The 2011 BMW 335d is a diesel-powered performance sedan that blends torque-rich power with impressive fuel economy. However, one common issue affecting this model—especially in North American versions with emissions control systems—is failure of the NOx (nitrogen oxide) sensors. These components are critical to emissions compliance and overall engine function. In this guide, we’ll cover what the NOx sensor does, signs of failure, how to replace it, and how much it costs.
What Is a NOx Sensor in the 2011 BMW 335d?
The NOx sensor monitors nitrogen oxide emissions in the exhaust and feeds data to the engine control unit (ECU) to help regulate combustion and emissions systems. Your 335d has two NOx sensors—one upstream (before the SCR catalyst) and one downstream (after the catalyst). These sensors work alongside the diesel exhaust fluid (DEF) system to meet emissions regulations.
Why Do NOx Sensors Fail?
Over time, NOx sensors degrade due to constant exposure to high exhaust temperatures, soot buildup, and electronic wear. Sensor failure is especially common in higher-mileage diesels or cars exposed to stop-and-go city driving, which doesn’t allow the exhaust system to self-clean efficiently.
Symptoms of a Failing NOx Sensor
Common signs of a failed NOx sensor in the 335d include:
- Check Engine Light (CEL) with NOx-related fault codes
- Reduced engine performance (limp mode)
- Poor fuel economy
- Increased DEF consumption or SCR warning messages
- Failed emissions test or inability to reset emissions monitors
Typical Diagnostic Trouble Codes (DTCs):
- P229F – NOx sensor circuit performance (Bank 1, Sensor 2)
- P20EE – SCR NOx catalyst efficiency below threshold
- P2201 – NOx sensor circuit range/performance
Upstream vs Downstream NOx Sensor
| Sensor | Location | Function |
|---|---|---|
| Upstream NOx Sensor | Before the SCR catalyst | Measures raw exhaust gases directly after combustion |
| Downstream NOx Sensor | After the SCR catalyst | Monitors SCR system efficiency and ensures NOx reduction |
How to Confirm NOx Sensor Failure
To verify whether your 335d’s NOx sensor has failed:
- Scan the vehicle using a BMW-specific diagnostic tool (ISTA, INPA, or high-end OBD2 scanners like Autel or Foxwell)
- Check for the DTC codes listed above
- Compare live data from the upstream and downstream NOx sensors
- Evaluate SCR catalyst performance and DEF injection functionality
Can You Drive with a Bad NOx Sensor?
You can drive for a limited time, but prolonged use can lead to limp mode, failed emissions tests, and even permanent engine restrictions. BMW’s ECU is programmed to reduce power or initiate a no-start condition if emissions thresholds are not met over time.
How to Replace the NOx Sensor
Here’s a general overview of what’s involved in replacing a failed NOx sensor on a 2011 BMW 335d:
Tools Needed:
- 22mm oxygen sensor socket
- Torque wrench
- BMW diagnostic tool (for coding or resetting adaptations)
- Jack and jack stands or a lift
Replacement Steps:
- Let the exhaust cool completely
- Locate the faulty sensor (upstream or downstream)
- Disconnect the electrical connector from the sensor control module
- Use the sensor socket to unscrew the NOx sensor from the exhaust pipe
- Install the new sensor and torque to spec
- Reconnect the electrical harness
- Clear fault codes and perform a sensor reset using a scan tool
OEM vs Aftermarket NOx Sensors
- OEM (Original Equipment): Recommended for long-term reliability and compatibility. Brands include Bosch and Continental.
- Aftermarket: Less expensive, but quality can vary significantly. Some may not work properly with BMW’s ECU or cause reoccurring fault codes.
Cost to Replace a NOx Sensor
| Sensor Type | Parts Cost | Labor Cost | Total Estimated Cost |
|---|---|---|---|
| Upstream NOx Sensor | $400–$600 | $150–$250 | $550–$850 |
| Downstream NOx Sensor | $400–$600 | $150–$250 | $550–$850 |
How to Extend the Life of Your NOx Sensors
- Use high-quality diesel fuel with detergents
- Avoid short trips—let the exhaust system reach full temperature
- Ensure proper DEF levels and functionality
- Clean or replace other sensors (oxygen, temperature) when doing exhaust maintenance
Final Thoughts
The 2011 BMW 335d is a powerhouse diesel sedan, but NOx sensor failure is a known issue that can limit performance and emissions compliance. The good news is that replacement is straightforward, and using OEM sensors paired with proper coding usually resolves the issue without further complications. If your 335d is showing signs of sensor failure, it’s best to address it promptly before limp mode kicks in or emissions faults cascade into larger problems.
Tip: Always pair NOx sensor replacement with a full emissions system diagnostic to make sure there are no DEF injector, SCR, or temperature sensor issues lurking behind the scenes.