OBD-II Diagnostic Trouble Code
P229F — NOx Sensor Bank 1 Sensor 2
P229F means the bank 1 sensor 2 NOx sensor (outlet NOx sensor after the SCR) is reporting implausible values or failed a self-diagnostic. The ECM no longer trusts it for SCR efficiency monitoring.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
NOx Sensor Bank 1 Sensor 2
Code
P229F
Category
Powertrain
Subsystem
Aftertreatment
Truck-relevant
Yes
What this means for truckers
P229F (NOx Sensor Bank 1 Sensor 2) — the downstream NOx sensor (after SCR catalyst) is reading bad. This is one of the most common DEF/SCR codes on 2010+ commercial diesels. Causes: failed sensor (most common, ~$500-900), DEF contamination, or actual SCR catalyst failure if upstream NOx is normal but downstream stays high. Replace the sensor first — it's a wear item. After replacement, perform an SCR adaption with the OEM tool.
Before you replace parts — try this first
P229F on the Aftertreatment system is most often failed or contaminated outlet nox sensor — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Emissions light, possible false SCR efficiency codes and an eventual inducement derate. Driveability is often normal until derate begins.
Common causes
- CommonFailed or contaminated outlet NOx sensor ($450–$1200)
- OccasionalExhaust leak near the outlet sensor ($150–$600)
- OccasionalAmmonia slip from overdosing or poor DEF quality skewing readings (NOx sensors also respond to ammonia) ($100–$800)
- RareECM software issue ($150–$400)
How to fix
Check for exhaust leaks near the outlet sensor
When to use: First check
Test DEF concentration with a refractometer
When to use: When SCR efficiency codes also appear
Compare inlet and outlet NOx readings at idle and under load
When to use: To see if the sensor responds
Replace the outlet NOx sensor
When to use: When the sensor reading is clearly wrong
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P2200 — NOx Sensor Circuit Bank 1
- → P229E — NOx Sensor Circuit Bank 1 Sensor 2
- → P22A4 — Reductant Heater B Control Circuit/Open
- → P22A5 — Reductant Heater B Control Circuit Low
- → P22A6 — Reductant Heater B Control Circuit High
Frequently Asked Questions
Can I drive with code P229F?
Drive with caution. P229F is severity 4 — stop within ~50 miles. Most commercial diesel codes at this severity (turbo, fuel rail, transmission solenoid) can damage adjacent expensive components if left unrepaired. Get to a service location before highway-speed loads.
How much does it cost to fix P229F on a commercial truck?
Total repair cost ranges $100–$1200 depending on the root cause. The cheapest fix is usually the first one listed below — Failed or contaminated outlet NOx sensor — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P229F fail a DOT inspection?
Yes — P229F is an emissions-related code, and DOT inspectors check emissions compliance. An active emissions code with a check engine light can result in an out-of-service violation under FMCSA Part 393. The truck typically can't be moved until the code is cleared. Repeated emissions violations escalate to CSA scores.
What's the difference between P229F and related codes?
P229F is closely related to: P2200 (NOx Sensor Circuit Bank 1), P229E (NOx Sensor Circuit Bank 1 Sensor 2), P22A4 (Reductant Heater B Control Circuit/Open), P22A5 (Reductant Heater B Control Circuit Low). These codes share the same subsystem (aftertreatment) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P229F is set without the companions, the issue is more isolated.
How do I prevent P229F from coming back after a repair?
For P229F (NOx Sensor Bank 1 Sensor 2), prevention centers on the aftertreatment system. Run quality DEF (verify supplier — BlueDEF, PEAK, Yara Air1). Don't skip parked regens — when the dash asks for one, do it. Avoid extended idle time which fouls the DPF. Replace NOx sensors at the OEM interval (typically 400K miles).