Novak Conversions Jeep Wrangler JK shifters

C0021 Brake Booster Performance code after replacing booster

C

Cosmo5754

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Apr 7, 2026
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Michigan
I recently replaced the power brake booster, brake booster vacuum sensor, brake light switch, and air pump. After completing this I did a brake flush and bled all of the calipers. I am still getting a stiff brake pedal and a C0021 code. After replacing the brake booster with a second new unit there was no improvement.

While on the test drive after the second brake booster I got a U0414 code and the car started acting sluggish. Car seems to be in limp home mode and the transmission refused to let me shift it using the auto-stick, but I confirmed that the 4WD is still functional. After letting it sit for an hour that has gone away.

I have replaced everything in the power braking system minus the master cylinder so I don't really know where to go from here. Thank you!

*Edit* Before replacing the parts I had the C0021 code and the stiff brake pedal. I replaced the vacuum pump and brake booster at that time problem was solved minus the brake light switch but after replacing that everything worked. It then gave a code for the yaw rate sensor so I also replaced that. Then the car was down for a bit for a failed head gasket so after finishing that job the brake issues returned.

2014 Wrangler Unlimited Sahara 3.6
 
Last edited:
What were the issues BEFORE you replaced the parts?

By stiff pedal you mean no power boost?

I’d be looking at the vacuum lines and check valves for a vacuum leak.
 
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Mopar said:
What were the issues BEFORE you replaced the parts?

By stiff pedal you mean no power boost?

I’d be looking at the vacuum lines and check valves for a vacuum leak.
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Before replacing the parts i had the c0021 code and the stiff brake pedal. I replaced the vacuum pump and brake booster at that time problem was solved minus the brake light switch but after replacing that everything worked. It then gave a code for the yaw rate sensor so I also replaced that. Then the car was down for a bit for a failed head gasket so after finishing that job the brake issues returned.

I believe the stiff pedal means no power boost but this is my first time dealing with brake stuff. When I push the pedal not much happens and it feels like the pedal is pulsing. It is also quite hard to push the pedal.

I checked all of the vacuum lines for leaks and none of them seem to be leaking. I forgot to mention that the T fitting that connects the vacuum pump, brake booster check valve, and intake manifold broke when removing it. I picked up a new one at autozone and added hose clamps to make sure it wouldn't leak.
 
I’d be looking at the check valves between the booster and the engine and the booster and vacuum pump.

Pull the vacuum line to the booster off with the engine running. Is there vacuum? Measure it with a vacuum gauge. What’s the reading?
 
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