The Ram 3500 Big Horn with the 6.7L Cummins turbo-diesel is a heavy-duty workhorse. Its instrument cluster provides critical alerts for engine, emissions, and drivetrain systems. Understanding these lights ensures reliability and safety under heavy loads.
RAM Cummins 3500 Big Horn Warning Light Symbols
| Symbol | Name | Color | What it means | What to do |
|---|---|---|---|---|
| Master Warning (Stop) | RED | A critical engine or brake fault has been detected in your Ram 3500 Big Horn; continued operation may cause severe damage. | Stop the vehicle safely, turn off the engine, and consult your Cummins service manual or contact a Ram dealer. | |
| Check Engine (MIL) | AMBER | The ECM has detected an emission or performance issue specific to your 6.7L Cummins, such as a faulty sensor or EGR valve. | Have the OBD-II codes read by a technician; you may continue driving if no other warnings, but schedule service soon. | |
| Glow Plug / Preheat | AMBER | The grid heater or glow plugs are active; wait until light turns off before cranking your Cummins diesel in cold weather. | Turn the ignition to Run and wait for the light to extinguish (typically 5-15 seconds), then start the engine. | |
| Water in Fuel | AMBER | Water has accumulated in the fuel/water separator on your Ram 3500, risking injector damage and poor performance. | Drain the water separator located on the driver-side frame rail; refer to the owner's manual for the procedure. | |
| DPF / Regeneration | AMBER | The diesel particulate filter is actively regenerating to burn off soot; this is normal and may increase exhaust temperature. | Continue driving at highway speeds if possible; do not shut down the engine during active regen if indicated. | |
| DEF / AdBlue | AMBER | Diesel exhaust fluid level is low or quality is poor; your Ram 3500 Big Horn will eventually limit speed if ignored. | Refill DEF immediately with API-certified fluid; if light stays on, have the DEF system inspected. | |
| Engine Oil Pressure | RED | Oil pressure in your Cummins 6.7L is critically low, risking bearing failure and engine seizure. | Stop the engine immediately; check oil level and add if low; if pressure remains, do not restart and call for service. | |
| Battery / Charging | RED | The charging system (alternator or battery) is failing on your Ram 3500; electrical power may drop soon. | Check battery connections; if light persists, have the alternator and battery tested; avoid long drives. | |
| Coolant Temperature | RED | Engine coolant temperature has exceeded safe limits on your Cummins; possible thermostat failure or low coolant. | Pull over, turn off engine, let cool; check coolant level and radiator cap; do not open hot system. | |
| Transmission Temp | RED | The 68RFE automatic transmission fluid temperature is dangerously high, often from overloading or heavy towing. | Stop towing immediately, shift to neutral, idle engine to cool transmission; check fluid level and external coolers. | |
| Brake System | RED | A fault exists in the hydraulic brake system, such as low fluid or worn pads on your Ram 3500. | Stop safely; do not drive until brakes are inspected; check brake fluid reservoir and look for leaks. | |
| ABS | AMBER | The anti-lock brake system has a malfunction; normal braking still works but ABS may not engage on slippery roads. | Drive cautiously; have the ABS module and wheel speed sensors checked by a Ram dealer. | |
| Parking Brake | RED | The parking brake is engaged or a fault exists in the parking brake system on your Ram 3500 Big Horn. | Release the parking brake fully; if light stays on, have the switch or actuator inspected. | |
| Tire Pressure (TPMS) | AMBER | One or more tires on your Ram 3500 are significantly under-inflated, affecting load capacity and fuel economy. | Check all tire pressures and inflate to the recommended values on the driver door jamb; reset system if needed. | |
| Airbag (SRS) | AMBER | The supplemental restraint system has detected a fault; airbags may not deploy in a collision in your Ram. | Have the airbag system diagnosed immediately; do not ignore as it affects safety. | |
| Seat Belt | RED | The driver seat belt is not fastened; the chime will sound until it is secured in your Ram 3500. | Buckle your seat belt immediately; the light should go out. | |
| High Beam | BLUE | High-beam headlights are turned on; use caution when approaching other vehicles in your Ram 3500. | Dim your headlights for oncoming traffic to avoid glare. | |
| Low Fuel | AMBER | Fuel level is low; your 31-gallon tank needs refueling soon to avoid air in the fuel system. | Fill up with diesel as soon as possible; do not run the tank dry. | |
| Reduced Power (Limp Mode) | AMBER | The ECM has reduced engine power to protect the Cummins from damage, often due to a sensor or emission issue. | Drive at reduced speed to a repair facility; have the fault codes read and addressed. | |
| High Exhaust Temp (HEST) | AMBER | Exhaust gas temperature is elevated during DPF regeneration; this is normal but avoid parking over flammable materials. | Continue driving until regeneration completes; do not shut down engine. | |
| Door Open | RED | A door, hood, or tailgate is ajar on your Ram 3500 Big Horn; the dome light may stay on. | Check and fully close all doors and the tailgate. |
21 symbols shown. Icons follow ISO 2575 dashboard standards; exact graphics vary by model year.
Most Important Warning Lights
Engine Overheat (Coolant Temp Red)
Common causes
- Low coolant level or coolant leak
- Failed thermostat or water pump
- Clogged radiator or cooling fan failure
What to do
Pull over immediately and shut off your Cummins engine. Let it cool, then carefully check coolant level in the overflow tank. If low, top off with 50/50 coolant and water. If the light persists, do not drive; contact a Ram dealer.
Low Oil Pressure
Common causes
- Insufficient engine oil level
- Worn oil pump or clogged oil pickup
- Diluted oil from fuel or coolant
What to do
Stop the engine instantly. Check oil level with the dipstick; add 15W-40 oil as needed. If level is fine and light remains on, do not restart—call for a tow to avoid catastrophic engine damage.
Transmission Overheat
Common causes
- Extended heavy towing or hauling up grades
- Low transmission fluid level or leak
- Clogged external transmission cooler
What to do
Safely pull over and shift to Neutral. Let the engine idle to cool the 68RFE transmission. Check fluid level with the dipstick (engine running, warm). If low, add ATF+4. Reduce load and avoid overdrive until temps drop.
Brake System Failure
Common causes
- Low brake fluid level due to worn pads or leak
- Failed master cylinder or brake booster
- Brake line rupture
What to do
Stop immediately. Do not drive. Check brake fluid reservoir; if low, add DOT 3 fluid. If the light stays red, the hydraulic system may need professional repair. Use engine braking if needed to slow down.
DEF System Malfunction
Common causes
- Low DEF fluid or incorrect fluid used
- Faulty DEF quality sensor or pump
- Clogged DEF injector (crystal buildup)
What to do
Refill with fresh API-certified DEF. If the warning persists, the system may need servicing. Continued driving will eventually result in speed limitation and possible no-start after shutdown.
Frequently Asked Questions
Why does my glow plug light flash or stay on longer than usual?
In cold weather, the grid heater may cycle longer; a flashing light can indicate a faulty grid heater relay or glow plug module on your 6.7L Cummins. Have it checked if it stays on after the engine starts.
Can I drive with the DPF regeneration light on?
Yes, you should continue driving at highway speeds to allow passive or active regen to finish. If you shut down the engine during active regen, the soot load may not clear, and the light will reappear.
What does the water in fuel light mean, and how do I fix it?
It means water has accumulated in the fuel/water separator on the driver-side frame rail. You need to drain it by turning the valve at the bottom; collect water until clean diesel appears. Do this every oil change.
My reduced power light came on while towing. What now?
The ECM detected an issue like high EGT, low fuel pressure, or a sensor failure. Reduce speed and load; if the light stays on, have the codes read. Limp mode protects the Cummins engine from damage.
Why does my check engine light come on after fueling?
A loose or missing fuel cap can trigger the MIL on the Ram 3500. Tighten the cap until it clicks; the light may take a few drive cycles to turn off. If it persists, use a scanner to clear the code.
What should I do if the transmission temp warning appears while driving?
Stop and let the 68RFE cool by idling in Neutral. Check fluid level (warm, engine running). Avoid overdrive and reduce speed. If it happens frequently, inspect the external cooler for blockage or fan operation.