2018–2024 GMC Terrain

2018-2024 GMC Terrain — Check Engine Light / OBD2 Diagnosis

By Tom Becker ✓ Certified Mechanic · Published Sep 21, 2026 · Updated Oct 22, 2026

🔧 Easy 15-30 minutes $25 DIY

Save $95 by doing it yourself! Shop cost: $120

Quick answer: Check Engine Light / OBD2 Diagnosis on a 2018–2024 GMC Terrain is rated Easy and takes about 15-30 minutes. You'll need 2 tools for this job, and parts cost about $25. This guide walks you through the full job step by step, including the mistakes to avoid.

Quick Video Tutorial

Parts You Need

Total Cost

$25.00

Part Name	Part Number	Price	Buy
OBD2 scanner	—	$25.00	Amazon

About prices: shown prices are typical retail averages and change frequently; cheaper alternatives may be available. Confirm the current price before buying.

💡 Pro tip: Order all parts at once to save on shipping

Step-by-Step Instructions

Follow these steps carefully

Read the lamp and power messages

Start the 2018-2024 GMC Terrain and watch the check-engine lamp plus any reduced-engine-power message. Steady lamp means a stored fault to diagnose soon; flashing means an active misfire, so ease off and pull over safely to protect the converter.

Hook up the scanner at the OBD port

With the ignition off, locate the 16-pin OBD-II port under the driver's-side dashboard, left of the steering column on the 2018-2024 GMC Terrain. Seat the scanner plug straight in, then turn the ignition to ON without starting and confirm communication and live data.

Scan codes, freeze-frame, and boost PIDs

Run a full scan on the 2018-2024 GMC Terrain and record confirmed, pending, and permanent codes with freeze-frame (RPM, load, coolant temp, speed). GM documented that a separated charge-air-cooler outlet tube on the 1.5L sets P0101, P0172, P0299, P0506, and P1101 with reduced power. The other signature fault is P0496, EVAP purge valve stuck open, whose calling card is hard starting right after refueling. Also common: P0420 catalyst efficiency and P0455/P0456 EVAP leaks. Add live boost, MAF, and fuel-trim PIDs. Do not clear codes yet.

Check the documented failure points

For P0299 or reduced power on the 2018-2024 GMC Terrain 1.5L, pop the hood and check the charge-air-cooler outlet tube where it meets the throttle body; a separated or loose tube with oily residue is the documented cause, and reseating and clamping it properly often resolves the whole code set. For P0496, test the purge valve, which should be sealed when unpowered. For EVAP leak codes, tighten or replace the gas cap and inspect vapor hoses. For misfires, check plugs, coils, and wiring before suspecting anything deeper.

Fix, clear, and complete a drive cycle

Repair the confirmed fault on the 2018-2024 GMC Terrain, then clear codes with the scanner. Run a GM-style drive cycle: cold start, a few minutes of idle with the A/C on, steady highway cruising, and city stop-and-go over 50-100 miles so all monitors complete.

All Done!

Test your repair before closing everything up. If something doesn't work right, double-check your steps.

⚠️ Common Mistakes to Avoid

• Replacing parts based on a code without checking wiring, connectors, or vacuum leaks
• Clearing codes before recording confirmed, pending, and permanent codes
• Using a scanner that cannot communicate with the vehicle protocol
• Treating a stored code as proof that the named sensor itself failed

Tools You'll Need

OBD-II scanner compatible with the vehicle Flashlight Notebook or phone for codes and freeze-frame data Safety glasses

Related Trouble Codes

P0300 Random/Multiple Cylinder Misfire Detected P0301 Cylinder 1 Misfire Detected P0171 System Too Lean (Bank 1) P0101 Mass Airflow Sensor Range/Performance P0174 System Too Lean (Bank 2) P0562 System Voltage Low P0302 Cylinder 2 Misfire Detected P0303 Cylinder 3 Misfire Detected

⚠️ Safety First!

• Informational DIY. EZ Car Fix is not a shop.
• A flashing check-engine light can indicate a damaging misfire; reduce load and seek professional service promptly.
• Never work under a vehicle supported only by a jack.
• Do not clear codes before recording them and the freeze-frame data.