Clearing Codes Before an Emissions Test: Rejection and How Long to Drive

This lesson draws on Nevada DMV’s OBD page, EPA’s 2001 OBD inspection guidance, California’s Bureau of Automotive Repair, New York DMV’s readiness brochure and the BlueDriver and Innova product pages. How we research explains the method. It covers US state programs; inspections in other countries work differently.

What clearing codes does to readiness monitors

Readiness monitors are self-tests the car’s computer runs on its emission systems, such as the catalyst, the evaporative (fuel vapor) system and the oxygen sensors. EPA’s guidance says each one reads “ready” once it has run and “not ready” until then. Three of them run all the time; the rest need specific driving conditions.

Clearing codes wipes all of them at once. EPA says that clearing the fault code memory also sets every readiness code to “not ready”, and that clearing is all-or-nothing: you can’t erase one code and keep the rest. EPA and New York’s DMV both list a disconnected or replaced battery as another cause.

That is one more reason to charge a battery without disconnecting it in the weeks before a test.

Why a cleared car is rejected

Nevada gives 1996 and newer vehicles, except heavy-duty ones, an OBD II check instead of a tailpipe test. Its DMV says a cleared computer gets the car “Rejected” for being not ready, and adds: “A rejection is not a failure.” States may still test a car with a few monitors not ready:

Program	Model years	Monitors allowed “not ready”
EPA guidance (states that follow it)	1996-2000	Up to two
EPA guidance (states that follow it)	2001 and newer	One
California Smog Check, gasoline	1996-1999	Any one
California Smog Check, gasoline	2000 and newer	Evaporative system only

EPA counts only the monitors that need driving conditions. A freshly cleared car usually has more monitors unset than these limits allow, so the station sends it away.

New York words it differently: a car with a required monitor not ready does not pass, but if that is the only problem and the current inspection has already expired, the inspector’s computer issues a 10-day extension. Rules vary, so check your state’s current page.

How long to drive after clearing codes

No single number fits every car. Nevada’s DMV says to drive “under varying conditions for up to one week” so the computer can reset its monitors. EPA’s guidance says a week of normal driving is enough in most cases, and that in under 1% of cases a car may need a repair shop to set its monitors using the maker’s procedure.

New York’s DMV says most cars need two drive cycles separated by a cool-down period, in a mix of city and highway driving. Its generic cycle includes steady stretches at 55 mph (89 km/h). The exact cycle depends on the make, model and monitor.

A scanner that reads readiness takes the guessing out: check the monitors before you book the test, not after a rejection.

Permanent codes: the ones a scanner can’t erase

California’s Bureau of Automotive Repair (BAR) says permanent diagnostic trouble codes cannot be reset by disconnecting the battery or cleared with a scan tool. The only way to clear one is to fix the fault, then drive until the monitor that found it runs again without finding a problem. The code then clears itself.

Since July 1, 2019, California fails 2010 and newer vehicles with a permanent code stored, whether or not the check engine light is on (some 2010 models don’t support these codes). BAR ignores a permanent code once the car has completed at least 15 warm-up cycles and 200 miles (322 km) since its OBD information was last cleared.

BAR says most cars finish their self-tests well before that. What other states do with a stored permanent code varies, so check your state’s current rule.

Check readiness before you go

Never clear codes to get through a test: get the fault fixed, then drive the cycle. If the check engine light is flashing rather than steady, skip the drive cycle and get the car serviced: can an OBD2 scanner read ABS and airbag codes explains why a flashing light is urgent.

BlueDriver says its Smog Check feature lets you “check if the vehicle is ready for a smog or emissions test.”

BlueDriver

BlueDriver Pro

A Bluetooth OBD2 adapter for iOS and Android with repair reports, freeze frame and smog check readiness, and no subscription fees. Made for 1996+ vehicles sold in North America.

Read the full guide

Key specs

No subscription fees
1996+ vehicles sold in North America
Repair reports, freeze frame, smog check
1-year limited warranty

Mid-range

Paid link

View on Amazon

Innova says the 5210 re-checks monitors automatically after engine restarts to show whether the car is ready for a smog test, and gives step-by-step drive cycle instructions.

Innova

Innova 5210

A handheld code reader for engine and ABS codes with a battery and charging system test. It does not read SRS (airbag) codes and is built for North American vehicles and OBD protocols.

Key specs

Engine and ABS codes; no SRS (airbag)
Battery and charging system test
North American OBD protocols only

Mid-range

Paid link

View on Amazon

A “ready” result is not a guaranteed pass: an active code with the check engine light on, or a permanent code in California, still fails. Both tools are built for North American vehicles; Bluetooth vs wired scanners covers other markets, and BlueDriver vs FIXD compares features.

Buying a used car? Since clearing is all-or-nothing, a car with the check engine light off but many monitors “not ready” may have had its codes cleared recently. It is a clue, not proof: New York’s DMV lists recent repairs, a disconnected battery and a software update among the causes too.

Sources

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