P20EC

Code

Generic P — Powertrain

SCR NOx Catalyst - Over Temperature Bank 1

Brand: Generic

Type: P — Powertrain

Views: UK: 27 EN: 38 RU: 29

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Page language: EN

Causes

Actual SCR catalyst temperature above safe threshold (excessive exhaust heat)
Faulty or out-of-spec SCR/exhaust temperature sensor (Bank 1)
DEF/AdBlue dosing too high or wrong dosing strategy causing exothermic reaction
Blocked or restricted exhaust / failed DPF resulting in elevated temps
Exhaust leak causing hot gases to impinge on catalyst housing or sensor
Wiring/connectors open, shorted, corroded or poor ground to temp sensor

Symptoms

Malfunction Indicator Lamp (MIL) illuminated
Reduced engine power or limp mode depending on calibration
Service messages related to SCR/NOx system or high exhaust temperature
Increased DEF consumption or DEF system warnings
Unusual exhaust odor, visible smoke, or evidence of exhaust leaks
Possible increased NOx emissions / failed emissions test

What to check

Retrieve freeze frame and live data for exhaust/ SCR temperature sensors and DEF dosing commands
Check for additional related DTCs (NOx sensor, DEF system, DPF, exhaust leaks)
Visual inspection of exhaust system, catalyst housing and heat shields for damage or soot
Inspect wiring and connectors for the SCR/exhaust temperature sensor Bank 1 for corrosion, breaks or heat damage
Verify DEF quality, level, and injector/valve operation
Compare upstream and downstream temperature sensor readings under load and idle

Signal parameters

Exhaust/SCR temperature sensor normal range typically ~200–650°C during operation; over-temperature trip commonly set above ~700°C (consult OEM spec)
Temperature sensor types vary: thermistor/RTD or thermocouple — expected output may be 0–5 V (RTD/thermistor circuits) or millivolts for thermocouples; confirm with service data
Typical NOx/DEF system commands: DEF dosing pulse width and injector duty should correlate with engine load and NOx readings
Look for abrupt jumps, stuck readings, or readings that exceed sensor/ECM maximum values in live data

Diagnostic algorithm

Read and record all stored and pending codes, freeze frame and live data. Note engine load, speed and sensor temps when code set.
Perform a visual inspection of the exhaust system, SCR catalyst area, heat shields and wiring harness for heat damage, leaks or contamination.
Verify DEF quality and level. Check DEF injector/solenoid operation and plumbing for leaks or clogging.
Monitor exhaust/SCR temperature sensors (upstream and downstream of SCR) during cold start, steady idle and a controlled loaded run to reproduce the condition safely.
Inspect and test the temperature sensor Bank 1 electrical circuit: check connector condition, continuity to ECM, reference voltage and signal wiring for short to power/ground.
If sensor output is suspect, test sensor per manufacturer resistance/voltage chart or substitute a known-good sensor to confirm behavior.
Check for exhaust restrictions: measure backpressure, inspect DPF and catalyst for clogging or melted substrate. Use borescope if available to inspect internal substrate.
Verify ECM commands for DEF dosing match expected NOx levels and that dosing is not excessive; check NOx sensor readings upstream/downstream for plausibility.
If wiring and sensors are good but over-temperature persists, suspect catalyst damage or internal restriction. Replace SCR catalyst or repair exhaust as required.
After repairs, clear codes and perform a controlled road test and recheck live data to confirm temps remain within specification.

Likely causes

Defective exhaust/SCR temperature sensor (Bank 1)
Excessive DEF dosing or DEF system malfunction producing local overheating
Restricted exhaust (DPF/catalyst) causing high backpressure and temperature
Wiring harness connector damage or corrosion at sensor
Actual catalyst degradation/damage from prior overheating or contamination

Fault status

⚠️ Status

SCR NOx Catalyst Over Temperature (Bank 1) — high exhaust/catalyst temperature detected. Inspect sensor, wiring, DEF dosing, and exhaust system to prevent catalyst damage.

🔴 Repair difficulty: Hard

⏱️ Diagnostic time: 2.0-6.0 hours

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P20EC — SCR NOx Catalyst - Over Temperature Bank 1