P2203: NOx Sensor Circuit High (Bank 1 Sensor 1)

P2203 flags a raw NOx value from the upstream sensor on bank 1 that lies above anything the engine can actually emit. The limits differ: GM sets 1,500 or 2,500 ppm on the 6.6L Duramax (LML) and 2,500 ppm on the 3.0L LZ0 (Type B, two trips), VW 1,655 ppm on the 2014 Touareg TDI. On 2023 and 2024 Silverado and Sierra 1500 pickups with the LZ0, GM bulletins 23-NA-193 and 24-NA-094 trace the code to software and prescribe an ECM update. Volvo and Mack class P2203 on the inlet sensor as a likely sensor failure and replace that sensor alone. P2202 is the lower limit, P2201 a plausible-looking but wrong reading, P22A1 the downstream counterpart. The MIL is on; SCR control loses a reliable input.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

• Malfunction indicator lamp on
• Implausibly high upstream NOx value in live data
• Possible over-dosing of AdBlue or SCR efficiency codes

Most likely causes

• Upstream NOx sensor reading high due to internal failure
• ECM software anomaly (GM 3.0L LZ0, bulletins 23-NA-193 and 24-NA-094)
• Sensing element contaminated by oil ash or DEF deposits

What to check, in order

Start with the simplest. You can tick off each step.

1. Read the codes and the NOx data before and after the SCR catalyst
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Check the sensor's connector and wiring
4. Look for exhaust leaks ahead of the sensor
5. Confirm that the AdBlue/DEF is of good quality
6. Replace the sensor only after ruling out power, ground, and leaks

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P2203 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 2: Fuel injection
• 03: Specific fault in that system

Frequently asked questions

Can I keep driving with code P2203?

You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.

What causes code P2203?

The most common causes are: upstream NOx sensor reading high due to internal failure; eCM software anomaly (GM 3.0L LZ0, bulletins 23-NA-193 and 24-NA-094); sensing element contaminated by oil ash or DEF deposits.

Related codes

• P2200NOx Sensor Circuit (Bank 1 Sensor 1)
• P2201NOx Sensor Circuit Range/Performance (Bank 1 Sensor 1)
• P2202NOx Sensor Circuit Low (Bank 1 Sensor 1)
• P2204NOx Sensor Circuit Intermittent (Bank 1 Sensor 1)
• P22A1NOx Sensor Circuit High (Bank 1 Sensor 2)
• P2216NOx Sensor Circuit High (Bank 2)

Sources

• gsi.ext.gm.com: 25OBDG04A
• gsi.ext.gm.com: 14OBDG04 Engine Diagnostics
• static.nhtsa.gov: MC 10074008 2280
• static.nhtsa.gov: MC 10245255 0001

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.