SCR system faults and efficiency checks in Stuttgart

What does SCR system diagnostics involve?

SCR stands for selective catalytic reduction. The system uses a reductant derived from AdBlue to reduce nitrogen oxides in the catalyst. Dosing, exhaust temperature, the catalyst and NOx readings must work together for this to happen.

Diagnostics therefore goes beyond the fault memory. It assesses dosing demand, system pressure, temperature readings, NOx signals, possible exhaust leaks and the operating state in which the fault was detected.

• Record fault status and operating conditions
• Include AdBlue dosing and the operating temperature range
• Check NOx readings before and after the catalyst for plausibility
• Assess the exhaust path and catalyst performance separately

How SCR and AdBlue are connected

AdBlue is the operating fluid; SCR is the catalytic process and the associated system. The urea solution is dosed into the hot exhaust, where ammonia forms and participates in nitrogen-oxide reduction in the SCR catalyst.

A fault in AdBlue delivery can therefore affect SCR performance. Conversely, a full AdBlue tank does not prove that dosing, temperature and catalyst performance are correct.

Why an efficiency fault does not automatically condemn the catalyst

SCR efficiency is inferred from several signals and operating conditions. Implausible NOx sensors, insufficient or incorrect dosing, unsuitable exhaust temperatures and leaks can affect the assessment.

Only after these factors have been checked for plausibility can catalyst condition be assessed properly. Premature replacement can be expensive and leave the actual fault unchanged.

• Dosing demand and system pressure
• Exhaust temperatures and operating state
• NOx signal before and after SCR
• Exhaust leaks, sensors and catalyst condition

Interpreting P20EE and similar SCR codes

P20EE is a commonly searched indication that SCR performance has been assessed as too low. The exact wording, status and test conditions must nevertheless be interpreted for the specific vehicle.

The code alone cannot distinguish dosing, sensors, temperature, an exhaust leak and the catalyst. Making that distinction is precisely the purpose of system diagnostics.