GMC T Series Warning Lights: What They Mean

Red: stop or act now

Hydraulic Brake System Warning Light

Master cylinder fluid is low or the hydraulic brakes have a problem; a warning tone also sounds.
What to do: Pull off the road and stop carefully. If the light stays on, have the vehicle towed for service. PDF page 102

Air Brake System Warning Light

Brake reservoir air pressure has dropped below 60 psi. The parking brake may apply suddenly if pressure falls further.
What to do: Drive only to the nearest safe point and stop. Stop as soon as you can if the warning tone sounds. PDF page 103

Other warnings and indicators

Service Transmission Warning Light

An Allison automatic transmission malfunction was detected; the transmission stays in one gear and may not reach all gears or reverse.
What to do: Do not shift. Drive directly to an authorized dealer, or have the vehicle towed if you cannot. PDF page 101

Low Coolant Warning Light

Coolant is low and the engine may overheat; a warning tone sounds for 10 seconds. It may also show when automatic engine shutdown has begun.
What to do: Have the vehicle serviced as soon as you can. PDF page 106

Antilock Brake System (ABS) Warning Light

The vehicle needs service if the light stays on or comes on while driving; you lose the antilock feature.
What to do: Have the vehicle serviced. If the brake or low air light is also on, the regular brakes have a problem. PDF page 104

Trailer Antilock Brake System Warning Light

If it comes on and stays on, there is a problem in the trailer's antilock brake system.
What to do: Have the system repaired immediately. PDF page 104

Parking Brake Warning Light

Comes on when the parking brake is set, and stays on if the parking brake does not release fully. PDF page 103

Range Inhibit Warning Indicator

On an Allison transmission, a flashing bar, light, or gear indicator means range shifts may not occur.
What to do: See the Allison Automatic Transmission Operator's Manual for shift inhibit limitations. PDF page 101

Wait to Start Light

The glow plugs are on while the key is in ON/RUN or START.
What to do: Wait until the indicator turns off before turning the key to START. PDF page 106

Summarized in our own words from the 2010 GMC T Series owner's manual (AI-assisted; every light name was matched against its cited page 2026-10-18). Your vehicle may not have every light, and other years can differ. A red warning that stays on usually means stop safely and check: your manual has the final word.