Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
Your car's computer thinks that the engine is not getting enough fuel on one side, which can make it run poorly.
The engine control module (ECM) has detected that the air-fuel mixture on bank 2 is too lean, meaning there is too much air and not enough fuel.
Code P1091 indicates that the fuel trim system for the pre-catalyst on bank 2 is running too lean. This means the ECM has detected an air-fuel ratio imbalance where the mixture is too lean, potentially due to issues like vacuum leaks, faulty sensors, or fuel delivery problems.
• Check Engine Light is on
• Poor engine performance
• Rough idling
• Vacuum leaks
• Faulty oxygen sensor
• Fuel injector issues
ECM: Engine Control Module
Fuel Trim: Adjustments made by the ECM to the air-fuel mixture
• Check for vacuum leaks around the intake manifold.
• Inspect the oxygen sensors for proper operation.
• Test the fuel injectors for proper fuel delivery.
• Repair any vacuum leaks found.
• Replace faulty oxygen sensors.
• Clean or replace fuel injectors as needed.
• OBD-II scanner
• Vacuum gauge
• Multimeter
$150 - $500
Moderate
• BMW 3 Series
• Audi A4
• Mercedes-Benz C-Class
• Regular maintenance checks
• Timely replacement of air filters
• Use quality fuel
Engine Performance
• Do not ignore the Check Engine Light.
• Have the vehicle inspected by a professional if you notice performance issues.
Check with your vehicle manufacturer for warranty coverage on emissions-related components.
This code has been commonly associated with vehicles experiencing lean conditions due to vacuum leaks or sensor failures.
Moderate – It should be addressed soon to prevent potential engine damage.
Driving with a lean condition can cause engine damage and increase emissions, which may not comply with local regulations.
• Engine at operating temperature
• Idle and under load conditions
Bank 2 refers to the side of the engine opposite bank 1, typically on V-type engines.
Q: What does 'too lean' mean?
A: It means there is too much air and not enough fuel in the engine's air-fuel mixture.
Q: Can I drive with a P1091 code?
A: It's not recommended, as it can lead to poor performance and potential engine damage.
High-End · Full-system diagnostics
A full-system bidirectional platform aimed at advanced DIY users and professional diagnostic workflows.
Network faults often require full-system access instead of a generic code reader alone.
View on Amazon
Universal · Digital multimeter
A solid entry-level multimeter for voltage, continuity, resistance, and basic electrical troubleshooting.
Helpful for checking voltage drops, grounds, and basic circuit health when modules are not communicating.
View on Amazon
Universal · Wire connectors and repair kit
Useful when diagnosis points toward damaged wiring, corroded terminals, or connector repairs.
Useful if diagnosis points toward damaged connectors, wiring, or terminal repairs.
View on AmazonThese are optional diagnostic tools, not guaranteed fixes. As an Amazon Associate I earn from qualifying purchases.
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 Amazon
Fitment Required · Vehicle-fitment part
Relevant when catalyst-efficiency or emissions faults persist after proper diagnosis.
Catalyst-efficiency faults sometimes point toward converter problems after proper testing.
Catalytic converters are highly vehicle-specific and should not be purchased by code alone.
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.