P2040: Reductant Injection Air Pressure Sensor A Circuit High

With P2040 the injection air pressure sensor A reports a voltage above its upper limit, more than a real air pressure could produce. On Deutz's EMR L1 controller this is P2040-00 under SPN 4335, defined as a short circuit to battery. In practice that means the signal wire touches a powered conductor, the sensor ground is missing so the output floats up to the reference, or the sensor itself has failed high. A wrongly high reading is dangerous for the strategy because it could pretend that air is available when it is not, so the controller treats the signal as invalid and blocks DEF injection. Low readings are covered by P2039, implausible but in-range values by P2038. Check the ground pin first, then the harness for chafing against powered lines.

Symptoms

• Emission warning lamp on
• DEF injection blocked
• Air pressure value shown at maximum

Most likely causes

• Signal wire shorted to battery or another powered line
• Sensor ground open so the output floats high
• Sensor failed with output stuck high

What to check, in order

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

1. Check the AdBlue/DEF level and quality, and do not add additives or water
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Look for white crystals around the injector and the tank
4. Check connectors and heaters with the scanner
5. Run the dosing test available on your diagnostic tool
6. Replace parts only when the test points to that component

Good to know

• Repair difficulty: medium
• It can trigger limp mode.
• It affects emissions.

What each character of P2040 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 0: Subsystem depends on the code range
• 40: Specific fault in that system

Frequently asked questions

Can I keep driving with code P2040?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P2040?

The most common causes are: signal wire shorted to battery or another powered line; sensor ground open so the output floats high; sensor failed with output stuck high.

Related codes

• P2037Reductant Injection Air Pressure Sensor A Circuit
• P2039Reductant Injection Air Pressure Sensor A Circuit Low
• P2038Reductant Injection Air Pressure Sensor A Circuit Range/Performance
• P209DReductant Injection Air Pressure Sensor B Circuit High

Sources

• serdia.deutz.com: DTCList EMRL1 MainECU EN
• law.cornell.edu: 13 CCR 1968.2
• energy.gov: 2005 deer hesser

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.