Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
The car's computer is saying that a sensor in the exhaust is detecting too much air and not enough fuel. It's like when you blow on a fire and it doesn't have enough wood to burn.
The NOx sensor is detecting a lean condition, meaning there is too much air and not enough fuel in the exhaust gases.
The P123C code indicates that the NOx sensor on Bank 1 is reporting a lean condition. This means the air-fuel mixture is not balanced, with excess air or insufficient fuel. This can affect engine performance and emissions.
• Check engine light on
• Poor fuel economy
• Rough engine idle
• Faulty NOx sensor
• Exhaust leak
• Fuel injector issue
NOx Sensor: A sensor that measures nitrogen oxides in the exhaust gases.
Lean Condition: A situation where there is too much air and not enough fuel in the combustion process.
• Scan for codes and freeze frame data
• Inspect the NOx sensor and wiring
• Check for exhaust leaks
• Replace faulty NOx sensor
• Repair exhaust leaks
• Check and clean fuel injectors
• OBD-II scanner
• Wrench set
• Multimeter
$200 - $600
Moderate
• BMW 3 Series
• Audi A4
• Volkswagen Passat
• Regular maintenance checks
• Use quality fuel
• Inspect exhaust system periodically
Sensor Issue
• Do not ignore the check engine light
• Seek professional diagnosis if unsure
Check if the vehicle's emissions warranty covers sensor replacement.
This code has been associated with sensor failures in vehicles with high mileage.
Moderate – can lead to increased emissions and decreased performance if not addressed.
Driving with a malfunctioning NOx sensor may cause the vehicle to fail emissions tests.
• Engine at operating temperature
• Vehicle at idle
The NOx sensor is located in the exhaust system, usually near the catalytic converter.
Q: Can I drive with a P123C code?
A: It's possible, but not recommended as it can affect fuel efficiency and emissions.
Q: How do I know if the NOx sensor is bad?
A: A diagnostic scan showing P123C and symptoms like poor fuel economy or rough idle can indicate a bad NOx sensor.
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Universal · Digital multimeter
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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.