Information About OBD Testing

Below are notes from CARB regarding testing

Background

During the initial phase of the Clean Truck Check program in 2023, only vehicles that had their Malfunction Indicator Lamp (MIL) failed the OBD test. The program did not fail vehicles with active PDTCs or for a lack of OBD Monitor readiness (i.e., completion of internal diagnostic tests). One lesson learned from this early phase of the program testing is that in the absence of minimum readiness requirements, code clearing was prevalent. About 20% of vehicles had OBD codes cleared immediately before, or within a few days, of the test. As readiness criteria and PDTCs are incorporated into the pass/fail determination in 2024, this code clearing will cause a vehicle to fail the OBD test.

Readiness

Readiness is determined by looking at how many of the OBD system diagnostics have been completed and the amount of vehicle operation since codes were cleared (OSC – Operation Since Clear). The OBD system diagnostics are broken down into categories called “Monitors.” Each Monitor must go through a series of diagnostic tests to determine if those systems are working properly. The OBD system reports whether each Monitor has had time to complete its tests; if so, the Monitor is considered “Ready.” The specific number and types of Monitors vary based on OBD protocol (J1939 vs J1979), fuel type, vehicle make, and model year. Based on the tests performed to date, most vehicles have all Monitors ready when compliance tests are submitted. However, vehicles that have had codes cleared recently will have multiple Monitors not ready (incomplete). If too many monitors are incomplete, the vehicle will fail the readiness check. The readiness limits will be phased in and become more stringent over time.

Operation Since Clear

OBD systems store information regarding vehicle operation since codes were cleared. Most vehicles have OSC for:

· Warm-Up Cycles (WUCs) since clear

· Distance traveled, in km, since clear

· Engine run time, in minutes, since clear

Typically vehicles track WUCs and either minutes or distance since clear. Readiness criteria will be applied to the OSC variables provided by an individual vehicle. As they are phased in, the criteria thresholds for these variables will become more stringent over time.

PDTCs

Permanent Diagnostic Trouble Codes (PDTCs) are very similar to regular DTCs. However, unlike regular DTCs, they cannot be reset by disconnecting the vehicle’s battery or cleared using an OBD scan device. The only way to clear a PDTC is to fix the underlying problem with the vehicle that originally activated the PDTC, and then allow the vehicle sufficient drive time to re-run the Monitor that identified the problem in the first place. When the Monitor runs without identifying a problem, the PDTC will clear itself.

Phase 2 Minimum OSC 

1. Minimum of 5 Warm-Up Cycles(15 if at least one PDTC is present) 

2. If the vehicle OBD system does not collect Warm-Up Cycles, the CTC test will check for a  minimum of 100 miles OSC (300 if at least one PDTC). 

3. If neither WUC nor mileage are in the test record, the CTC test will check for a minimum of 200 minutes of OSC (600 if at least one PDTC). 

   
   

Getting ready for the CTC OBD test 

Things to do to have your best chance at passing the CTC OBD test: 

• If your Malfunction Indicator Light (right) is on, get your vehicle repaired as soon as possible.    

• Don’t wait - Do the OBD test early in the inspection compliance window so there will be time to complete the test if there are problems 

• Avoid clearing codes. OBD systems require substantial vehicle operation to be able to complete their emission control diagnostic tests. 

• A vehicle that has not been operated enough will fail the test.