No, generally, you cannot pass emissions in Colorado if your check engine light (CEL) is illuminated. While the presence of an illuminated check engine light itself cannot be the sole criterion for failing an emissions test in Colorado, the underlying issues that trigger the light almost invariably lead to a failure.
The check engine light indicates that your vehicle's On-Board Diagnostics (OBD-II) system has detected a problem with your vehicle's engine, transmission, or, most commonly, its emissions control system. During an emissions test in Colorado, the testing equipment connects to your car's OBD-II port to read critical information.
Why a Check Engine Light Leads to Failure
Even though the light isn't the only reason for failure, the conditions it signals are:
- Active Diagnostic Trouble Codes (DTCs): When the check engine light is on, it means there are active DTCs stored in your vehicle's computer. These codes point to specific malfunctions, many of which directly impact emissions (e.g., issues with oxygen sensors, catalytic converter efficiency, or the evaporative emissions system). Any active emissions-related DTC will result in an automatic failure of the emissions test.
- Incomplete Readiness Monitors: After a check engine light is cleared (either by fixing the issue or manually), the vehicle's computer needs to run through various "readiness monitors" to confirm all emissions control systems are functioning correctly. This process usually requires specific driving conditions over a period. If the check engine light was recently on and cleared, these monitors might be "incomplete" or "not ready." Vehicles with too many incomplete readiness monitors (typically more than one, depending on the model year) will fail the emissions test in Colorado.
Understanding Colorado's Emissions Testing
Colorado's emissions program, often referred to as Air Care Colorado, utilizes an OBD-II test for most vehicles model year 1996 and newer. This test specifically checks:
- Diagnostic Trouble Codes (DTCs): Are there any active fault codes?
- Readiness Monitor Status: Are all critical emissions monitors complete?
- Malfunction Indicator Light (MIL) Status: Is the check engine light illuminated? (Though, as noted, the underlying reason for the light is the primary failure point).
Here's a quick overview of how a CEL typically impacts your test result:
| Check Engine Light Status | OBD-II Scan Status | Typical Emissions Test Result |
|---|---|---|
| Illuminated | Active Emissions-Related DTCs present | FAIL |
| Illuminated | Incomplete Readiness Monitors (due to recent reset) | FAIL |
| Off | No Active DTCs, All Readiness Monitors Complete | PASS (if no other issues) |
| Off (recently reset) | Too many Incomplete Readiness Monitors | FAIL |
Steps to Take Before Your Emissions Test
If your check engine light is on, or has been on recently, it's crucial to address it before heading to the emissions testing station to avoid an automatic failure.
- Get a Diagnosis: The first step is to have the diagnostic trouble codes read from your vehicle's OBD-II system. Most auto parts stores offer this service for free, or you can use a personal OBD-II scanner. This will provide a "P-code" (e.g., P0420 for catalytic converter inefficiency) that points to the specific problem.
- Repair the Underlying Issue: Based on the diagnostic code, identify and fix the root cause of the problem. This might involve replacing a faulty sensor (like an oxygen sensor), repairing a vacuum leak, addressing an evaporative emissions system issue, or, in more severe cases, replacing a catalytic converter.
- Clear the Codes: After the repair, clear the diagnostic codes. This will turn off the check engine light.
- Complete a Drive Cycle: This is a critical, often overlooked, step. After clearing codes, you must drive your vehicle under varying conditions (highway, city, idling) to allow the readiness monitors to complete their self-checks. This process can take several days and hundreds of miles of typical driving. Consult your vehicle's owner's manual or an automotive technician for a specific drive cycle procedure for your make and model.
- Re-Verify: Before heading to the testing station, use an OBD-II scanner (or visit an auto parts store) to ensure the check engine light remains off and all necessary readiness monitors are in a "ready" or "complete" state. If any emissions-related monitors are still incomplete, you'll need to drive more.
Addressing the check engine light proactively saves you time, money, and the hassle of retesting. It also ensures your vehicle is operating efficiently and minimizing its environmental impact.
