P20EE: SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 1
What to do
Do not agree to a catalyst replacement on this code alone. It says the emissions system is not converting enough, which has several possible causes — and the catalyst is usually not the first one. Faulty sensors, poor quality fluid, or a blocked injector all produce this and cost a fraction as much.
Ask what else is stored. If there is a fault about reductant dosing, quality or pressure, that is very likely the actual problem.
Mention where the reductant fluid has come from. Weak or wrong fluid is a common cause, and months of it is also what wears a catalyst out early.
Do not leave it. The system escalates towards limiting the vehicle's speed and eventually refusing to restart — and fixing it at that stage is far more disruptive.
What this code means
The selective catalytic reduction catalyst is not converting enough nitrogen oxide.
This is the headline code of the whole SCR system — the outcome measurement that everything else in this block exists to make possible. The module compares NOx before and after the catalyst and concludes the conversion rate is too low.
Which makes it the least specific code in the family and the one most often misread. It names the result, not the cause, and the catalyst itself is only one of several explanations — often not the likeliest.
Work outward from the cheapest.
- The NOx sensors. They are the measurement, so a lazy or drifted downstream sensor reports poor conversion that is not happening. Verify them before condemning a catalyst.
- Dosing. Fluid quality, a blocked or crystallised injector, low pressure, an empty tank. If reductant is not arriving, no catalyst can convert anything — and every one of those has its own code in this block.
- Exhaust leaks between the sensors, which admit air and skew the reading.
- The catalyst itself, aged, thermally damaged, oil-poisoned or sulphur-poisoned — and this is where a catalyst that has been fed weak fluid for months ends up.
Read every stored code before deciding. A dosing fault alongside this one is almost certainly the cause, and replacing the catalyst without fixing it destroys the replacement too.
Expect escalation. A system that cannot meet its target moves through the inducement stages towards a restart lockout.
Likely causes
Start here
- Faulty NOx sensor · moderate
- Reductant diluted or incorrect fluid · moderate
- Crystallised reductant injector · moderate
Less common
- Aged or contaminated SCR catalyst · specialist
- Exhaust leak upstream of the sensor · moderate
Symptoms
Components involved
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.
Community experience
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- The car — make, model, year and engine. The same fault behaves differently on each.
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