SCR Conversion Efficiency Below Threshold

Common Symptoms

• Instrument cluster displays fault code 4364-17 with a warning lamp
• Yellow or amber SCR warning light stays on during operation
• Engine may derate to reduced power or limited RPM after extended fault time
• DEF tank appears full but the system continues to flag low conversion efficiency
• Exhaust may have a faint ammonia smell if DEF is over-dosing
• AGCO EDT (Electronic Diagnostic Tool) shows NOx outlet values close to or exceeding NOx inlet values
• Fault persists after a key cycle and does not clear on its own

Probable Causes (Ranked by Likelihood)

• DEF fluid is contaminated, diluted, or incorrect concentration (urea not at 32.5%) Very Likely
• DEF dosing injector partially clogged or not delivering correct spray pattern into the SCR catalyst Very Likely
• SCR catalyst degraded, poisoned by excess sulfur, or physically damaged internally Likely
• Downstream NOx sensor failed or reading incorrectly, falsely reporting poor conversion Likely
• DEF supply pump worn or losing pressure, causing under-dosing Possible
• Exhaust temperature too low for SCR reaction (extended low-load or idle operation cooling the ATS) Possible
• Upstream NOx sensor drift causing ECM to miscalculate conversion ratio Less Likely

Step-by-Step Diagnostic Procedure

1. Check the DEF tank first. Drain a small sample into a clear container. Good DEF is clear to slightly yellow with no cloudiness, sediment, or oily sheen. If it looks off, drain and refill with fresh DEF meeting ISO 22241 spec.

2. Use a DEF refractometer (available at most farm supply stores for under $30) to verify urea concentration. The reading should be 32.5% plus or minus 1.5%. A reading outside that range means the fluid is bad regardless of how new it looks.

3. Inspect the DEF dosing injector at the SCR inlet pipe. Look for white crystalline buildup around the tip. A partially blocked injector is one of the most common causes of this code. If buildup is present, the injector tip can often be soaked in warm distilled water to dissolve deposits.

4. With AGCO EDT connected, navigate to the ATS diagnostics screen and compare live NOx inlet (upstream sensor) vs. NOx outlet (downstream sensor) values at operating temperature. A healthy SCR should show outlet NOx at least 70-90% lower than inlet. If the gap is narrow, the catalyst or dosing system is the problem. If the outlet reads near zero while inlet reads normal, suspect the outlet NOx sensor.

5. Check exhaust temperature at the SCR inlet using AGCO EDT live data. The SCR reaction requires catalyst temperature above approximately 200 degrees Celsius to begin working and is most efficient above 250 degrees Celsius. If you have been running light loads or extended idle, run the tractor under load for 20-30 minutes and recheck.

6. Inspect the DEF supply line from the tank to the dosing module for kinks, cracks, or loose fittings. Check the DEF filter (located in the supply module on most AGCO Power installations) for restriction. A plugged filter will reduce DEF flow and cause under-dosing.

7. If all fluid and injector checks pass, connect AGCO EDT and run the SCR dosing actuator test to confirm the pump is building correct pressure (typically 5-9 bar depending on model). If pump pressure is low, the DEF pump module likely needs replacement. This step requires AGCO EDT and dealer-level access.

8. If the code returns after correcting DEF quality and clearing with AGCO EDT, suspect the downstream NOx sensor or a degraded SCR catalyst. Sensor replacement is a moderate DIY task. Catalyst replacement is advanced and should be confirmed by a dealer technician before purchasing parts.

Frequently Asked Questions