Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
Your car has a sensor that checks how clean the air coming out of the engine is. Right now, it thinks there's too much air and not enough fuel in the mix, which isn't good for the engine or the environment.
The NOx sensor in Bank 1 is detecting a lean condition, meaning there is too much air and not enough fuel in the exhaust gases.
The P123D code indicates that the NOx sensor, which monitors nitrogen oxide levels in the exhaust, is reporting a lean condition. This can result from an imbalance in the air-fuel mixture, often due to a malfunctioning sensor or issues in the fuel delivery system.
• Check Engine Light is on
• Poor fuel economy
• Rough engine idle
• Faulty NOx sensor
• Exhaust leaks
• Fuel delivery issues
NOx Sensor: A sensor that measures nitrogen oxide levels in the exhaust gases.
Lean Condition: A situation where there is more air than fuel in the air-fuel mixture.
• Use an OBD-II scanner to confirm the code.
• Inspect the NOx sensor and wiring for damage.
• Check for exhaust leaks or fuel delivery issues.
• Replace the NOx sensor if faulty.
• Repair any exhaust leaks.
• Address fuel delivery issues, such as replacing the fuel filter.
• OBD-II scanner
• Multimeter
• Wrench set
$200 - $600
Moderate
• Volkswagen Jetta
• Audi A4
• BMW 3 Series
• Regular maintenance of the exhaust system
• Timely replacement of fuel filters
• Routine checks of sensor connections
Sensor Issue
• Avoid driving long distances until the issue is fixed.
• Regularly monitor fuel economy for any changes.
Check with your vehicle manufacturer for warranty coverage on emissions-related components.
This code has been reported in vehicles with high mileage or those that have not undergone regular maintenance.
Moderate – Addressing this issue promptly can prevent further engine or emissions system damage.
Ignoring emissions-related codes can result in failing emissions tests and increased pollution.
• Engine at operating temperature
• Vehicle driven under normal conditions
The NOx sensor is typically located in the exhaust system, near the catalytic converter.
Q: Can I drive with a P123D code?
A: It's not recommended to drive for extended periods as it may cause further damage to the emissions system.
Q: How do I reset the Check Engine Light after fixing the issue?
A: You can use an OBD-II scanner to clear the code after repairs are completed.
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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.