Gaziantep NOx Sensor Fault
NOx sensor · SCR efficiency · GaziantepA NOx sensor fault usually shows up as an AdBlue warning. To know whether the sensor really is faulty, you have to look at how the two sensors behave relative to each other.
What the NOx sensor does
The NOx sensor measures the quantity of nitrogen oxides in the exhaust gas. From that value the control unit sets the AdBlue dosing and calculates the efficiency of the after-treatment system.
The sensor has its own electronic module and has to heat up to a certain temperature before it can read correctly. That is why values taken immediately after a cold start are not used for assessment.
The difference between upstream and downstream sensors
Most cars have two NOx sensors and their jobs are different.
- Upstream (before the catalyst)
Measures the raw NOx the engine produces. How much is dosed is largely set by this value.
- Downstream (after the catalyst)
Measures the NOx leaving the system. Efficiency is calculated from the difference between the two sensors.
- Why does the difference matter?
If either sensor drifts, the difference is corrupted; the system can see low efficiency even when dosing is correct, or conversely miss a real fault.
How a wrong NOx reading affects the SCR system
- Incorrect dosing
An upstream sensor reading high causes more AdBlue to be injected than needed, which speeds up crystallisation.
- False efficiency entry
A drifting downstream sensor creates a low-efficiency entry while the system is healthy.
- Countdown
When the efficiency entry repeats, the car goes into the warning and countdown process.
- Unnecessary part replacement
When the pump or injector is replaced before the sensor is verified, the problem stays exactly the same.
NOx sensor fault symptoms
- AdBlue warning
The warning can appear even with a full tank and working dosing.
- Engine warning light
Together with an entry relating to efficiency or the sensor circuit.
- Countdown
When the efficiency entry repeats.
- Rising AdBlue consumption
A false high reading raises the dosing.
- A fault that appears after warm-up
Codes logged once the sensor has heated up and started measuring.
What do we look at in live data?
- Simultaneous values from both sensors
How the two sensors behave relative to each other is followed on a warm engine at steady load.
- Cold start behaviour
Sensor readiness times and initial readings are compared.
- Relation to dosing
We check whether the downstream value responds as the AdBlue injection quantity changes.
- Efficiency calculation
We check whether the efficiency figure the control unit calculates is consistent with the measured difference.
- Wiring
Sensor module supply and communication line, connector contact.
How do we assess the NOx sensor?
The aim is to separate whether the problem is the sensor or the dosing, and to leave sensor replacement as the last step.
- Code and entry frequency
Whether it is an efficiency entry or a circuit entry, and over how many cycles it has repeated.
- Warm-up check
We follow whether the sensors reach their ready-to-measure state.
- Comparative reading
Upstream and downstream values are logged together under the same conditions.
- Correlation with dosing
We verify that the sensor responds in the expected direction as injection changes.
- Wiring measurement
Supply, ground and data line are checked.
- Verification
After repair, monitoring is carried out over enough driving for the efficiency calculation to settle.
How is the solution decided?
On the NOx side, sensor replacement is not the first step but the last one, after verification.
- Wiring repair
If there is a supply, ground or data line problem, the circuit is repaired rather than the sensor.
- Fixing the actual fault on the SCR side
A dosing or pressure problem can make the sensor look guilty; that is solved first.
- Sensor replacement
Renewed if comparative measurement shows the sensor has drifted.
- Matching and adaptation
The new sensor is registered to the system to the manufacturer's procedure.
Gaziantep NOx sensor solution
To establish in Gaziantep whether the NOx sensor really is faulty using comparative live data, get in touch with Nero Performance.