Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
This means that the car's computer thinks that one side of the engine is not getting enough fuel when the car is not idling. It's like the engine is trying to run on a diet and it's not getting enough food.
The code indicates that the fuel mixture is too lean, meaning there's too much air and not enough fuel in the combustion chamber, specifically for Bank 1 when the engine is off idle.
P21F4 is triggered when the Engine Control Module (ECM) detects that the air-fuel ratio for Bank 1 is too lean when the engine is operating above idle speed. This can result from various issues such as vacuum leaks, fuel delivery problems, or faulty sensors.
• Check Engine Light (CEL) is on
• Poor engine performance
• Increased fuel consumption
• Vacuum leak in the intake manifold
• Faulty oxygen sensor
• Fuel injector malfunction
ECM: Engine Control Module
CEL: Check Engine Light
• Check for vacuum leaks around the intake manifold
• Test the oxygen sensor for proper operation
• Inspect fuel injectors for blockages or failures
• Repair any vacuum leaks found
• Replace faulty oxygen sensor
• Clean or replace fuel injectors
• OBD-II scanner
• Vacuum gauge
• Multimeter
$100 - $500
Moderate
• Ford F-150
• Chevrolet Silverado
• Toyota Camry
• Regular maintenance of the fuel system
• Ensure air intake system is sealed properly
• Use quality fuel
Engine Performance
• Do not ignore the Check Engine Light
• Seek professional diagnostic if unsure
• Regularly maintain your vehicle's fuel system
Check with your vehicle manufacturer for warranty coverage on emissions-related components.
This code has been associated with issues in vehicles that have high mileage or have not been maintained regularly.
Moderate – Addressing this issue promptly can prevent further engine performance problems.
Ignoring this code can lead to increased emissions and may not pass vehicle inspection in some regions.
• Engine running above idle
• Warm engine temperature
The components related to this code are located in the engine bay, including the intake manifold and oxygen sensors.
Q: What does 'Bank 1' mean?
A: Bank 1 refers to the side of the engine where cylinder number 1 is located.
Q: Can I drive with a P21F4 code?
A: It's not recommended to drive for extended periods with this code, as it can lead to poor engine performance and increased emissions.
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Fitment Required · Vehicle-fitment part
Relevant when the code or supporting diagnostics point toward upstream or downstream oxygen-sensor faults.
This code family often involves upstream or downstream oxygen-sensor diagnosis.
Verify exact sensor position and vehicle fitment before buying.
View options on AmazonThese parts may be related to this fault category, but a code by itself does not confirm exact fitment or guarantee that replacing a part will solve the problem.