P229E: NOx Sensor Circuit Bank 1 Sensor 2
What to do
Ask for the connector and wiring under the vehicle to be inspected before the sensor is replaced. These sensors are expensive, they hang underneath in the road spray, and a corroded connector is a far more common cause than a failed element.
Ask whether any communication or network faults are stored alongside. These sensors have a small module of their own that talks to the car over a data line — if that conversation fails, a perfectly good sensor is reported as faulty.
Mention any recent work underneath the vehicle, since a disturbed or unlatched connector produces this exactly.
The car will usually drive normally, so plan the repair around your MOT — the emissions self-checks will not complete while it is stored.
What this code means
A general electrical fault has been detected in the downstream NOx sensor circuit.
Sensor 2 sits after the SCR catalyst and measures what is actually leaving the vehicle, which makes it the more consequential of the pair for emissions monitoring: it is the number that says whether the catalyst worked.
These are not simple sensors electrically. A modern NOx sensor carries its own small control module bolted to the harness, which manages the heater, runs the measurement and reports the result over the vehicle's data network rather than as a voltage. So "circuit" here covers a supply, an earth, a heater and a communication link — and a fault on any of them lands on this code.
That architecture is why a network fault can present as a sensor fault. If the sensor's module cannot talk, the engine module reports the sensor as faulty — while the sensing element is perfect. Look for communication codes stored alongside.
Location supplies the ordinary causes. The sensor and its module hang under the vehicle in the exhaust stream, exposed to road salt, water and stone damage, and their connectors corrode accordingly.
A general circuit code is the least specific member of its family. Where a range/performance or heater code is stored alongside, that one carries the useful information — this one simply says something in the circuit is wrong.
Likely causes
Start here
- Corroded or loose connector · easy
- Faulty NOx sensor · moderate
- Faulty NOx sensor control module · moderate
Less common
- Damaged data bus wiring · moderate
- Water ingress into connector · easy
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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