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 special sensor, which checks how much harmful gas is coming out of the car, isn't working right. It's like when a thermometer doesn't show the right temperature.
The NOx sensor measures the levels of nitrogen oxides in the exhaust gases. If the signal is too low, it means the sensor might not be working properly or the exhaust levels are not as expected.
The P123E code indicates that the signal from the NOx sensor on Bank 1 is below the expected range. This could be due to a malfunctioning sensor, wiring issues, or actual low NOx levels in the exhaust.
• Check Engine Light is on
• Decreased fuel efficiency
• Possible engine performance issues
• Faulty NOx sensor
• Wiring or connection issues
• Exhaust leaks
NOx: Nitrogen Oxides, harmful gases produced during combustion.
Bank 1: The side of the engine with cylinder 1, usually the side with the first cylinder in the firing order.
• Scan the vehicle for codes and freeze frame data.
• Inspect the NOx sensor and its wiring for damage.
• Test the NOx sensor signal with a multimeter.
• Replace the NOx sensor if faulty.
• Repair or replace damaged wiring or connectors.
• Check and fix any exhaust leaks.
• OBD-II scanner
• Multimeter
• Wrench set
$200 - $500
Moderate
• Volkswagen Passat
• Audi A4
• Mercedes-Benz C-Class
• Regular maintenance of the exhaust system
• Periodic inspection of sensors and wiring
• Use high-quality fuel
Sensor Issue
• Have the vehicle inspected by a professional.
• Avoid driving long distances until the issue is resolved.
Check with the vehicle manufacturer for warranty coverage on emissions-related components.
This code has been more common in vehicles with higher mileage or those that frequently drive in stop-and-go traffic.
Moderate – It's important to address this issue soon to avoid potential engine damage and to ensure emissions compliance.
Driving with a faulty NOx sensor can lead to increased emissions, which may not comply with environmental regulations.
• Engine at operating temperature
• Vehicle in closed-loop operation
The NOx sensor is typically located in the exhaust system, near the catalytic converter on Bank 1.
Q: What does a NOx sensor do?
A: A NOx sensor measures the levels of nitrogen oxides in the exhaust gases to help control emissions.
Q: Can I drive with a P123E code?
A: It's not recommended to drive for long periods with this code, as it can affect emissions and engine performance.
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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.