SCR Catalyst
The catalyst that converts oxides of nitrogen into nitrogen and water, using ammonia released from the reductant fluid injected ahead of it. Selective catalytic reduction is a chemical process rather than a filter, which is the distinction that matters: nothing accumulates in it to be cleaned out, and it fails by losing activity rather than by blocking.
It is judged entirely by the NOx sensors either side of it, so a sensor reading incorrectly condemns a perfectly good catalyst — verifying them comes before replacing anything here. It loses activity through thermal damage, oil contamination and sulphur poisoning, and a sufficiently aged one behaves indistinguishably from a substitute of the wrong specification.
Its faults escalate rather than sit still. A system that cannot meet its NOx target moves through the legally mandated inducement stages — warnings, then a speed limit, then a refusal to restart — so a stored efficiency code is a deadline rather than an advisory.
Also known as: scr, selective catalytic reduction catalyst, denox catalyst, urea catalyst, adblue catalyst
Referenced by 65 codes
Codes involving this component (65)
- P241D SCR Inducement - Forced Engine Shutdown Stop driving Critical
- P207F Reductant Quality Performance High
- P208E Reductant Injection Valve Stuck Closed Bank 1 Unit 1 High
- P20EE SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 1 High
- P20F4 Reductant Consumption Too Low High
- P20F6 Reductant Injection Valve Stuck Open Bank 1 Unit 1 High
- P214D SCR NOx Catalyst Outlet Temperature Too High During Particulate Filter Regeneration High
- P225D NOx Sensor Performance - Signal Biased/Stuck Low Bank 1 Sensor 1 High
- P24FE SCR Catalyst Incorrect High
- P225C NOx Sensor Performance - Signal Biased/Stuck High Bank 1 Sensor 1 Moderate
- P229E NOx Sensor Circuit Bank 1 Sensor 2 Moderate
- P229F NOx Sensor Circuit Range/Performance Bank 1 Sensor 2 Moderate
- P22A7 NOx Sensor Heater Sense Circuit Range/Performance Bank 1 Sensor 2 Moderate
- P22FA NOx Sensor Performance - Slow Response High to Low Bank 1 Sensor 1 Moderate
- P249C Excessive Time To Enter Closed Loop Reductant Injection Control Moderate
- P249E Closed Loop Reductant Injection Control At Limit - Flow Too High Moderate
- P201A Reductant Injection Valve Circuit Range/Performance Bank 2 Unit 1
- P202D Reductant Leakage
- P202E Reductant Injection Valve Circuit Range/Performance Bank 1 Unit 1
- P202F Reductant/Regeneration Supply Control Circuit Range/Performance
- P204F Reductant System Performance Bank 1
- P205F Reductant System Performance Bank 2
- P208F Reductant Injection Valve Stuck Closed Bank 2 Unit 1
- P20EC SCR NOx Catalyst — Over Temperature Bank 1
- P20ED SCR NOx Pre-Catalyst — Over Temperature Bank 1
Other components in Exhaust and Emissions
- Oxygen Sensor 326
- EGR Valve 150
- Catalytic Converter 142
- NOx Sensor 118
- Reductant Injector 107
- Exhaust Gas Temperature Sensor 95
- Reductant Tank 95
- Diesel Particulate Filter 76
- EGR Cooler 62
- Exhaust Gas Differential Pressure Sensor 58
- Fuel Tank Pressure Sensor 58
- Exhaust Pressure Control Valve 56
- Reductant Heater 55
- Reductant Pump 51
- EVAP Vent Valve 43
- Exhaust Aftertreatment Fuel Injector 42
- Charcoal Canister 33
- EGR Position Sensor 32
- Secondary Air Injection Pump 32
- Reductant Temperature Sensor 29
- EVAP Leak Detection Pump 29
- EVAP Purge Valve 27
- Particulate Matter Sensor 24
- Reductant Control Module 24
- Reductant Level Sensor 23
- Secondary Air Switching Valve 22
- EGR Temperature Sensor 21
- Secondary Air Check Valve 21
- Catalyst Heater 20
- Catalyst Temperature Sensor 20
- Fuel Filler Cap 18
- EGR Cooler Bypass Valve 16
- Reductant Quality Sensor 16
- Exhaust Back Pressure Sensor 15
- Purge Flow Sensor 13
- Direct Ozone Reduction Catalyst 13
- EGR Vacuum Solenoid 9
- Fuel Vapour Temperature Sensor 5
- Fuel Additive Control Module 4
- Fuel Additive Tank 4
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Working on this component
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about SCR Catalyst.