I failed my smog check. Now what?

This week

General guidance for this symptom. A warning light that is flashing or an overheating engine: stop driving and call.

Likely causes

• The check engine light is on

  For 1996 and newer gasoline vehicles the Nevada test reads the OBD-II computer, and a malfunction indicator that is commanded on is a fail by itself. The stored code says what set it. Get the code read, fix the cause, and let the light go out on its own or be cleared after the repair. Our check engine light page covers the common codes.

  Multi-point maintenance check
• Readiness monitors not ready

  The computer runs self-tests on the emissions systems as you drive. After a battery disconnect, a dead battery or a code clear, those tests start over, and until enough of them have finished the DMV calls the car not ready rather than passed or failed. The fix is a week of mixed driving, not a part.

  Smog check and emissions test
• A failed emissions component

  Oxygen sensors, the catalytic converter, the evaporative system and the EGR valve are the usual culprits on cars past 100,000 miles. These are repairs, and under Nevada's station rules a test-only station like a quick lube does not repair emissions parts. A repair shop fixes it; we retest it.

  Smog check and emissions test
• Neglected basics

  A clogged engine air filter, old spark plugs and a dirty fuel system do not fail a test on their own, but they push a borderline older car over the line and set the codes that do. An air filter and a fuel system cleaning before the retest are cheap insurance on a high-mileage car.

  Engine air filter
• A gas cap that does not seal

  The evaporative monitor checks for vapor leaks, and a worn or loose cap sets a code that fails the test. A new cap and a drive cycle is the whole repair.

  Fuel system cleaning

The report you got in the bay says why. Read it before you do anything else, because the three usual reasons have three different fixes and only one of them involves a repair shop. A lit check engine light is a code to chase. Not ready monitors are a drive cycle to finish. A failed oxygen sensor or converter is a repair. We test at our two Reno service centers, Mae Anne Avenue and Longley Lane, and we have watched plenty of cars fail for the second reason when nothing was wrong with them at all.

The Nevada rules that apply to a fail

Nevada tests gasoline vehicles 1968 and newer and diesels up to 14,000 pounds, once a year at registration renewal, but only for vehicles registered in the Washoe County urban area (Reno, Sparks, Washoe Valley and nearby) and the Las Vegas valley in Clark County. New vehicles are exempt for their first three registrations, and hybrids for their first five model years. A passing test is valid for 90 days, so a car that passes on the retest has 90 days to finish the renewal (Nevada DMV emission page, read 10/15/26).

Carson City, Douglas, Lyon and Storey County vehicles are not tested at all, so a Gardnerville or Dayton registration never fails a smog check. If you moved to Reno from one of those counties, the first renewal at your Washoe address is when the test starts.

Getting monitors ready without guessing

If the report says not ready, do not clear anything and do not disconnect the battery. Drive the car for several days through a mix of cold starts, steady highway cruising on I-580 or I-80, and stop-and-go city driving. Most monitors complete within a week of normal use; the evaporative monitor is the stubborn one and often needs a cool overnight and a fuel tank between a quarter and three-quarters full. The EPA's OBD guidance explains why each monitor needs its own conditions. Then come back and we test again. Our check engine light and smog guide has a drive cycle that works for most cars.

What we do after a fail

• Walk you through the report line by line so you know which of the three reasons it was
• Check the engine air filter, the gas cap, the oil level and the obvious fluids, which are ours to fix
• Offer a fuel system cleaning on a high-mileage car when carbon and deposits are a likely contributor
• Point you to a repair shop for codes, sensors, converters and evaporative leaks, with the report in hand
• Retest the car at Mae Anne Avenue or Longley Lane and send the pass to the DMV electronically so you can renew online the same day

We do not tune, replace sensors or do waiver repairs. Saying that up front saves you a wasted trip.

A note on older cars at altitude

Engines tuned at sea level run a little rich at 4,500 feet, and a tired oxygen sensor makes that worse. Reno's thin air is part of why a 2004 pickup with 180,000 miles that passed in Sacramento can struggle here. Clean air filter, fresh plugs, a fuel system cleaning and a long warm-up drive before the test give the computer its best chance. None of that is a trick. It is just a car in the condition the test expects.