P20EE Code: SCR NOx Catalyst Efficiency Below Threshold Bank 1
The SCR catalyst isn't removing enough NOx; DEF quality, dosing, a NOx sensor or the catalyst is at fault.
At a glance
Severity: Moderate
- Meaning
- SCR NOx Catalyst Efficiency Below Threshold Bank 1
- System
- Emission Controls · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $100–$5,000
- DIY difficulty
- Moderate
A selective catalytic reduction (SCR) system sprays diesel exhaust fluid (DEF, a 32.5 % urea solution) into the exhaust, where it turns into ammonia and converts NOx into nitrogen and water across the SCR catalyst. The ECM measures NOx before the catalyst with NOx sensor 1 and after it with NOx sensor 2, and calculates how much of it was removed. P20EE sets when that conversion rate stays below the calibrated threshold over several drive cycles. Although the code names the catalyst, the catalyst itself is rarely the first culprit: watered-down or old DEF, a crusted dosing injector, weak DEF pressure or a drifting NOx sensor produce the same low number. Because it is an emissions-critical fault, most trucks start a countdown to a speed limit if it isn't repaired.
Can I keep driving? The truck drives normally at first, but watch the dash: most systems display a countdown and then restrict speed, sometimes down to about 5 mph, after a set distance or a restart. Get it diagnosed before that countdown runs out, especially before a long trip or towing.
How to read P20EE
- P Powertrain
- 2 Generic (SAE)
- 0 Fuel/air & emissions (aux.)
- EE Specific fault
Symptoms
- Check engine light, often with a 'DEF system fault' or 'Emissions system fault' message
- Mileage countdown to a speed limitation shown in the instrument cluster
- Speed limited to 55 mph, then much lower, once the countdown ends
- DEF consumption lower than usual, or unchanged tank level over many miles
- White crusty deposits at the DEF injector or tailpipe
- No drivability change until the derate starts
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Poor DEF quality: diluted with water, contaminated or degraded by heat and age
- DEF dosing injector partly clogged by urea crystals or sticking
- NOx sensor 1 or sensor 2 drifting, so the calculated efficiency is wrong
- Low DEF delivery: weak pump, clogged pump filter, leaking line or frozen line with a failed heater
- SCR catalyst aged, overheated, or coated with soot, oil or DEF deposits
- Exhaust leak near a NOx sensor, or excess engine-out NOx from an EGR fault
- ECM software that needs a calibration update
Diagnosis step by step
Read all codes and the countdown status
Note any DEF quality, pressure, heater or NOx sensor codes stored with P20EE; they usually point straight at the root cause. Record the remaining miles before speed limitation so you know how much time you have.
Tools: OBD-II scanner
Test the DEF itself
Draw a sample from the tank and check it with a DEF refractometer; it should read close to 32.5 % urea. Anything well below that, a cloudy sample or a smell of fuel means draining the tank, flushing and refilling with fresh, sealed DEF.
Tools: Gloves
Run a DEF dosing quantity test
Most OEM-level scan tools can command a purge or dosing test into a measuring cup over a fixed time. Too little fluid, a dripping injector or a poor spray pattern points to the injector, pump filter or lines rather than the catalyst.
Tools: OBD-II scanner, Gloves
Compare the two NOx sensors
At a warm idle with dosing inhibited, both NOx sensors see the same gas and should read within a few ppm of each other. During steady highway cruising with dosing active, sensor 2 should read far lower than sensor 1; if it barely drops, dosing or the catalyst is weak, and if the idle cross-check doesn't match, suspect a sensor.
Tools: OBD-II scanner
Inspect injector and catalyst inlet for crystals
Pull the dosing injector and look for white urea crystals on the tip and in the mixer. Heavy deposits mean clean the mixer and injector boss, then look for why they formed: low exhaust temperature from lots of idling, a leaking injector or poor DEF.
Tools: Flashlight, Gloves
Check for software and run the verification
Ask a dealer whether a calibration update exists for your engine; several manufacturers have revised SCR monitoring. After the repair, clear the codes and complete an SCR efficiency self-test or a steady highway drive to confirm the monitor passes.
Tools: OBD-II scanner
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Drain, flush and refill DEF tank with fresh fluid | Yes | $100–$300 |
| Replace or clean DEF dosing injector | No | $350–$900 |
| Replace NOx sensor (upstream or downstream) | No | $400–$900 |
| Replace DEF pump / tank module with heater | No | $900–$2,500 |
| Replace SCR catalyst | No | $2,000–$5,000 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Ford
P20EE is one of the most talked-about codes on the 6.7L Power Stroke. Technicians typically check DEF quality, NOx sensor readings and available PCM updates before the expensive SCR catalyst is considered.
Ram
On Ram trucks with the 6.7L Cummins, NOx sensors are a frequent replacement item and a drifting sensor commonly sets P20EE together with a NOx sensor code, so compare both sensors before replacing the catalyst.
Chevrolet
On Duramax pickups, P20EE often comes with DEF heater or NOx sensor codes. Fix those first; the efficiency code usually clears once dosing and sensing are correct.
Emissions / smog test impact
Depends
In states that OBD-test light-duty diesels, P20EE with the check engine light on fails the test; others exempt diesels. Removing or defeating the SCR system is federal tampering, fails any test that checks it and can bring heavy fines.
Frequently asked questions
Why is my truck counting down to 5 mph?
Federal rules require diesels to 'induce' the driver to fix SCR faults, so the ECM limits speed after a set distance or restart. The countdown stops once the fault is repaired and the SCR monitor passes again.
Can bad DEF really cause P20EE?
Yes. DEF that has been diluted, contaminated or cooked in a hot garage for a long time has less urea, so less NOx is converted. Buy sealed fluid, don't store it above roughly 86°F for long, and never top up with water.
Does the SCR catalyst need replacing?
Usually not first. DEF quality, the dosing injector, pump pressure and NOx sensor readings are cheaper and more often to blame; the catalyst is replaced only after those check out.
Does a lot of idling matter?
It does. At idle the exhaust is often too cool for good conversion and DEF deposits can form in the mixer. Regular loaded driving keeps the SCR system hot and clean.
Sources
- SAE J2012 – Diagnostic Trouble Code Definitions
- ISO 15031-6 – Road vehicles, diagnostic trouble code definitions (EOBD)
- US EPA – On-Board Diagnostics (OBD) program
- Manufacturer public service information (cross-checked)
- NHTSA Complaints API
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