C0021: Brake Booster Performance
A generic chassis trouble code, same meaning on every brand.
Only if the pedal still feels completely normal - and then only to a shop. If the pedal has gone hard or stopping takes real leg force, do not drive it: that is a tow, because the next stop may take more distance than you have.
Symptoms
- Brake pedal harder than usual
- Longer stopping distances
- Brake warning light
- Hissing or a whining vacuum pump on some cars
Likely causes, most common first
- Vacuum or hydraulic brake booster losing performance
- Failed booster vacuum pump on engines without manifold vacuum
- Leaking booster diaphragm or check valve
- Sensor reporting genuine low boost pressure
Read C0021 yourself before paying for diagnosis
A basic Bluetooth OBD-II scanner ($20-40) plugs into the port under the dashboard and shows this code with live sensor data on your phone - useful both for confirming the fix and for talking to a shop from an informed position.
What is going on and how it gets diagnosed
C0021 says the brake booster - the assist that turns your leg's effort into real clamping force - is not delivering the boost the system expects. Without assist the hydraulics still work, but pedal effort multiplies several times over, and drivers who have never felt an unboosted pedal routinely describe it as 'the brakes failed' even though, technically, they were still there.
Where the boost comes from decides the suspects. Traditional engines feed the booster manifold vacuum through one hose and check valve - cheap parts that crack with age and are checked in minutes. Direct-injection, turbo and hybrid designs often make little vacuum and run an electric pump for the booster; those pumps wear out and are this code's classic cause on such cars. Either way the rule from the verdict stands absolute: a changed pedal ends the drive. Brakes are the one system where 'it still sort of works' is not a plan.
Repair options and cost
Typical US independent-shop ranges, parts and labor. Dealers run higher; regional labor rates vary.
Owner reports mentioning C0021
“My brakes stoped working yesterday. The petal went to the floor and there were NO brakes. I was able to put the car in park and stop its movement. The vehicle is at the dealership. This is what they told me- BCSM AND ELECTRONIC BRAKE BOOSTER FAILURE. C0021”2023 Jeep Grand Wagoneer · Sep 2025
“17427 miles 68143491aa control anti-lock brake system replaced 68347098ab module anti-lock brake system replaced code found c0021-01 replacement of the hcu and module. 12/30/2019 At 80,460 mi same code and information appeared again. 12/23/2022 No ABS brakes /no speed control continues currently 111,400 miles an indicator lights stay on.”2018 Ram 2500 · Aug 2024
“Electrical short on brake booster performance error code c0021 while operating vehicle . Brake pedal became stiff and higher than where I had it . Vehicle making hissing noises and will no longer start due to this error. Safety concern this is not part of a recall list. I nearly crashed. I had to utilize emergency brake.”2019 GMC Sierra 1500 · Nov 2023
Show 6 more owner reports
“I received P258B warning and further diagnosis came to know that C0021-02 and C1252-92 warnings. While applying breaks i am getting hissing noise and break pedal pushes me back where i have to apply lot of pressure to stop the vehicle. This is happening only when i press the pedals hard. I did diagnosis and noticed that the break vaccum pump fuse gone and replaced the same again it blown. Then realized that that the Vaccum pump it self is not working and the same is ordered and replaced. Now the vaccum pump issue is fixed but still the break pedals are hard and getting hissing noise while applying breaks. Now i may need to change the break booster to get this fixed.”2017 Chrysler Pacifica · Jul 2023
“Upon inspection, Power Brake Booster failure, my safety is at Risk due to not having brakes, the issue it has been a confirmed in older models from 2013-2014, recall R63, it has been inspected by an expert and a message pops saying Service Electronic Brake System with code C0021”2015 Dodge Dart · Mar 2023
“In july 2020 the service electronic braking system and service antilock braking system lights and messages started to randomly appear and go away on the dashboard at around 24,000 miles the dealer changed the abs module and controller under warranty on aug 07 2020. Now at 67000 miles on Feb 01 2023 the same messages are happening with code c0021-01 this will be the second brake control module that is failing that will have to be replaced on this vehicle another guy I work with has experienced the same failure on a 2016 ram 2500 around 30000 miles and we are told parts are on back order”2018 Ram 2500 · Feb 2023
“Purchased new vehicle with less than 4,000 miles was running fine when parked. When it was cranked to go home listed esc failure, service steering column lock, trailering system failure, lane departure disabled, speed limited to 60 mph. I was able to get home, but the brakes were so hard that it was extremely difficult to stop. Local dealer it was purchased through "fixed it" less than 1,500 miles later I'm having the same issue! Obd has the codes c0021, c0021, p25a2, p25a2, p0606. I do not feel safe driving this truck! If that issue were to happen approaching an intersection I would have no way of stopping in time. Ridiculous.”2019 Chevrolet Silverado 1500 · Jan 2020 · 1,500 mi
“After remote start mychev app vehicle stationary four dash messages service esc service trailer brake brake system failure - 62 mph top speed steering assist reduced drive with care  lights on dash that were illuminated ABS check engine brake traction control tried to drive. Brakes and steering limited could not drive unsafe turned off ignition restarted threw five codes including p0606 c0021 p25a2 called onstar, they said all systems showing failed take to dealer called gm- they said to have it towed remote start again- same messages dealer appt tomorrow.”2019 Chevrolet Silverado 1500 · Jan 2020 · 13,000 mi
“Tl* the contact owns a 2019 chevrolet silverado 1500. While driving 60 mph, the check engine, stability control, traction control, and ABS warning indicators illuminated. Also, the "limited to 62 mph" message was displayed and the vehicle did not respond when the brakes were applied. The contact had to downshift for the vehicle to stop. The contact called onstar and was informed that codes: 925a2, p0606, and c0021 regarding the ecm, charging system, air bags, and transmission were detected. The vehicle was taken to gilbert chevrolet (3550 us-441, okeechobee, fl 34974, (863) 261-8947), but was not diagnosed or repaired. The failure mileage was 423.”2019 Chevrolet Silverado 1500 · Oct 2019 · 423 mi
Cars that report C0021 most often
Based on mentions of this code in NHTSA owner complaints.
Frequently asked questions
Can I drive with C0021?
Only if the pedal still feels completely normal - and then only to a shop. If the pedal has gone hard or stopping takes real leg force, do not drive it: that is a tow, because the next stop may take more distance than you have.
How much does it cost to fix C0021?
Typical US repair costs range from $40 for the simplest fix (vacuum hose and check valve service) to $1,200 in the worst case. The right number depends on which cause your car has - see the causes list.
Related codes
The codes closest to C0021. They often appear together, and telling them apart is usually what narrows the diagnosis.
Definition per SAE J2012 standard nomenclature. Symptoms, causes and costs are editorial guidance - always confirm with a diagnosis of your specific vehicle.