P2039: Reductant Injection Air Pressure Sensor A Circuit Low

P2039 is stored when the signal of injection air pressure sensor A sits below the lowest voltage a working sensor can deliver. Deutz documents it on its EMR L1 controller as P2039-00 under SPN 4335 and lumps two causes together: a grounded signal and an open circuit, since the input is pulled low when the signal wire is interrupted. A loss of the 5 V reference is another common reason for a reading near zero. Because the controller can no longer confirm that atomizing air is present, it stops DEF injection; Deutz engines then run without NOx reduction until the inducement logic of the machine steps in. The opposite fault is P2040, and electrical faults of the sensor supply fall under P2037.

Symptoms

• Emission warning lamp on
• DEF injection stopped
• Power derate after a while on machines with inducement

Most likely causes

• Signal wire open or shorted to ground
• 5 V reference to the sensor lost
• Sensor output stuck low

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. With the connector unplugged, check whether the signal wire is making contact with ground.
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 P2039 means

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

Frequently asked questions

Can I keep driving with code P2039?

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

What causes code P2039?

The most common causes are: signal wire open or shorted to ground; 5 V reference to the sensor lost; sensor output stuck low.

Related codes

• P2037Reductant Injection Air Pressure Sensor A Circuit
• P2040Reductant Injection Air Pressure Sensor A Circuit High
• P2038Reductant Injection Air Pressure Sensor A Circuit Range/Performance
• P209CReductant Injection Air Pressure Sensor B Circuit Low

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.