Every time a GMC vehicle detects a powertrain or EVAP related problem, it will inform the driver by turning on the check engine light on the dashboard.
GMC check engine light can be quite frustrating, as the yellow warning light doesn't provide much information about the problem.
In this article, we take a look at common problems that trigger the check engine light to come on in GMC vehicles and how to read the fault codes yourself with an OBD-II scanner.
Depending on the cause of the check engine light on your GMC, you may notice any of the following problems:
- Engine power is reduced
- GMC runs rough
- Vehicle won't idle, keeps dying
- Engine misfire present
- Lack of power
- Poor throttle response
- Smoke from the exhaust
- Transmission won't shift gears
- The vehicle goes in limp home mode/emergency mode.
- Check engine light stays on or flashes
In some cases such as an EVAP leak or loose gas cap, you may not notice performance issues.
Don't ignore GMC check engine light because even if there are no symptoms present, fuel consumption may be higher when check engine light stays on.
How to Diagnose GMC Check Engine Light
The check engine light does not provide much information. To find out why your GMC check engine light is on, read the fault codes with an OBD-II scanner.
- Locate the OBD-II port under the dashboard. Plug the scanner into the OBD-II port.
- Turn on the ignition but do not start the engine.
- Allow the OBD-II scanner to turn on and press enter to connect with the vehicle.
- Select Read Codes or Read Faults depending on the scanner.
- Write down all the fault codes and research them to better understand the possible cause of check engine light on your GMC.
Check engine light can be triggered by a number of problems. Some of the most common causes include:
- Faulty MAF sensor
- Bad oxygen sensor
- Loose or defective gas cap
- Clogged catalytic converter
- Worn spark plugs
- Defective ignition coil
Vacuum Leaks: Misfires are quite a common problem on GMC trucks. Tracing down the source of the problem can be tricky, as it usually happens sporadically. In addition to checking spark plugs and ignition coil, you should also inspect the intake for eventual vacuum leaks. Code: P0300
EGR Valve: A carbon buildup is a common cause of various EGR related problems. While this will not affect engine performance in general, it might cause higher fuel consumption. If not too extreme, carbon buildup can be cleaned with EGR cleaner spray. Related Fault Code: P0401
Catalytic Converter: Higher mileage GMC trucks are likely to experience some emission control issues. In most cases, this is caused by a worn-out catalytic converter, especially if the truck suffered from a prolonged misfire. Another possibility is a faulty downstream O2 sensor. Related Fault Codes: P0420 and P0430
EVAP leak: EVAP system is a frequent failure point. In addition to faulty vent or purge valve, these problems can be caused by various EVAP leaks. Finding a leak, usually as a result of cracks in EVAP lines, is best done with a smoke generating machine. Related Fault Codes: P0442, P0445, and P0449.
Variable Timing Issues: Most newer VORTEC engines have variable valve timing for better performance. This system depends on good oil pressure within VVT solenoids. If oil changes are neglected, screens on these solenoids get clogged with oil sludge and other contaminants. This obstructs oil flow, and throws off the timing, resulting in a lack of acceleration and hesitation. In most cases, solenoids and screens can be cleaned. Related Fault Code P1345 (crankshaft/camshaft correlation)
GMC Check Engine Light Flashing
One thing that must never be ignored is the check engine light that is flashing, especially if accompanied by engine shaking and juddering.
A check engine light going on and off intermittently signals an engine misfire, which can cause damage to the catalytic converter and other engine components.
It is not recommended to drive if the check engine light is flashing.
What to do when GMC light comes on?
With a 'check engine' light on, the first thing you need to find out is what caused it. You can either take your car to the workshop for a full diagnosis or get your own OBD-II scanner. Your local auto parts store may also read the codes free of charge.
OBD-II scanners come in all shapes, sizes, and prices to choose from.
The choices range from something simple as a Bluetooth OBD adapter that connects to an application on your smartphone, over dedicated hand-held scan tools to powerful diagnostic tools.