P20EE
- Powertrain
- Manufacturer-specific
- Emissions
Diagnose soon
There is no mechanical danger and the engine is not at risk. But this is an automatic emissions inspection failure, and many diesels respond to a confirmed selective catalytic reduction fault with a staged inducement that ends in a severe speed limit. That countdown is the practical urgency here, not engine damage.
- Safe to drive
- Usually drives normally at first. Many diesels start an inducement countdown ending in a severe speed derate, so address it before that.
NOx Catalyst Efficiency Below Threshold (Bank 1)
The engine computer compared how much NOx goes into the selective catalytic reduction catalyst against how much comes out, and the catalyst is not removing enough. That can be the catalyst itself, but it is more often the diesel exhaust fluid, the dosing system, or one of the NOx sensors.
The selective catalytic reduction system injects urea-based diesel exhaust fluid into hot exhaust, where it decomposes to ammonia and reacts with NOx across the catalyst. The module calculates conversion efficiency from the upstream and downstream NOx sensor readings, or from modelled engine-out NOx against the downstream sensor. P20EE confirms when that efficiency stays below the calibrated threshold across a qualifying evaluation window. Bank 1 is the designation of the exhaust bank being evaluated on engines that have more than one.
How the car detected itThe monitor only runs when its prerequisites are satisfied: catalyst temperature inside the active reduction window, dosing active and unfaulted, fluid quality not flagged, both NOx sensors reporting plausibly, and a reasonably steady load. Under those conditions the module integrates NOx across a sustained period and computes the ratio removed. It typically needs a long, steady highway drive to complete, which is why the code may not return quickly after a clear.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Usually drives normally at first. Many diesels start an inducement countdown ending in a severe speed derate, so address it before that.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- On most diesels this begins an inducement sequence measured in distance or engine hours, ending with the vehicle limited to a very low speed. Being derated to walking pace on a highway is a genuine hazard as well as an enormous inconvenience. If the underlying cause is hydrocarbon contamination from a leaking injector or excessive regeneration fuelling, continued operation keeps poisoning the catalyst, converting an inexpensive repair into an expensive one. The vehicle will also fail any emissions inspection.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | SCR catalyst aged or poisoned | Very common | — |
| 2 | NOx sensor drift | Common | — |
| 3 | Exhaust leak between upstream and downstream NOx sensors | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Every other stored code, especially dosing, fluid quality, fluid heater, NOx sensor, and exhaust temperature faults
This is an outcome code. The module only runs the efficiency test when it believes the supporting systems are healthy, so a companion fault usually names the real problem and must be fixed first.
Available manufacturer software updates and technical service bulletins
Selective catalytic reduction strategies have been revised repeatedly by most manufacturers. A calibration update sometimes resolves this outright, and it is far cheaper than any hardware.
The diesel exhaust fluid actually in the tank, tested with a refractometer
Concentration should sit close to 32.5 percent urea. Water, washer fluid, diluted fluid, or fluid that sat hot or beyond its shelf life all cause real conversion failures, and the test takes under a minute.
Upstream and downstream NOx sensor readings once both sensors are hot and reporting valid data, but before dosing begins
Once both sensors are hot and reporting valid data, but before dosing starts, they are sampling the same untreated exhaust and should read closely together. A large split under those conditions points straight at a drifted sensor and avoids condemning the catalyst. Readings taken at a cold key-on are not valid, because the sensors do not measure NOx until their heaters have brought them up to temperature.
The exhaust between the two NOx sensors for leaks
Air drawn in through a leak dilutes the downstream sample and fakes poor conversion. A visual and audible check of clamps, welds, and the catalyst housing costs nothing.
Read all codes and freeze frame across all modules
Tools Manufacturer-level scan tool
Capture the conditions the code set under, then list every stored and pending fault. Resolve any dosing, fluid quality, heater, NOx sensor, or exhaust temperature code before pursuing this one.
ExpectedA clean supporting system, or a named companion fault that becomes the first repair.
Check for calibration updates and bulletins
Tools Manufacturer service information subscription, programming tool
Query the manufacturer for outstanding software updates covering selective catalytic reduction strategy and NOx sensor handling on this vehicle. Apply them before touching hardware.
ExpectedEither the module is already at the latest calibration, or an update is applied and the fault is re-evaluated over a full drive cycle.
Test the diesel exhaust fluid quality
Tools Diesel exhaust fluid refractometer
Draw a sample from the tank and measure urea concentration with a refractometer. Note storage age and whether the vehicle has been topped from an opened container kept in heat.
ExpectedConcentration close to 32.5 percent urea and a clear, colourless sample with no sediment.
Compare both NOx sensors after warm-up, before dosing starts
Tools Scan tool live data
Wait until both sensors have completed their dew-point release, come up to operating temperature, and are reporting valid data, then read upstream and downstream NOx at a warmed-up idle or light cruise with the reductant injector inactive. Both sensors are sampling the same untreated exhaust at that point. Do not compare them at a cold key-on: NOx sensors output no valid measurement until their heaters are enabled, so readings taken then are defaults and will condemn good sensors.
ExpectedBoth sensors reading closely together once valid. A meaningful split with no dosing occurring implicates the drifted sensor, and drift does not set a circuit code.
Log conversion efficiency during a steady highway cruise
Tools Scan tool with data logging, safe highway route
With the catalyst at operating temperature and dosing active, record upstream NOx, downstream NOx, commanded dose quantity, and catalyst temperature over a sustained steady drive.
ExpectedDownstream NOx dropping well below upstream once the catalyst is hot and dosing. Downstream tracking upstream almost exactly means no reduction is occurring.
Verify that fluid is actually being delivered
Tools Scan tool with bidirectional dosing test, factory pressure specifications
Check dosing pump pressure build and hold, commanded versus actual dose quantity, and measured fluid consumption against distance travelled. A pump that cannot build or hold pressure, or an injector that will not flow, starves the catalyst regardless of its condition.
ExpectedPump reaching and holding commanded pressure, and consumption consistent with commanded dose.
Inspect the decomposition tube, mixer, and injector for urea crystallisation
Tools Hand tools, borescope
Remove the dosing injector and inspect the injection point and mixing element. Hard white urea deposits block or misdirect the spray so the reductant never mixes properly with the exhaust.
ExpectedA clean injector tip and an unobstructed mixing path with no crystalline buildup.
Pressure or smoke test the exhaust between the NOx sensors
Tools Exhaust smoke machine
Include the catalyst housing, clamps, welds, and sensor bungs. Any leak in this section admits air and corrupts the downstream reading.
ExpectedNo leaks anywhere between the upstream and downstream sensor locations.
Condemn the catalyst only on documented evidence
Tools Complete data log, factory service information
Conclude the catalyst is the fault only when fluid quality is verified, dosing is confirmed delivering, both NOx sensors are verified accurate, no exhaust leak exists, and efficiency still fails a properly run monitor. Document each of those before ordering.
ExpectedEvery alternative documented as ruled out, with a repeated failing efficiency measurement under valid conditions.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Drain the tank and refill with fresh specification diesel exhaust fluid. Cost depends heavily on whether the tank has a drain: many designs require siphoning around the in-tank pump and heater module or removing the tank, and any contaminated fluid left behind re-contaminates the fresh fill | Refractometer shows urea concentration outside specification, or the sample is contaminated | $80–$450 | DIY |
| Clean urea crystallisation from the injector, decomposition tube, and mixer | Visible hard deposits found at the injection point or in the mixing element on inspection | $200–$700 | DIY |
| Replace the dosing injector or dosing pump | Pump cannot build or hold commanded pressure, or the injector fails a flow test, during bidirectional testing | $250–$1,200 | DIY |
| Replace a NOx sensor. Many NOx sensors must be configured or calibrated to the module after fitting, and a sensor swapped in without that step will not resolve the fault, so confirm the requirement for the specific vehicle before buying the part | Large disagreement between the two sensors while both are hot, reporting valid data, and sampling untreated exhaust before dosing, or a sensor failing to respond correctly during a logged cruise | $350–$1,200 | Shop |
| Repair an exhaust leak between the upstream and downstream NOx sensors | Smoke or pressure test identifies escaping gas within that section | $150–$600 | DIY |
| Apply a manufacturer calibration update | An outstanding software update or bulletin exists addressing selective catalytic reduction efficiency on this vehicle | $80–$300 | Shop |
| Replace the selective catalytic reduction catalyst | Fluid quality, dosing delivery, both NOx sensors, and exhaust integrity all verified good, with efficiency still failing a valid monitor run | $1,500–$5,000 | Shop |
NOx sensors are surprisingly expensive parts and many require programming after fitting, so the labour is not trivial either. Catalyst pricing varies by an order of magnitude between a light-duty passenger diesel and a heavy-duty truck, and warranty coverage on emissions components is often longer than the base vehicle warranty, so check coverage before paying.
- Replacing the selective catalytic reduction catalyst first. It is the most expensive part in the system and rarely the actual failure on a vehicle with reasonable mileage.
- Topping up the fluid tank without ever testing what is already in it. Contaminated fluid is a leading cause and a top-up dilutes the evidence.
- Comparing the two NOx sensors before their heaters have brought them to operating temperature. Until the dew-point release is complete they are not measuring NOx at all, and the default values look like a huge split.
- Assuming a NOx sensor is fine because it has no circuit code. Sensors drift out of accuracy long before they fail electrically.
- Ignoring a hydrocarbon source such as a leaking injector or excessive post-injection fuelling. That contamination will poison a replacement catalyst too.
- Clearing the code and expecting a fast re-check. This monitor needs a long, steady, warm drive to run at all.
- Overlooking exhaust leaks at the sensor bungs themselves, which are a small and easily missed source of dilution.
- SCR Catalyst
- NOX Sensor Upstream
- NOX Sensor Downstream
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
Found something wrong? Tell us — we publish corrections.