California Smog Test Types Explained: Idle, 2500 RPM, 15 MPH & 25 MPH Tests
California Smog Testing: Why Some Cars Get the Idle Test and Others Get the 15/25 MPH Dyno Test
If you own or are shopping for an imported vehicle in California, one of the most common smog questions is simple: why do some vehicles get tested at idle and 2500 rpm, while others are driven on the dyno at 15 mph and 25 mph?
The answer depends on the vehicle, the model year, the smog program area, and whether the car can safely be tested on the emissions dynamometer.
The Two Main Tailpipe Test Types
For many older gasoline vehicles, California uses one of two tailpipe emissions test procedures:
- Two-Speed Idle Test (TSI): The vehicle is tested at approximately 2500 rpm and again at idle.
- ASM Loaded-Mode Test: The vehicle is tested on a dynamometer at 15 mph and 25 mph under load.
These tests are most commonly relevant to 1976-1999 gasoline vehicles. Newer 2000-and-newer gasoline vehicles are usually inspected through the OBD-II based system, although there are exceptions.
What Is the 15 MPH and 25 MPH Smog Test?
The 15 mph and 25 mph test is called the Acceleration Simulation Mode, or ASM, test. This is the loaded-mode test performed on a smog dynamometer.
The ASM test has two main parts:
- 50/15 test: 50% load at 15 mph
- 25/25 test: 25% load at 25 mph
Because the vehicle is tested under load, the ASM test can check emissions behavior that may not show up during a simple idle test. This is especially important for measuring oxides of nitrogen, commonly listed as NOx on the smog report.
When Does California Use the ASM 15/25 MPH Test?
In general, the ASM test is used for many 1976-1999 gasoline vehicles registered in California Enhanced Smog Check areas.
These are areas where California requires a more involved emissions test because of air quality requirements. If the vehicle is eligible for ASM and can safely be tested on the dyno, the smog station will usually perform the 15 mph and 25 mph loaded-mode test.
What Is the Idle and 2500 RPM Test?
The idle and 2500 rpm test is called the Two-Speed Idle, or TSI, test.
During this test, the vehicle is checked at a higher rpm, usually around 2500 rpm, and then again at idle. The test measures tailpipe emissions without placing the vehicle under road-load conditions on the dyno.
When Does California Use the Two-Speed Idle Test?
The Two-Speed Idle test is generally used for 1976-1999 gasoline vehicles in Basic and Change-of-Ownership smog program areas. It is also used in Enhanced Areas when the vehicle is not compatible with the ASM dynamometer test.
That last part is important for many imported vehicles.
Why Some Vehicles Cannot Use the Dyno Test
Not every vehicle can safely be run on a two-wheel-drive smog dyno. If the vehicle is incompatible with ASM testing, the smog station should use the Two-Speed Idle procedure instead.
Common reasons a vehicle may not be compatible with ASM testing include:
- Full-time all-wheel drive or four-wheel drive
- Traction control that cannot be disabled
- The vehicle does not safely fit on the dynamometer
- Certain heavy-duty or special-use vehicle configurations
For example, many Nissan Skyline GT-R models use the ATTESA all-wheel-drive system. Because of that, they are generally not treated like a normal two-wheel-drive car for ASM dyno testing. In those cases, the Two-Speed Idle test may be the appropriate tailpipe test instead of the 15/25 mph dyno test.
Quick Reference: Which Test Applies?
| Vehicle Type | Typical Test |
|---|---|
| 1976-1999 gasoline vehicle in an Enhanced Area | ASM 15 mph / 25 mph dyno test, if compatible |
| 1976-1999 gasoline vehicle in a Basic or Change-of-Ownership area | Two-Speed Idle test |
| 1976-1999 gasoline vehicle that cannot safely be dyno tested | Two-Speed Idle test |
| 2000-and-newer gasoline vehicle | Usually OBD-II based inspection |
What This Means for Imported Vehicles
For imported vehicles, especially Japanese performance cars, the test type can matter. A rear-wheel-drive vehicle may be treated differently than an all-wheel-drive vehicle. A 1990s Nissan Silvia, for example, may be tested on the dyno in an Enhanced Area if it is otherwise eligible and safe to test. A Skyline GT-R, because of its AWD system, may be handled differently.
The important thing is that the test procedure should match the vehicle. The smog station follows the California Bureau of Automotive Repair procedures and the prompts from the emissions inspection system.
Final Thoughts
California smog testing can seem confusing because not every vehicle gets the same test. The short version is: older gasoline vehicles may receive either a loaded dyno test or a two-speed idle test, depending on where the vehicle is registered and whether it can safely be tested on the dyno.
For buyers of imported vehicles, this is one more reason to understand the car before purchase. Drivetrain layout, model year, emissions equipment, and California compliance all matter.
If you are looking for a legally imported Japanese vehicle, or have questions about California compliance, Import a Vehicle can help guide you through the process.
