P20EE fault code: low SCR catalyst efficiency
Plain-language definition
SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 1
What does it mean?
Many newer diesel cars have an SCR (selective catalytic reduction) system to reduce the nitrogen oxides (NOx) in the exhaust. The system injects AdBlue (a urea solution) from a separate tank into the exhaust. In the hot exhaust, AdBlue turns into ammonia; the ammonia then reacts with NOx in the SCR catalyst and turns most of it into nitrogen and water vapour.
Using information from the NOx sensors, the engine computer calculates by how much the SCR catalyst is reducing the amount of NOx. If this reduction stays below a set threshold, it stores P20EE. "Bank 1, Catalyst 1" in the definition shows that the code concerns the first SCR catalyst in the exhaust system.
Most of the time there's no noticeable difference while driving; the check engine light or an AdBlue or emissions warning may come on. If the problem continues, some cars show a countdown warning on the instrument cluster.
How serious is it?
This is a Warning-level code, but if the problem isn't fixed, on some cars the engine may not restart once the countdown runs out. That's why it's important to find the cause early.
Severity is a guide; if the warning light is flashing or you notice symptoms while driving, have the car checked at a workshop.
Common general causes
- Poor-quality, diluted or old AdBlue; the wrong fluid put in the tank
- One of the NOx sensors measuring incorrectly
- A clogged AdBlue injector, or crystals (urea) building up around it
- Too little injection (dosing): a very low AdBlue level, or problems with the pump, lines or heater
- An aged, contaminated or damaged SCR catalyst
- Less often, an exhaust leak or an exhaust temperature sensor giving wrong values
The list shows general causes; a workshop finds the actual cause on your car with a check.
How does it appear in the 2dakika report?
SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 1
If the code is read as a confirmed (stored) fault, the report shows it with the "Warning" severity; in pending or past records, codes that don't request the warning light and aren't critical appear as "Info". If the engine computer stored them, the conditions when the fault was recorded (engine temperature, rpm, speed) are shown too.
Read this code with the OBD-X1, and clear it from the app after the repair.
Clearing the code doesn't fix the fault; if the problem persists, the code comes back.