Kubota P1153 – Engine Control Module (ECM): Closed Loop High (NG)

This code indicates that the engine is running in a lean exhaust condition while operating on Natural Gas (NG) in closed-loop mode, and the Engine Control Module (ECM) has reached its maximum fuel trim adjustment limit (CL_BM < -35%) trying to correct it. The ECM will typically disable closed-loop mode and initiate a 'Power Derate 2' system action to protect the engine.

• WG1605-E3
• WG1605-N-E3
• WG1605-GLN-E3
• WG1903
• WG2503
• WG3800

1. 1

   Check for other DTCs and diagnose those first

2. 2

   Inspect Oxygen Sensor wiring for mispositioning, contact with hot exhaust, or short to ground

3. 3

   Check for vacuum leaks in the intake manifold or crankcase causing unmetered air

4. 4

   Verify natural gas fuel pressure from the tank and the regulator output

5. 5

   Check for exhaust leaks upstream of the oxygen sensor causing a false lean reading

6. 6

   Verify natural gas fuel quality, looking for high inert gas content

7. 7

   Inspect system grounding for the ECM and engine to the battery

1. 1

   Visual & Electrical Inspection

   Inspect the HEGO (Heated Exhaust Gas Oxygen) sensor wiring harness. Repair any melted wires or shorts to ground.

2. 2

   Smoke Test / Leak Check

   Perform a smoke test on the intake manifold and crankcase to locate and seal any vacuum leaks. Inspect the exhaust manifold and piping upstream of the O2 sensor; tighten flanges or replace gaskets if exhaust leaks are found.

3. 3

   Fuel System Test

   Hook up a pressure gauge to the natural gas fuel line to verify the Direct Electronic Pressure Regulator (D-EPR) is outputting the correct fuel pressure to the mixer/injector. If pressure is dropping under load, the regulator or fuel supply valve may be faulty.

4. 4

   Clean Grounds

   Remove, clean, and retighten the main engine and ECM ground straps to the chassis/battery.

5. 5

   Replace Sensor

   If all mechanical (vacuum/exhaust leaks), fuel pressure, and electrical ground tests pass, the HEGO/UEGO sensor itself is likely non-responsive or degraded. Replace the oxygen sensor with a known good OEM part, clear the codes, and run the engine in closed-loop mode to verify the repair.