Cummins/Onan DFM 300 troubleshooting: 7 fault codes and 17 symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan DFM 300 — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Code | Meaning | Cause | Remedy |
|---|---|---|---|
| PRE HI ENGINE TEMP | *2. PRE HI ENGINE TEMP lamp lights. Engine continues to operate. | 2. Indicates engine has begun to overheat and engine temperature has risen to approximately 205°F (97°C). If generator is powering non-critical and critical loads and cannot be shutdown, use the following: a. Reduce load if possible by turning off non-critical loads. b. Check air inlets and outlets and remove any obstructions to airflow. c. Open doors or windows in generator area to increase ventilation. If engine can be stopped, follow procedure in step 3. | |
| HI ENG TEMP | *3. HI ENG TEMP lamp lights. Engine shuts down. | 3. Indicates engine has overheated (engine temperature has risen above 215°F/102°C) or coolant level is low (sets with coolant level sensor). Allow engine to cool down completely before proceeding with the following checks: a. Check coolant level and replenish if low. Look for possible coolant leakage points and repair if necessary. b. Check for obstructions to cooling airflow and correct as necessary. c. Check for a slipping fan belt and tighten if loose. d. Reset control and restart after locating and correcting problem. Contact an Onan Dealer or Distributor if none of the above. | |
| PRE LO OIL PRES | *4. PRE LO OIL PRES lamp lights. Engine continues to operate. | 4. Indicates engine oil pressure has dropped to 20 psi (138 kPa). If generator is powering critical loads and cannot be shut down, wait until next shutdown period and then follow step 5 procedure. If engine can be stopped, follow procedures in step 5. | |
| LO OIL PRES | *5. LO OIL PRES lamp lights. Engine shuts down NOTE: See also step 6. | 5. Indicates engine oil pressure has dropped to 14 psi (97 kPa). Check oil level, lines and filters. If oil system is okay but oil level is low, replenish. Reset control and restart. Contact an Onan Dealer or Distributor if oil pressure is not in the range of 50 to 70 psi (345 to 483 kPa). | |
| OVERCRANK | *6. OVERCRANK lamp lights and engine stops cranking. | ||
| FAULT 1 (red) | Undedicated fault. May be factory programmed as a shutdown or non-shutdown, and as a timed or non-timed (normally set for timed shutdown). | @ FAULT 1 (red) an undedicated fault. May be factory programmed as a shutdown or non-shutdown, and as a timed or non-timed (normally set for timed shutdown). | |
| FAULT 2 (red) | Same features as Fault 1 (normally set for timed shutdown). | @ FAULT 2 (red) same features as Fault 1 (normally set for timed shutdown). |
| Symptom | Cause | Remedy |
|---|---|---|
| Green RUN lamp lights following engine start-up | Indicates all engine systems are normal. No corrective action required. | |
| PRE HI ENGINE TEMP lamp lights. Engine continues to operate | Engine has begun to overheat and engine temperature has risen to approximately 205°F (97°C) | If generator is powering non-critical and critical loads and cannot be shutdown, reduce load if possible by turning off non-critical loads; check air inlets and outlets and remove any obstructions to airflow; open doors or windows in generator area to increase ventilation; if engine can be stopped, follow procedure in step 3 |
| HI ENG TEMP lamp lights. Engine shuts down | Engine has overheated (engine temperature has risen above 215°F/102°C) or coolant level is low (sets with coolant level sensor) | Allow engine to cool down completely before proceeding: check coolant level and replenish if low; look for possible coolant leakage points and repair if necessary; check for obstructions to cooling airflow and correct as necessary; check for a slipping fan belt and tighten if loose; reset control and restart after locating and correcting problem; contact an Onan Dealer or Distributor if none of the above |
| PRE LO OIL PRES lamp lights. Engine continues to operate | Engine oil pressure has dropped to 20 psi (138 kPa) | If generator is powering critical loads and cannot be shut down, wait until next shutdown period and then follow step 5 procedure; if engine can be stopped, follow procedures in step 5 |
| LO OIL PRES lamp lights. Engine shuts down | Engine oil pressure has dropped to 14 psi (97 kPa) | Check oil level, lines and filters; if oil system is okay but oil level is low, replenish; reset control and restart; contact an Onan Dealer or Distributor if oil pressure is not in the range of 50 to 70 psi (345 to 483 kPa) |
| OVERCRANK lamp lights and engine stops cranking; or engine runs, shuts down, and LO OIL PRES lamp lights | Possible fuel system problem | Check for empty fuel tank, fuel leaks, or plugged fuel lines and correct as required; check for dirty fuel filter and replace if necessary (see MAINTENANCE section); check for dirty or plugged air filter and replace if necessary (see MAINTENANCE section); refer to Step 5; reset the control and restart after correcting the problem; contact an Onan Dealer or Distributor for service if none of the above |
| Engine runs and then shuts down, OVERSPEED lamp lights | Engine has exceeded normal operating speed | Contact an Onan Dealer or Distributor for service |
| SWITCH OFF lamp flashes | Run/Stop/Remote switch is in the Stop position which will prevent automatic starting if an automatic transfer switch is used | Move the Run/Stop/Remote switch to the Remote position for automatic starting |
| LO FUEL lamp lights. Engine continues to run | Diesel fuel supply is running low | Check fuel supply and replenish as required |
| LO FUEL lamp lights | Engine has run out of fuel | Check fuel level and replenish as required |
| LO ENG TEMP lamp lights. Set is in standby mode but is not operating (lamp lights when engine coolant temperature is 70°F (21°C) or lower; since the lamp goes out after the engine warms up, there should be no cause for alarm even during initial generator set operation) | Engine coolant heater is not operating or is not circulating coolant | Check for the following conditions: coolant heater not connected to power supply — check for blown fuse or disconnected heater cord and correct as required; check for low coolant level and replenish if required, look for possible coolant leakage points and repair as required; contact an Onan Dealer or Distributor if none of the above |
| The FAULT 1 or FAULT 2 fault lamp lights. Engine shuts down immediately, engine runs for several seconds and then shuts down, or engine continues to run | The standard undesignated fault functions are programmed to shut down the set when a fault is sensed. Fault 1 is timed delayed while Fault 2 is immediate. The nature of the fault is an optional selection that is determined when the set installation is designed. The undesignated fault functions may also be programmed for non-shutdown or non-time delay | |
| Fault lamp lights but no fault exists. Engine gauges show oil pressure, engine temperature, and frequency (speed) are within normal limits | The monitor board or a sensor may be at fault | Contact an Onan Dealer or Distributor for service |
| Engine starts from generator control panel but will not start automatically or from a remote panel (Note: The Run/Stop/Remote switch must be in the Remote position for automatic or remote starting) | Remote circuit breaker is tripped | Reset breaker and restart; contact an Onan Dealer or Distributor if breaker trips after resetting |
| Engine will not crank | Possible fault with control or starting system | Check for the following conditions: fault lamp on — correct fault and reset control; poor battery cable connections — clean the battery cable terminals and tighten all connections; discharged or defective battery — recharge or replace the battery; contact an Onan Dealer or Distributor for assistance if none of the above |
| No AC output voltage | Field breaker is tripped | Reset breaker; contact an Onan Dealer or Distributor if voltage build up causes breaker to trip |
| Green RUN lamp does not light following engine start-up | Possible Start/Disconnect relay failure | Contact an Onan Dealer or Distributor for assistance |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Engine has begun to overheat and engine temperature has risen to approximately 205°F (97°C). See the table for the matching fixes.
Documented causes: Engine has overheated (engine temperature has risen above 215°F/102°C) or coolant level is low (sets with coolant level sensor). See the table for the matching fixes.
Documented causes: Engine oil pressure has dropped to 20 psi (138 kPa). See the table for the matching fixes.