A U.S. smog test checks OBD codes, readiness monitors, the check-engine light, fuel cap sealing, visual emission parts, and sometimes tailpipe gases.
Across the states, emissions inspections share the same core goal. Catch excess pollution and flag fixes before the car loads the air with extra hydrocarbons, CO, or NOx. The exact steps vary by model year, fuel, and weight. Newer vehicles lean on an electronic scan. Older vehicles add tailpipe sampling. Diesels get separate smoke checks in some programs. The list below shows what techs review and why it matters to your pass.
What They Check During A U.S. Smog Test: Full Walkthrough
Modern inspections start at the diagnostic port. The tool talks to the engine computer to learn if emission devices ran their self checks and if any faults remain. The light on the dash must work and stay off. Inspectors also look for missing or tampered parts. Some regions still measure exhaust at idle or on a dynamometer for pre-OBD cars. Many shops also run a quick check of the tailpipe and the crankcase.
Common Inspection Elements
| Check Item | What The Inspector Confirms | Notes |
|---|---|---|
| OBD Scan | Vehicle reports no emission codes and proper comms | Core step for 1996+ light-duty gasoline and many diesels |
| Readiness Monitors | Required monitors show “ready” state | Too many “not ready” flags lead to an automatic fail |
| Check-Engine Light | Bulb proves out, then stays off | A lit light or a missing bulb fails the test |
| Gas Cap | Cap present and seals | Some programs pressure test; others do a visual fit check |
| Evaporative System | Hoses, purge lines, and canister connected | Leaks vent fuel vapor and raise smog-forming compounds |
| Catalytic Converter | Correct unit, intact, no tamper signs | Wrong or hollowed units trigger faults and high tailpipe readings |
| Oxygen Sensors | Present and plugged in | Missing or lazy sensors harm fuel control and converter life |
| EGR / Air Injection | Hardware present where required | Reduces NOx or speeds catalyst light-off on many engines |
| PCV / Crankcase | No smoke from crankcase vent during idle check | Part of the visible smoke procedure in some states |
| Tailpipe Test | HC/CO/NOx within cutpoints on older cars | ASM or two-speed idle only for pre-OBD groups |
| Diesel Smoke | Opacity within the limit | Applies to many non-OBD diesel trucks |
OBD, Readiness, And The Dash Light
On cars with OBD-II, the scan checks two things. First, stored and pending codes tied to emissions. Second, whether self tests completed since the last reset. Those self tests, called readiness monitors, track the catalyst, oxygen sensor, evaporative leak, and EGR. One or two monitors can be incomplete on some older cars and still pass. Too many not ready flags means a retest after more driving.
Clearing codes or disconnecting the battery resets the monitors. That wipes proof that the car ran its self checks. A steady drive cycle is needed to set them again. City and highway time helps. Some states give published limits on how many incomplete monitors they’ll allow by model year. If the shop’s reader can’t talk to the car at all, that’s a fail until the port, fuse, or network issue gets fixed.
Why Readiness Fails
The monitor may wait for a cold soak, a set speed, or a certain fuel level. If the tank is near empty or topped off, the evaporative check may not run. A stuck thermostat, a lazy O2 sensor, or a weak battery can stall monitor logic too. Driveability issues can also block a pass, even with the light off. The fix is part maintenance, part patience, and part steady driving.
Visual And Functional Checks
Inspectors confirm that core parts are present and hooked up. That includes the catalyst, oxygen sensors, PCV valve, and any add-on devices listed for the engine family. Missing or welded-in parts without an approved label can lead to a fail. Many programs include a brief smoke check at idle and during a snap of the throttle. Some regions still pressure test the gas cap or confirm the cap seals by fit.
Where tailpipe gear is used on older groups, the station measures HC and CO at two speeds, or runs a loaded test on rollers. The car must meet set cutpoints for its year and weight. That gear isn’t used on most late model gasoline cars. Those rely on the OBD scan and the visual check to confirm clean running.
What’s Different For Diesel
Light-duty diesels often follow the same OBD scan rules as gasoline cars. Many non-OBD diesel trucks see a smoke test and a quick check of emission parts. Opacity must be under a set limit. Some states also run roadside screens for heavy trucks.
State Differences, Shared Basics
Every state that runs an I/M program leans on the same core principles. Scan first on newer cars. Confirm the dash light works. Check that parts are present. Use a tailpipe rig on pre-OBD cars. Use smoke checks where diesel rules call for it. Policy details change by region, but the checklist above will look familiar across the map.
You can read a plain overview of I/M programs and where they run on the EPA I/M page. California also publishes a clear outline of what a Smog Check includes on the BAR Smog Check page. Those two sources give you the big picture and the nuts-and-bolts list many shops follow.
What Causes Fails And How Shops Explain Them
Techs write a printout that points to the cause. The most common reasons are a lit dash light, incomplete readiness, or a missing part during the visual check. For older platforms, the tailpipe may show HC or CO above the cutpoint. On diesel trucks, opacity can bust the limit. The quick table below maps each fail to plain fixes.
| Fail Item | Common Cause | What Usually Fixes It |
|---|---|---|
| Lit Check-Engine Light | Stored emission code | Repair the fault, clear codes, complete drive cycle |
| Not Ready Monitors | Recent reset or drive cycle not complete | Mixed highway and city drive with correct fuel level |
| No OBD Communication | Blown fuse or broken port wiring | Restore power to the port or fix network wiring |
| Gas Cap Fail | Cracked seal or wrong cap | Replace cap and rerun test |
| Visual Tamper | Missing or non-approved catalyst or sensors | Install approved parts for the engine family |
| High Tailpipe HC/CO | Rich mix, misfire, or tired converter | Fix misfire and fuel trim, verify converter health |
| Diesel Smoke Over Limit | Injector, turbo, or EGR issues | Repair tune and air path, confirm clean opacity |
How To Prep Your Car So It Passes Cleanly
Start with maintenance. Fresh oil and a clean air filter help. Fix any dash light well before inspection day. If you recently replaced a battery or cleared codes, plan a few days of mixed driving. Keep fuel between a quarter and three-quarters for the evaporative check. Warm the car before you pull into the lane. Bring the correct fuel cap for your make.
Avoid clearing codes right before the visit. The scan will see a reset and the monitors won’t be ready, even if the car runs fine. Bring any repair receipts in case the shop needs quick context.
Drive Cycle Basics
Readiness often sets during steady cruising, gentle stops, and a cold start the next day. Many owners see full readiness after a week of normal trips. Cold weather and short hops slow things down. Some states allow a small number of incomplete monitors by year, so a pass is still possible.
After A Fail
Ask for the printout. It lists the code or the failed step. Fix the root cause. Leaks in evaporative lines, a loose intake boot, or a cracked vacuum tee can pull the system out of range. A smoke test can spot hidden leaks. After repairs, complete a calm drive cycle and return for a retest.
Older Vehicles And Tailpipe Rigs
If your car predates wide OBD use, the station measures exhaust with a probe. Results are compared to cutpoints for your model year and weight. Many shops use a two-speed idle method, and some regions still use a loaded dyno run. Cars in this group also get the visual and functional checks listed earlier.
Diesel Trucks And Opacity Rules
Light-duty diesels with OBD face a scan and a visual check. Heavy or older diesel trucks often see a smoke test. The meter reads how much light the smoke blocks, and the number must stay under the limit for weight and year. Program names and weight cutoffs differ by state.
What Inspectors Don’t Check
Smog programs don’t rate ride comfort or cosmetic mods. They don’t judge oil life or brake wear during an emissions pass. If the car is unsafe to test, the shop can reject the visit for safety reasons. That’s a stop, not a fail.
Final Notes Before You Head In
Know Your Test Type
Find out if your car will get an OBD scan only, a tailpipe test, or both. That depends on year, fuel, and weight. Knowing the plan steers your prep.
Set Readiness
If you cleared codes or lost battery power, give the car time. Mix freeway and town miles. Don’t top off the tank right before you go.
Pass The Visual
Use approved emission parts for your engine family. Keep clamps, hoses, and sensors installed and tidy. Bring the correct cap and make sure it seals.
Arrive Warm
Take a 15-minute drive first. A warm catalyst and stable idle help both the scan and any tailpipe work on older groups.
With those steps done, most cars pass cleanly.