P056D

What it means

P056D means the reductant control module, which runs the selective catalytic reduction (SCR) dosing system, has seen its supply voltage outside the expected range. The module monitors its own power supply and sets the code when voltage is too low, too high or unstable. The exact thresholds and whether it is a separate module or part of the engine control unit depend on the vehicle.

Symptoms

• Engine warning light on, often with an exhaust fluid or emissions warning message
• Reductant dosing reduced or stopped, so nitrogen oxide emissions rise
• Possible power or speed limit, or a restart-inhibit countdown, on some vehicles
• Other low-voltage or communication fault codes stored at the same time
• Slow cranking or a weak battery as well, if the supply is at fault
• Fault that comes and goes, particularly on cold starts or over bumps

Possible causes

• Weak, discharged or ageing battery giving low or unstable system voltagelikely
• Charging system fault, such as a worn alternator or faulty regulator, giving low or high voltagelikely
• Corroded, loose or damaged battery terminals, earth straps or main power connectionslikely
• Damaged wiring, high resistance or poor connector contact on the reductant control module power and earth circuitspossible
• Blown fuse or faulty relay supplying the reductant control module or dosing systempossible
• Internal fault in the reductant control modulerare
• Outdated control unit software where the manufacturer has published a fix for false voltage faultsrare

How to diagnose

1. Read all stored codes and freeze frame data, noting battery voltage and engine state when the code set, and look for other voltage or communication codes
2. Check the battery state of charge and condition with a proper load or conductance test, after charging if it is low
3. Measure battery voltage with the engine off, at idle and at raised revs to confirm the charging system is within specification
4. Inspect battery terminals, earth straps and main power connections for corrosion, looseness or heat damage
5. Check the fuses and relays for the reductant control module and dosing system against the wiring diagram
6. Inspect the module connector and harness for water ingress, chafing, bent pins or corrosion, as the module is often underneath the vehicle
7. With the module connected, voltage-drop test its power and earth circuits while it is active
8. Check for any manufacturer software updates or technical bulletins for the model and engine before condemning the module

How to fix

• Charge or replace the battery if it fails testing
• Repair or replace the alternator or regulator if charging voltage is out of range
• Clean and tighten terminals and earth points, and replace any damaged straps or cables
• Repair damaged wiring or connectors, or replace faulty fuses and relays after finding why they failed
• Update the control unit software if a published update addresses the fault
• Replace the reductant control module only after supply and earth have been proven good, and code it to the vehicle as required

Can I keep driving?

Nearby codes

• P056CGenericCruise Control Cancel Signal
• P056EGenericCold Start Turbocharger/Supercharger Boost Control A Performance