OBD-II trouble code
P229F: NOx Sensor Circuit Range/Performance (Bank 1 Sensor 2)
The downstream NOx sensor (bank 1, sensor 2) is producing an in-range but illogical signal. This sensor, located after the SCR catalyst, verifies how well the system reduced NOx — so a fault can point at the sensor or at real SCR performance.
Quick facts
- System
- Powertrain
- Category
- Auxiliary Emissions Controls
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $200 – $700
- DIY difficulty
- Intermediate DIY
What does P229F mean?
Diesel emissions systems commonly use two NOx sensors: an upstream sensor before the SCR catalyst and a downstream sensor after it. Comparing the two tells the engine control module how effectively the selective catalytic reduction (SCR) system converted nitrogen oxides. P229F is a range/performance fault for the downstream NOx sensor (bank 1, sensor 2): its signal is electrically valid but out of range or illogical for conditions. (Some manufacturers describe this position as bank 2; the defining point is that it is the second NOx sensor in the monitoring pair.)
As with other NOx performance codes, the leading cause is an aged, contaminated, or drifting sensor, since these sensors endure high heat and soot. But because the downstream sensor's whole job is to verify SCR effectiveness, P229F can also reflect a genuine SCR/DEF performance problem — if the catalyst isn't reducing NOx, the downstream reading will legitimately look 'wrong.' Wiring, connector, and exhaust-leak issues round out the causes. Diagnosis weighs sensor condition against actual SCR performance and DEF dosing, so a good sensor isn't replaced when the real issue is the emissions system.
Common causes
- Aged, contaminated, or drifting downstream NOx sensor
- Soot fouling of the sensing element
- Genuine SCR/DEF underperformance making the downstream reading look wrong
- Exhaust leak near the sensor
- Degraded wiring or a corroded connector
- Heater-circuit weakness preventing a stable reading
- Rarely, an ECM/SCR controller fault
Symptoms
- Check engine light with an emissions/SCR message
- Possible reduced fuel economy
- SCR efficiency or DEF dosing faults stored alongside P229F
- Reduced power (inducement) if emissions control stays impaired
- Failed emissions test
Diagnostic steps
- 1.Scan and record all codes; note companion NOx and SCR-efficiency codes (such as P2200, P2201, P204F).
- 2.Identify the downstream (post-SCR) NOx sensor and inspect its connector and wiring for heat/corrosion damage.
- 3.Check for exhaust leaks near the sensor.
- 4.Compare upstream vs. downstream NOx readings to judge whether the SCR system is actually reducing NOx.
- 5.Evaluate DEF quality/level and SCR dosing before condemning the sensor.
- 6.If the downstream sensor is confirmed drifting/contaminated with a healthy SCR system, replace it and clear codes.
Repair cost
$200 – $700
Downstream NOx sensor replacement typically runs $250-$650 with labor. If the reading is implausible because the SCR system is genuinely underperforming, that underlying issue must be fixed too. Compare upstream and downstream data before replacing the sensor.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with nox sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.