California Air Resources Board

Modified OBD System – Frequently Asked Questions 

Why did I receive a test result indicating that my OBD system has been modified? 

The On-Board Diagnostic (OBD) system is an integrated vehicle monitoring and reporting system   designed primarily for emissions control, performance optimization and maintenance efficiency.  

Some aspect(s) of your vehicle’s OBD, engine, or emissions system have been modified independent of an authorized repair facility (e.g., dealership) or engine manufacturer. Subsequently, your vehicle’s OBD test data does not match a certified configuration created by the engine manufacturer.  

How should my vehicle be repaired? 

Restore the OBD system to the certified manufacturer configuration, along with any associated repairs that may be needed. Repairs could include, but are not limited to, having an authorized repair center update the OBD system with the latest certified engine software available from the manufacturer.  

Failure to appropriately repair the OBD system will result in a failing inspection, subsequent repairs, and follow-up Referee inspections. The “Referee Referral” test message will not be cleared until the vehicle/engine passes a Clean Truck Check OBD emissions test.   

Buyer Beware: Please note that under California law, business entities are generally not covered by consumer protection statutes, which are intended to protect individual consumers. Beware of predatory and/or repair shops that are unable to properly diagnose and perform the repairs needed.  

Please consult an authorized service center (e.g., dealership) to confirm if the OBD software is correct for the applicable vehicle and engine model year. An authorized service center should have the appropriate resources and expertise to fully and/or reliably diagnose and repair vehicles with modified OBD systems.  

Please note that repairs to correct a modified OBD system may require an unknown number of repairs that may cause excessive downtime and cost. 

It is important and beneficial to work with a reputable and authorized repair facility: 
 

1. Check customer review sites like Yelp, Automobile Club of Southern California (AAA), Better Business Bureau (BBB), Google reviews, and/or social media. 

2. Ask other vehicle owners, heavy-duty fleets, and industry experts, such as your vehicle’s engine manufacturer and/or authorized dealer. 

3. Vehicles non-compliant with Clean Truck Check cannot operate on California public roads and highways. 

I purchased the vehicle used and I don’t know what could be modified. What should I do? 

Please review the above section, “How should my vehicle be repaired?” 

Please have the vehicle inspected by an authorized repair center (e.g., dealership), to confirm if the OBD software is correct for the applicable vehicle and engine model year.  

The vehicle needs to be in an Original Equipment Manufacturer (OEM) configuration to operate legally in California. 

How do I make a Referee appointment? 

How to request a CTC Referee Appointment 

Please submit a Referee Appointment Request form in your CTC-VIS entity account (click Main Menu -> Forms -> Online Forms tab -> Referee Appointment Request form) to request a Referee inspection 

Please select the Referee Action, State-Directed Referee Inspection. The checklist of required supporting documents is located at, https://ww2.arb.ca.gov/clean-truck-check-referee-referral-required-documents 

I want to contest this result. What should I do? 

If you wish to contest this result, please make an appointment with the Clean Truck Check Referee. 

Please submit a Referee Appointment Request form in your CTC-VIS entity account (click Main Menu -> Forms -> Online Forms tab -> Referee Appointment Request form) to request a Referee inspection. Please select the Referee Action, Test Result Dispute. The checklist of required supporting documents is located at, https://ww2.arb.ca.gov/clean-truck-check-referee-referral-required-documents 

To learn more about certified engine configurations/specifics:

Look up Your Engine Certification with the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) 

1. Find the Engine Family Name (EFN). Every federal EPA certified diesel engine has an emissions label (silver or black sticker) with: 

• Engine Family (e.g., FCPTH12.9M5A) 

• Model year 

• Emissions standard (e.g., 0.2 g/bhphr NOx) 

• Aftertreatment type (DPF, SCR) 

• Tell the owner to check: 

  ✔ On the valve cover 

  ✔ On the timing cover 

  ✔ Under the hood on the driver's side frame rail 

  ✔ In the owner’s manual 

2. Federal EPA Certificates of Conformity Search https://www.epa.gov/importing-vehicles-and-engines/how-obtain-copy-certificate-conformity-heavy-duty-or-nonroad-engine 

3. CARB New Engine Certification: Executive Orders New Vehicle and Engine Certification: Executive Orders for Compression-Ignition and Heavy-Duty Engines and Vehicles

Emission Control Label Example 

Existing California law (AB 1009 of 2004) requires vehicles that operate in California, regardless of entry point, to run with engines that meet emissions standards at least as stringent as U.S. federal standards for the model year that the engine was manufactured. 

All heavy-duty vehicles need to have proof that the engine meets appropriate emissions standards by having the manufacturer emission control label (ECL) properly affixed on the engine and fully legible.  

Clean Truck Check - What to Do When an ECL is Missing/Illegible | California Air Resources Board 

Figure 1, Example Emission Control Label (ECL) that contains Engine Family Name (EFN), Engine certification Model Year (EMY), Engine Serial Number (ESN), Engine Model and some of the equipped Emission Control System (ECS) components. 

Figure 2, Example CARB Executive Order (EO). An EO is an official engine certification that confirms specific engines comply with California’s emissions standards. https://ww2.arb.ca.gov/sites/default/files/classic/msprog/nvepb/executi…