Why lamp color matters
Your truck uses colors to tell you how urgent a problem is. As a simple rule, amber means "get it checked soon" and red means "stop safely now." Knowing the difference can save an engine and keep you from a costly tow.
Lamp shapes and names vary by brand (Freightliner, Kenworth, Peterbilt, International, Volvo, Mack). Check your owner's manual, but the color logic is very similar across trucks.
The four engine lamps on the J1939 network
Truck modules can turn on four standard lamps:
- Amber warning lamp: a fault that needs attention
- Red stop lamp: a serious fault that can damage the engine or be unsafe
- MIL (malfunction indicator lamp): an emissions-related fault
- Protect lamp: a problem outside the engine itself, often in a system that protects the engine
Seeing a light on the dash? Snap it with DERATE and know if you can keep driving
Amber "Check Engine" lamp
This means the system found a fault that needs checking soon. In most cases you can finish your run carefully and book the shop. Overdrive's lamp guide says you can usually drive to the end of the shift, but not much more.
What to do:
- Note the fault code and when it came on.
- Watch your gauges: oil pressure, coolant temperature, boost.
- Drive gently and avoid heavy loads or long grades if you can.
- Schedule a check before the next trip.
MIL (emissions lamp)
The MIL usually means a problem in the emissions system: DEF, SCR, DPF, EGR or related sensors. It is often amber and shaped like an engine. It is not an emergency on its own. But if you ignore it, many emissions faults lead to a derate, where the engine cuts power or limits speed.
Red "Stop Engine" lamp
This is the most urgent lamp. It can mean low oil pressure, high coolant temperature, low coolant level or another fault that can wreck the engine.
- Solid red: find a safe place to stop as soon as you can and shut the engine down.
- Flashing red: on many trucks, engine protection can shut the engine down soon. Overdrive cites about 30 to 60 seconds of warning on some trucks. Get off the road right away.
Never keep driving on a red stop lamp to "make it to the yard." That is how engines and turbos get destroyed.
Other lamps you'll see with them
- DPF lamp: the filter needs regeneration. Flashing usually means a parked regen is needed soon.
- HEST lamp: exhaust is very hot during a regen. Usually normal.
- DEF lamp: DEF is low or there's a DEF fault. Combined with a stop lamp, the truck may be limited to very low speed. Overdrive gives an example of 5 mph.
Amber + red together
When amber and red lamps are both on, treat it as red. Often you'll also see a derate code such as SPN 1569 FMI 31, which means the engine is limiting torque to protect itself. That code is the result. The real cause is another code underneath it.
When to stop and call a mechanic
Stop and call for help if you see red, if oil pressure drops, if coolant temperature climbs, or if you smell coolant or burning. Amber lamps still need a diagnosis soon. Don't clear codes and keep driving without knowing why they came on.
DERATE reads the lamp and code from a photo of your dash and tells you if it's safe to keep going.
FAQ
Can I drive with the amber check engine light on?
Usually for a short time, carefully, to get to a safe place or a shop. Watch your gauges and get it checked soon.
What does a flashing red stop engine light mean?
The engine may shut down soon to protect itself. Get off the road safely right away.
Why did my truck derate with only an amber light?
Many emissions faults cause a derate without a red lamp. The amber lamp or MIL is the warning, and the derate comes if the fault isn't fixed.