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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20E6

P20E6Reductant Injection Air Pressure Too Low

P20E6 indicates that the powertrain control module (PCM) has detected that the air pressure in the reductant injection system is lower than the expected value for the current operating conditions. This system uses compressed air to help inject diesel exhaust fluid (DEF) into the exhaust stream for selective catalytic reduction (SCR), and low air pressure can prevent proper DEF delivery and cause emissions non-compliance.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering and auxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms.
  • Reduced engine power or limp mode may be activated to protect the emissions system.
  • A message on the dashboard indicating low reductant pressure or a DEF system fault.
  • Possible rough running or misfire if the low pressure affects DEF injection quantity.
  • Increased diesel exhaust fluid consumption in some cases due to improper injection.
  • In rare cases, a sulfur smell or white smoke from the exhaust if DEF is not properly atomized.

Common causes, most likely first

  1. A leak in the reductant injection air pressure line, including cracks, loose fittings, or damaged seals.
  2. A faulty air pressure sensor that is reading lower than actual pressure.
  3. A failing air compressor or air pump that supplies pressure to the reductant injection system.
  4. A clogged or restricted air filter or intake for the reductant air pump.
  5. A malfunctioning reductant injection air pressure regulator or control valve.
  6. Wiring issues such as a short, open, or high resistance in the air pressure sensor circuit.
  7. A clogged DEF injector or nozzle causing backpressure and low measured air pressure.

How to diagnose and fix it

  1. Perform a visual inspection of all reductant injection air lines and fittings for leaks, cracks, or loose connections.
  2. Use a scan tool to check live data for the air pressure sensor and compare it to a mechanical gauge if possible.
  3. Test the air compressor or pump for proper output pressure and inspect the air filter for blockage.
  4. Check the air pressure sensor circuit for continuity, shorts, and proper reference voltage.
  5. Inspect the reductant injection air pressure regulator and control valve for proper operation.
  6. Remove and inspect the DEF injector for clogging or carbon buildup, and clean or replace as needed.
  7. After repairs, clear the codes and perform a road test to verify the repair and ensure the monitor runs.

Common Misdiagnosis Traps for P20E6

  • A low air pressure reading can be caused by a simple loose hose clamp on the air line, which is often overlooked in favor of replacing expensive components.
  • The air pressure sensor itself can fail and report low pressure even when the system is functioning correctly, so always verify with a mechanical gauge.
  • In some vehicles, the reductant injection air pressure is derived from the turbocharger boost system, so a boost leak can set this code.
  • The code may be set by a clogged DEF injector creating backpressure that the sensor interprets as low air pressure, leading to misdiagnosis of the air pump.
  • Extreme cold weather can cause DEF to freeze and block the injector, temporarily causing low air pressure readings and setting this code.

Can I keep driving with P20E6?

It is generally safe to drive with this code as long as the vehicle is not in limp mode, but avoid heavy loads or high-speed driving because the emissions system may not be functioning correctly. Have the vehicle diagnosed promptly to prevent further damage to the SCR system and to ensure compliance with emissions regulations.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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