SCR Efficiency Below Threshold
The Selective Catalytic Reduction (SCR) system is not performing as expected.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
The SCR system, which helps reduce harmful nitrogen oxide emissions, is not working well. This happens when the system does not convert at least 90% of the NOx gases it receives from the engine into harmless nitrogen and water, causing higher pollution levels.
What the computer is actually seeing
The engine control unit (ECU) monitors the SCR system's output and input NOx levels. It expects the conversion efficiency to be above a certain threshold, often around 90%. If the output NOx levels exceed input levels significantly, indicating poor efficiency, it triggers SPN 3295.
What a healthy reading looks like
Input NOx levels should typically be below 2.0 grams per kilowatt-hour (g/kWh) for proper SCR function; output levels should be less than 0.2 g/kWh when the SCR is functioning correctly.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Check DEF level; it should be above the minimum threshold and the fluid should be clean. If low or contaminated, correct the issue before further testing.
- 22. Inspect all wiring and connectors to the SCR system and NOx sensors for damage or corrosion. Repair as necessary.
- 33. Test the NOx sensors. Use a scan tool to monitor real-time values. If one sensor shows fully open or stuck readings, replace that sensor.
- 44. Run a diagnostic scan after repairs and check for the reoccurrence of the code. If it returns, consider inspecting the SCR catalyst for blockage or damage.
Common causes
- Faulty SCR catalyst
- Contaminated DEF (Diesel Exhaust Fluid)
- Faulty NOx sensors
- Low DEF level or concentration
- SCR module failure
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with SPN 3295 active?
It’s not recommended to drive for long periods, as it can lead to increased emissions and potential fines.
Will this code come back after clearing?
If the underlying issue is not resolved, the code will likely return. Make sure to check all components of the SCR system.
What does it mean if my truck goes into limp mode?
Limp mode is usually triggered to protect the engine and emissions system from damage when critical failures are detected, such as persistent SCR issues.
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