Troubleshooting GMC Check Engine Light

Vehicle:   GMC
Difficulty: ⚫⚫⚪⚪⚪
DIY Cost: $30-$420
Est. Time: 1-3 hr

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.

gmc check engine light on gmc yukon
2008 GMC Yukon Check Engine Light On
gmc check engine light cel service engine soon
2012 GMC Terrain Check Engine Light On

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.

Symptoms

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
  • The vehicle won't idle; it 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 the GMC check engine light because even if there are no symptoms present, fuel consumption may be higher when the check engine light stays on.

How to Diagnose and Reset 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.

  1. Locate the OBD-II port under the dashboard. Plug the scanner into the OBD-II port.gmc obd2 port location
  2. Turn on the ignition but do not start the engine.
  3. Allow the OBD-II scanner to turn on and press enter to connect with the vehicle.diagnose gmc check engine light
  4. Select Read Codes or Read Faults depending on the scanner.
  5. Write down all the fault codes and research them to understand better the possible cause of check engine light on your GMC. 
  6. Fix all the current problems then go back to the main menu and select Clear Fault Codes. 

Common problems cause the GMC check engine light to come on? 

gmc maf sensor
GM Mass Air Flow (MAF) Sensor 

Check engine light can be triggered by many 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
check engine light on
GMC V6 VVT Direct Injection Engine
  • 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: The 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 due to 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)

Why is my GMC Check Engine Light Flashing On and Off? 

One thing that must never be ignored is the check engine light flashing, especially if accompanied by engine shaking and juddering.

A check engine light going on and off intermittently signals an engine misfire, causing 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 the GMC check engine light comes on?

gmc check engine light on

With a 'check engine' light on, you need to find out 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.

By YOUCANIC Automotive Experts
Nov 2020