P21FB: Fuel Control System B Too Rich at Idle (Bank 2)
The secondary fuel control loop B reports a rich condition at idle on bank 2. Idle rich often means injectors that leak at low duty cycle.
What P21FB means
P21FB is a generic OBD-II powertrain code in the fuel & air group. In plain English: the engine is getting too much fuel or too little air, and the computer has run out of room to correct it. Bank 2 is the side of a V or flat engine opposite cylinder 1.
- Check engine light
- Usually on
- Emissions-related
- Yes — can fail inspection
- Limp mode possible
- Not typical
- Drive cycle to clear
- Not required
Get it fixed
Post this code once and get quotes from local mechanics. Free to post.
Makes the repair and parts links below specific to your car. Kept in this browser only.
Symptoms
- Check engine light on
- Increased fuel consumption
- Black smoke from exhaust on acceleration
- Smell of unburned fuel, fouled spark plugs
Common causes
- 01Injectors leak on bank 2Most likely
Can I drive with P21FB?
Usually yes, for a short time.
The car will often drive normally, but get it diagnosed soon. Left alone it can raise fuel use and emissions and will fail an OBD inspection. If you notice rough running, warning lights flashing, overheating or a burning smell, stop driving.
Why we rate it moderate
- Turns on the check engine light
- Emissions-related: will usually fail an OBD inspection
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose P21FB
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just P21FB. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Check for related codes first
Look for P21F9. Related codes often share a cause; a circuit or sensor code usually needs fixing before a performance code like this one.
- 3
Read fuel trims and look for leaks
Compare short- and long-term fuel trims at idle and at 2,500 rpm. Trims that drop as rpm rises point to a vacuum leak; trims that stay high point to fuel delivery or a MAF reading low. Smoke-test the intake for leaks.
- 4
Test the likely causes in order
Start with the most likely cause and work down: Injectors leak on bank 2. Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and drive the car under the conditions in the freeze-frame data to confirm it does not return.
P21FB repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Moderate
- Good DIY job?
- Yes, with a scan tool
Needs a scan tool and some experience with testing.
Parts and systems involved
- Fuel injector
P21FB FAQ
What does P21FB mean?
P21FB means “Fuel Control System B Too Rich at Idle (Bank 2)”. The secondary fuel control loop B reports a rich condition at idle on bank 2. Idle rich often means injectors that leak at low duty cycle.
Is P21FB serious? Can I keep driving?
We rate P21FB as moderate severity. Usually yes, for a short time. The car will often drive normally, but get it diagnosed soon. Left alone it can raise fuel use and emissions and will fail an OBD inspection. If you notice rough running, warning lights flashing, overheating or a burning smell, stop driving.
What causes P21FB?
The most common causes, from most to least likely: Injectors leak on bank 2. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix P21FB?
Typical labor is about $90–$500 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix P21FB myself?
DIY difficulty is moderate. Many owners can handle it with a scan tool, a multimeter and basic hand tools. Always confirm the cause before buying parts.
Will P21FB clear itself?
If the fault was intermittent, the warning may go off once the computer stops seeing it, but the code stays in memory. Fix the cause; clearing the code alone doesn’t repair anything.