Damaged or corroded wiring harness or connector at the reductant injection air pressure control valve; Failed reductant injection air pressure control valve solenoid or internal coil; Blown fuse or poor ground connection supplying power to the valve circuit; Open circuit or high resistance in the valve control wire between the valve and the powertrain control module; Water intrusion or contamination in the connector causing low signal voltage; Faulty powertrain control module driver circuit, though this is less common

DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20A8

P20A8Reductant Injection Air Pressure Control Valve Circuit Low

Q: What does code P20A8 mean?

Reductant Injection Air Pressure Control Valve Circuit Low. P20A8 indicates that the powertrain control module has detected an abnormally low voltage or current on the electrical circuit for the reductant injection air pressure control valve, which is part of the diesel exhaust fluid dosing system. The computer expects a specific signal range from this valve circuit and has determined that the signal is below the acceptable threshold, which can prevent proper air-assisted reductant injection into the exhaust stream.

P20A8 indicates that the powertrain control module has detected an abnormally low voltage or current on the electrical circuit for the reductant injection air pressure control valve, which is part of the diesel exhaust fluid dosing system. The computer expects a specific signal range from this valve circuit and has determined that the signal is below the acceptable threshold, which can prevent proper air-assisted reductant injection into the exhaust stream.

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 triggered by the powertrain control module
A message on the dashboard indicating a problem with the emissions system or diesel exhaust fluid
Rough running or hesitation during regeneration events
Increased diesel exhaust fluid consumption in some cases
Failure to complete an active regeneration cycle, leading to soot accumulation

Common causes, most likely first

Damaged or corroded wiring harness or connector at the reductant injection air pressure control valve
Failed reductant injection air pressure control valve solenoid or internal coil
Blown fuse or poor ground connection supplying power to the valve circuit
Open circuit or high resistance in the valve control wire between the valve and the powertrain control module
Water intrusion or contamination in the connector causing low signal voltage
Faulty powertrain control module driver circuit, though this is less common

How to diagnose and fix it

Inspect the valve connector and wiring for corrosion, chafing, or loose pins, and repair as needed
Check the fuse and ground connections for the reductant injection air pressure control valve circuit
Measure resistance across the valve solenoid and compare to the manufacturer's specification to rule out an open coil
Backprobe the control wire at the valve and check for the expected voltage or duty cycle signal from the powertrain control module
Perform a wiggle test on the harness while monitoring the signal to detect intermittent faults
If the circuit tests good, substitute a known-good valve and clear the code to see if it returns
Verify the repair by clearing codes and running a reductant system monitor or test drive to confirm the code does not reset

What the low circuit code really means

A low circuit code often points to a wiring or connector problem rather than a failed valve, so inspect the harness thoroughly before replacing parts.
The reductant injection air pressure control valve works with the air pump and pressure sensor, and a fault in those components can sometimes cause a low signal on this circuit.
Some vehicles will not illuminate the check engine light for this code until the fault occurs on two consecutive drive cycles, so a single stored code may not indicate a hard failure.
Clearing the code without repairing the underlying circuit issue will usually result in the code returning after a few drive cycles, especially during active regeneration.
A weak battery or charging system can cause low voltage codes across multiple circuits, so always verify battery voltage and ground integrity before chasing the valve circuit.

Can I keep driving with P20A8?

You can usually continue driving to a repair shop, but avoid long trips or heavy loads because the emissions system may not function correctly and the vehicle could enter limp mode. Prolonged driving with this code can lead to increased diesel exhaust fluid consumption and potential damage to the aftertreatment system.

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.