Month: November 2026

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.

Trailer lights that do not work are one of the most common problems we see, and one of the easiest ways to get stopped by an officer. They are also a safety problem. If your brake lights and turn signals are out, the driver behind you has no warning.

The good news is that most trailer light problems come from a short list of causes. This guide walks you through them in the order that finds the problem fastest.

Before you start

You need a second person, or a light tester that plugs into the 7-way socket. Park on level ground, set the brakes and chock the wheels. Connect the trailer cable to the tractor and turn the key on. Then have your helper run the lights while you check the back of the trailer: running lights, left and right turn signals, brake lights and hazards.

Write down what works and what does not. The pattern tells you a lot. If every light is out, think power or ground. If only one side is out, think one circuit. If one bulb is out, think that bulb.

Step 1: Check the ground first

A bad ground is the number one cause of weak, dim or flickering trailer lights. Lights that work on their own but act strange when you turn on another light are a classic sign.

• Look at the ground wire at the plug and where it connects to the trailer frame
• Clean any rust or paint where the ground touches the metal
• Make sure the connection is tight and not corroded

Many trailer problems that look like a wiring disaster turn out to be a loose or rusty ground.

Step 2: Inspect the 7-way plug and socket

Look inside both the tractor socket and the trailer plug. Green or white powder on the pins is corrosion. Bent pins, melted plastic, and broken springs inside the socket are also common.

• Clean the pins with a contact cleaner and a small brush
• Replace a plug or socket that is cracked, loose or burned
• Make sure the socket cover closes when the plug is not in use
• Add dielectric grease to the pins after cleaning so moisture stays out

Use the wiring chart for your plug. Different trailers and tractors can use different layouts, so do not guess which pin does what.

Step 3: Follow the cable

The coiled cable between the tractor and trailer gets pulled, stepped on and pinched. Check it from end to end.

• Look for cuts, cracked insulation and flat spots
• Check where the cable enters the plug, since broken wires often hide there
• Check the cable for stretching near the coil ends
• Flex the cable gently while a helper watches the lights. Lights that flicker as you move the cable show you where the break is

Step 4: Check the bulbs and lamps

If only one light is out, look at it directly. Replace the bulb or, on LED lamps, check the connection and the lamp itself.

• Look for water inside the lamp or a loose connector
• Check the pigtail and the wire running to that lamp
• LED lamps usually need a clean ground just as much as bulbs do

One more thing: a wet lamp can work for a day and fail the next. If a lamp has water inside, replace it.

Step 5: Look at the trailer wiring

If the plug and the cable test good but lights are still out, the problem may be in the trailer harness. Wires run along the frame, through crossmembers and past axles. They rub, corrode and get cut by road debris.

• Check junction boxes for water or loose connections
• Look for wire that has rubbed through near the frame and axles
• Look for old repairs made with tape instead of proper connectors

Trailer wiring is easier to fix when you find the break early. A long run of bad wire is a job for a shop.

Step 6: Check the tractor side

If nothing works on the trailer but the lights are fine on a different trailer, the problem is on the tractor side. That could be a fuse, a relay or the wiring to the socket. Check the fuses first, then look at the wiring back to the socket.

What about the ABS light?

Many trailers have an ABS warning lamp. The ABS system needs a steady power supply from the tractor through the plug. If the ABS light stays on, a poor connection at the plug, a bad ground or a power problem is often the cause. Do not ignore it. An ABS fault can mean a wheel sensor or the trailer module, and it needs a proper scan to find.

When to bring the trailer to a shop

• You cleaned the ground and the plug and the lights are still out
• Lights work only when you wiggle the cable or the plug
• Fuses keep blowing
• The ABS light stays on
• You find melted wiring or burned connectors

Do not pull a loaded trailer with no brake lights. If the problem is on the road, we try to repair it on the spot. If it cannot be fixed roadside, we tow it to our own shop. We do not offer standalone towing.

Trailer light repair in Dallas

Bawa’s Truck & Trailer Repair is at 4700 Irving Blvd, Ste 200, Dallas, TX 75247. Our trailer repair and electrical diagnostics pages list what we handle. Call (214) 400-4228. We are open Monday to Saturday, 8 AM to 6 PM, and we tell you the price before the work starts.

A semi can weigh 80,000 pounds with a load, and every pound of it stops because of air. When an air brake system starts to fail, it rarely does it all at once. It gives you small signals first. Drivers who catch those signals in the yard pay for a small repair. Drivers who ignore them often pay for a tow, a failed inspection, or worse.

This guide covers the problems we see most at our shop on Irving Blvd in Dallas, how to tell what is going on, and when the right answer is to stop driving.

How an air brake system works, in plain words

The air compressor on the engine fills the air tanks. A governor tells the compressor when to stop, usually around 125 psi, and to start again when pressure drops to roughly 100 psi. An air dryer pulls moisture out of that air. When you press the brake pedal, air flows to the brake chambers at each wheel. The chamber pushes a rod, the rod turns a slack adjuster, and the brake shoes press against the drum.

The spring brakes work the opposite way. They are held off by air pressure. If the pressure drops too low, the springs apply the brakes. This is the safety design: lose air, and the truck stops. It is also why a leak is never a small thing.

Warning signs you can notice before a failure

Air pressure that builds slowly or drops fast

Watch your gauges when you start the truck. If the compressor takes a long time to build pressure, it may be worn or there may be a leak somewhere. If pressure falls quickly with the engine off, you have a leak.

A common test used for the CDL air brake exam is the leakage check. With the engine off and the brakes released, a single vehicle should not lose more than about 2 psi in one minute, and a combination about 3 psi. With the brakes fully applied, the limits are about 3 psi for a single vehicle and 4 psi for a combination. If your truck loses more, do not just top it off and go.

A hissing sound

A hiss near the wheels, under the cab, or at the glad hands after you set the brakes usually means air is escaping. Common sources are worn rubber seals on the glad hands, a loose fitting, a cracked or chafed air line, or a damaged brake chamber diaphragm.

One trick that helps: spray soapy water on a suspected fitting with the system pressurized. Bubbles show you where the air is going.

The low air warning

The low air buzzer and light must come on before pressure falls below 60 psi. If you see it in normal driving, something is wrong. Pull over safely and find out why. If pressure keeps dropping, the spring brakes can begin to apply, and that is usually between 20 and 45 psi.

A soft pedal or a longer stopping distance

If the pedal feels spongy, or the truck takes longer to stop than it used to, the brakes may be out of adjustment, the shoes may be worn thin, or air may be leaking. Out-of-adjustment brakes are one of the most common reasons trucks are put out of service at roadside inspections.

The truck pulls to one side

If the truck drifts left or right when you brake, one side is doing more work than the other. That can be a stuck brake, a bad chamber, a contaminated shoe, or uneven adjustment. Do not wait on this one. Uneven braking is hard on tires and dangerous on wet pavement.

Brakes that drag or smell hot

A hot smell, smoke, or a wheel that is much hotter than the others after a drive can mean a brake is not releasing. A seized slack adjuster or a weak return spring can cause it. Dragging brakes burn through shoes and drums and can start a fire.

Water or oil in the air system

If you see water when you drain the tanks, the air dryer may not be working. Moisture in the lines freezes in cold weather and corrodes valves in warm weather. If you see oil in the tanks, the compressor may be passing oil, which damages seals and valves down the line.

What a shop checks during an air brake inspection

When a truck comes in with brake complaints, we do not start by replacing parts. We start by finding the cause. A basic air brake check includes:

• Building pressure and checking the compressor build-up time and governor cut-in and cut-out
• Static and applied leakage tests to see how fast the system loses air
• Walking every air line, fitting and hose for chafing, cracks and wet spots
• Measuring brake chamber pushrod travel and checking the slack adjusters
• Pulling a wheel when needed to measure lining thickness and check drums for cracks and heat damage
• Checking the air dryer and draining the tanks to look for water and oil
• Testing the low air warning and the spring brakes

Then we tell you what we found and what it costs before we start the repair. That is how we work on every job, brakes or not.

When to stop driving

Some problems can wait until you reach a shop. These cannot:

• The low air warning will not go away, or pressure keeps falling while you drive
• You cannot build air back to the normal range
• The spring brakes apply on their own or will not release
• The pedal goes toward the floor, or the truck does not stop the way it should
• A brake is smoking or glowing hot
• You can hear or see a major air leak that does not stop

If any of these happen, get the truck off the road in a safe spot and call a shop. Driving on a failing air system is a risk to you and to everyone around you. If you break down, we try to repair the truck on the spot. If it cannot be fixed roadside, we tow it to our own shop for the repair. We do not offer towing as a separate service.

How to prevent most air brake problems

• Drain the air tanks as part of your regular routine so moisture does not build up
• Do a walk-around before each trip and listen for leaks with the brakes applied
• Replace worn glad hand seals before they fail
• Keep brakes adjusted and have them inspected on a regular schedule
• Fix small leaks early, because they put extra load on the compressor

A few minutes in the yard is cheaper than a day on the shoulder.

Need your brakes checked in Dallas?

If your semi or trailer has any of the signs above, bring it to Bawa’s Truck & Trailer Repair at 4700 Irving Blvd, Ste 200, Dallas, TX 75247. Our brake and suspension repair page lists what we do, and our truck repair team can look at the rest of the truck while it is in. If you are stuck on the road, read about truck roadside repair first, then call (214) 400-4228.

We are open Monday to Saturday, 8 AM to 6 PM. We tell you the price before we start any work.