A dash light on a semi is not a decoration. It is the truck telling you something changed, and the color tells you how fast you need to act. Drivers who know the difference between a light that means “watch it” and a light that means “stop now” save themselves a lot of money, and sometimes a lot of trouble on the road.
This guide explains the warning lights we get asked about most, what each one usually points to, and when to keep driving or park the truck. Lights and symbols vary by make and model year, so always check your truck’s owner manual for the exact meaning on your dash.
Red lights and amber lights: the basic rule
On most heavy trucks, a red light means stop as soon as it is safe. Pull off the road, shut the engine down if the light tells you to, and find out what is wrong. An amber (yellow) light means a fault has been found and the truck needs service soon. You can usually keep driving for a short while, but you should not ignore it, because amber faults often turn into red ones.
Many trucks also show a message on the driver display. Read it. It often gives a fault description that helps a shop find the problem faster.
Lights that mean stop driving
Engine stop or red engine warning
A red engine light usually means a serious problem such as very low oil pressure, high coolant temperature or another fault that can damage the engine. Do not keep going to finish the trip. Pull over and shut the engine down. Running an engine with no oil pressure or while overheating can ruin it in minutes.
Low oil pressure
If the oil pressure light or gauge shows a drop while the engine is running, stop. Check the oil level only after the engine has been off for a few minutes. Low oil, a failed oil pump or a bad sensor can all cause this, and only a shop can tell which.
High coolant temperature or low coolant
A coolant temperature warning means the engine is running hot. Pull over, let it idle for a moment if the manual allows, and then shut down. Do not open a hot radiator cap. Common causes are low coolant, a leak, a bad fan clutch, a failed water pump or a blocked radiator.
Low air pressure
The low air buzzer and light mean the air system pressure has dropped below a safe level. The brakes depend on this air. If the light stays on or the pressure keeps falling, stop in a safe place. Our guide on air brake problems on semi trucks explains the signs in more detail.
Brake system warning
A red brake light is not something to drive on. It can point to low air, a parking brake that is still applied, or a fault in the brake circuit. If you are not sure why it is on, do not move the truck loaded.
Amber lights that need service soon
Check engine or malfunction indicator
This amber light means the engine computer has stored a fault code. It could be something small like a sensor, or something that is getting worse like an injector or a turbo problem. The light alone does not tell you which. A scan tool reads the code, and a technician then confirms the cause. Do not clear codes and hope. The fault will come back.
DPF and regeneration lights
Modern diesel engines have a diesel particulate filter, or DPF, that traps soot. The truck burns that soot off in a process called regeneration. A DPF light means the filter is filling up. If the truck is allowed to finish a regen, the light may go away. If you keep ignoring it, the filter can plug, and the engine may lose power. A cleaning or replacement is a much bigger bill than catching it early.
A high exhaust system temperature light tells you the exhaust is hot during a regen. Park away from anything that can burn, such as dry grass, and do not park over it while it runs.
DEF low or DEF fault
Diesel exhaust fluid, or DEF, is used by the SCR system to clean up exhaust. A low DEF light is a reminder to refill. If you ignore it, or if there is a fault with the system, the truck can reduce engine power and later limit your speed. A bad DEF pump, a failed sensor, a clogged injector or poor quality fluid can also trigger these lights, even when the tank is full. If the lights stay on after a refill, the system needs to be checked.
ABS light on the tractor
When the ABS light stays on after the truck starts, the anti-lock system has found a fault. Your normal brakes still work, but you lose the anti-lock function. A wheel speed sensor, a damaged wire or a bad connection is often the cause. Get it checked soon.
ABS light on the trailer
Many trailers have their own ABS lamp, and it shows up in the cab. A poor connection at the 7-way plug, a bad ground or a power problem can cause it. Our trailer light check covers the plug and ground steps, and those same checks often help with the ABS lamp.
Battery or charging
A battery or charging light means the alternator is not charging properly, or a connection is bad. The truck will keep running on the batteries for a while, but it will eventually die. Head to a shop or pull over before the batteries run flat.
What to do when a light comes on
- Note the color, the symbol and any message on the driver display
- If it is red, pull over safely and shut the truck down if the manual says so
- If it is amber, finish the trip carefully if the truck runs normally, and plan service soon
- Do not keep driving with power loss, strange noises, smoke or a burning smell
- Write down when the light came on and what you were doing, such as climbing a hill, idling or loading, because it helps the technician
- Never clear codes just to turn the light off
How a shop finds the cause
A warning light is only the start. A technician connects a diagnostic tool, reads the fault codes, and then tests the system to confirm the real cause, such as a sensor, a wire, a pump or a part that has failed. Replacing parts without testing wastes money. We read the codes, tell you what we found and give you the price before any repair starts.
Truck dash light diagnosis in Dallas
If a light is on in your semi, box truck or trailer and you are in the Dallas area, you can bring it to Bawa’s Truck & Trailer Repair at 4700 Irving Blvd, Ste 200, Dallas, TX 75247. Our electrical diagnostics and truck repair pages list the work we do. If the truck breaks down on the road, see truck roadside repair. We try to repair on the spot, and if it cannot be fixed roadside we tow it to our own shop. We do not offer standalone towing. Call (214) 400-4228. We are open Monday to Saturday, 8 AM to 6 PM.