A failed Clean Truck Check doesn't have to ground your truck for long. Most failures come from a handful of predictable issues. Here's how to diagnose the cause, fix it, and get a passing result reported to CARB.
Most Common Reasons Trucks Fail
- Active check-engine light (MIL): An illuminated malfunction indicator lamp is an automatic fail on an OBD test.
- Stored emissions fault codes: Diagnostic trouble codes related to the emissions system will cause a failure even if the light is off.
- Incomplete OBD readiness monitors: If the truck was recently repaired or the battery disconnected, monitors may not be "ready" yet.
- DPF / DEF system faults: Diesel particulate filter or DEF dosing problems are a leading cause of failure on 2013+ engines.
- Excessive opacity (smoke): On pre-2013 engines, a tailpipe smoke test that exceeds CARB limits fails.
- Tampered or missing emissions equipment: Removed DPFs or defeat devices are a hard fail and a serious violation.
Not sure which test type applies to your engine? See Clean Truck Check vs smog check.
Step-by-Step: What to Do After a Fail
- Get the specific reason. Your test result lists the fault codes or opacity readings that caused the failure.
- Address the root cause. Have a qualified diesel technician repair the underlying emissions issue — not just clear the code.
- Drive a completion cycle. After repairs, drive the vehicle so the OBD readiness monitors reset to "ready."
- Re-test and report. Once repaired, re-test and submit a passing result to CARB to clear your compliance status.
Don't Just Clear the Code
Erasing a fault code without fixing the problem also erases your OBD readiness monitors — which causes an incomplete failure on re-test. Worse, driving with a known emissions fault can lead to the fines and registration holds the program is designed to enforce.
How Fast Can You Re-Test?
Once the repair is complete and readiness monitors are set, you can re-test the same day. Our mobile technicians can perform your re-test at your yard so a failed vehicle gets back on the road with minimal downtime. Learn more about our mobile testing services or the CARB compliance guide, then schedule a re-test or view pricing.