P229E on Mercedes-Benz: How to Diagnose & Fix
NOx Sensor Circuit (Bank 1 Sensor 2)
1 Mercedes-Benz model · 1 documented issue · 1 critical · $300-$900 repair
P229E on Mercedes-Benz by model: symptoms, diagnosis and fix
On Euro 6 W447 diesels the SCR emissions system depends on NOx sensors (before and after the catalyst) and the AdBlue dosing system. A failed NOx sensor, dosing fault, or low/contaminated AdBlue escalates from a yellow warning to a red warning and finally a 'No start in X miles' countdown, after which the engine will not restart once switched off. The vehicle commonly drops into limp mode (restricted speed/power) before that. NOx sensor failure is one of the most frequently cited W447 emissions faults.
Common Symptoms
- AdBlue warning light (yellow then red)
- 'No start in 500 miles' / engine-start countdown message
- Limp mode with reduced power
- Repeated emissions warnings after a basic code clear
How to Fix
Diagnose the specific fault: replace the failed NOx sensor (genuine/OE recommended) or repair the AdBlue dosing/heater/pump as required, top up with fresh AdBlue, then clear codes and reset the SCR warning scenario (e.g. 'Reset SCR Warnszenario'/soft reset in dealer-level diagnostics). Simply clearing codes without fixing the root cause makes the warning return.
Sources for this issue (3)
P229E code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P229E mean on a Mercedes-Benz?
P229E stands for "NOx Sensor Circuit (Bank 1 Sensor 2)." P229E indicates a fault in the NOx (nitrogen oxide) sensor circuit for Bank 1 Sensor 2, which monitors the nitrogen oxide content in the exhaust gas, typically downstream in the SCR/aftertreatment system on diesel engines. It sets when the engine control module detects an electrical problem (open, short, or implausible signal) in this sensor circuit. The vehicle may enter a reduced-power "derate" mode to limit emissions until the fault is repaired. Note: some manufacturers reassign this code (e.g., RAM uses it for a seat back heater sensor, VW for a seatbelt load limiter), so verify against the specific make. On Mercedes-Benz it is documented across 1 model: V-Class.
What are the most common causes of P229E?
The most common causes of P229E are: Failed or contaminated downstream NOx sensor (Bank 1 Sensor 2), Damaged, corroded, or loose NOx sensor wiring or connector, Open or short circuit in the NOx sensor harness, Faulty NOx sensor control module, Exhaust leak near the sensor affecting readings, Problem in the SCR/DEF aftertreatment system causing implausible NOx values. The specific cause varies by vehicle.
How much does it cost to fix P229E on a Mercedes-Benz?
Published repair estimates on Mercedes-Benz range from $300 to $900 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P229E on a different make?
View P229E across all makes →Join 600+ people using Au7o to better understand their vehicles.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.