P20EE

What the Code Means

P20EE flags reduced efficiency in the SCR catalytic converter — the selective catalytic reduction system that cuts nitrogen oxides in diesel exhaust by injecting AdBlue reagent.

The control module evaluates catalytic converter performance from the NOx sensor readings: if the difference between oxide concentration before and after the catalytic converter doesn't match the expected value, this code sets. Causes include a faulty NOx sensor, AdBlue dosing faults, or contamination of the catalytic converter itself.

Symptoms

• Check Engine light is on.
• Increased AdBlue consumption.
• Reduced engine power.

Diagnosis and Repair

Step 1. Check the SCR System and NOx Sensors

Start with the NOx sensors — a faulty one is a typical cause of this code. When its readings are off, the control module can wrongly judge the catalytic converter as inefficient.

Step 2. Check AdBlue Dosing

Next, verify AdBlue dosing. Reagent injection faults lead to incomplete reduction of nitrogen oxides and a lower calculated catalytic converter efficiency.

Step 3. Replace the SCR Catalytic Converter

If the injection system and sensors are working and efficiency doesn't recover, contamination of the catalytic converter is the likely cause. In that case, replace the SCR catalytic converter.

Secondary Problems

• Further loss of SCR catalytic converter efficiency.
• Possible damage to the DPF.

Don't put off diagnosis: ignoring the code leads to progressive degradation of the exhaust aftertreatment system and the risk of DPF failure.