Fuel Additive to Pass an Emissions Test

Two products come up again and again: CRC Guaranteed To Pass and Sea Foam. You use each one the same basic way, by adding it to the tank and letting it run through the engine as you drive. The instructions aren't identical, though, so a quick comparison is useful.

According to CRC, a single bottle treats 16 gallons of gasoline. The company also promises a refund of twice your money if the car still fails the smog check after you've used it, though that only applies when the car has no other mechanical faults.

It can, when the trouble is a dirty fuel system. CRC describes its formula as a detergent that strips carbon deposits off the injectors, intake valves and combustion chambers, and says this lowers carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust. CRC lists rough idling, hard starts, hesitation and loss of power as possible signs of a dirty fuel system, and Sea Foam says black exhaust smoke is a sign of unburnt fuel.

CRC is open that the product works by cleaning deposits, not by fooling the test, and that it's no substitute for real repairs. The company also says it can clear the P0420 and P0430 check engine codes tied to the catalytic converter. That said, O'Reilly Auto Parts staff state the product isn't designed for cleaning the catalytic converter.

A technician Sea Foam cites runs through typical failure causes no additive can repair. When one of these is to blame, only a repair will get you through the test.

• An engine that misfires
• Oxygen sensors that have gone bad
• Fuel vapor (EVAP) system leaks
• A faulty fuel cap
• A catalytic converter that has failed

The steps here come from CRC's directions as relayed by O'Reilly staff, plus Sea Foam's advice on timing.

1. For CRC Guaranteed To Pass, wait until your tank is about half full.
2. Empty the entire 12 fl oz bottle into the tank, then drive for 20 to 40 miles.
3. Keep going until the gauge shows about 1/8 of a tank before filling up again.
4. If you're using Sea Foam instead, its instructions are to add two cans to a low tank, drive about 20 miles, and finish the treatment at least a couple of days before the test.

For ongoing maintenance, CRC suggests adding the cleaner four times a year, or one bottle for every 3,000 miles.

The technician Sea Foam cites puts it simply: keeping your car well maintained is the surest route to passing. Most modern tests are OBD tests, where the tester connects to the car and reads what the car's computer says. The computer carries out its own checks, known as readiness monitors, and the test mainly verifies that they've run and passed.

A handful of easy checks matter a lot:

• A check engine light that's on means an automatic fail on an OBD test, so fix the underlying fault first.
• If you've driven less than about 200 miles since a battery change or cleared codes, the car's computer checks may not be ready for testing yet.
• A gas cap that's loose or gone will fail the cap check, so snug it down or get a new one.

Most states will still test a car when some monitors read not ready. Under EPA guidance most states use, model years 1996 to 2000 may show 2 not ready, while 2001 and newer cars may show 1.