Control Module is located on the driver's side frame rail. It has a large, multi-pin connector (often 38-way).. All power, ground, and communication signals for the brake control system pass through this connector. Technicians must ensure it is clean, dry, and fully seated. Pin-to-pin checks would be performed here if a wiring issue is suspected after software and mechanical checks are complete.

Real Owner Repair Stories

• Reddit user in r/gmcsierra (2019 GMC Sierra) — After replacing the battery, the truck displayed a 'Service ETC' message and the check engine light came on. Multiple codes were stored: U3006, P0606, C0021, C0002, and P25A2.
  ❌ Tried (didn't work) A mechanic tried to erase the codes but could not., The owner's friend tried to erase the codes and was also unsuccessful., The GMC dealership was unable to simply reset the light and wanted to perform further diagnosis.
  ✅ What actually fixed it The user did not post a definitive repair, but the story highlights a critical edge case: a simple battery replacement can trigger this specific cluster of non-erasable codes, strongly pointing to the software glitch that requires a BSCM reprogram. The inability for anyone, including the dealer, to simply clear the codes is a hallmark of this problem.

OEM Part Supersession History

• 22872386, 84031701 → 84472897 — Standard part evolution and potential design improvements.
  Heads up: Part 84472897 is the correct replacement for the specified original numbers on this platform.