How to pass the CARB smoke test

Failed the snap test? How to fix it

A failed snap test means the engine made too much smoke for its model year, or the truck failed the visual inspection. The fix is a repair, then a retest.

Start with what the result tells you:

• High smoke reading. Have a mechanic check the usual suspects: injectors, turbocharger, air filter and intake, exhaust leaks, and the particulate filter if the truck has one.
• Visual inspection fail. A missing, disconnected or tampered emissions part fails the truck even with a good smoke reading. The part has to be put back or repaired.
• Deleted equipment. Deleted or tampered systems fail and are illegal. The only path is to restore the equipment.

We do not do repairs, but we explain the result in plain English so your shop knows where to start. Bring the repair records to the retest if you have them.

Plan for this before the deadline. If an older truck is borderline, test early so there is time to fix it and retest. More on causes of smoke: DPF, deletes and smoke.

Fuel additives and a hard run: do they help?

They are not a fix. An older diesel passes the opacity test because its engine and emissions parts are in good shape, not because of a bottle or a trick.

Fuel additives. We cannot tell you that any additive will get a truck through a smoke test. If the injectors are worn or the turbo is failing, an additive does not repair them.

The "Italian tune-up." A good hard run before the test does one useful thing: it gets the engine fully warm, and a cold engine smokes more. That is the same reason we ask for the truck at operating temperature. It will not fix worn parts.

What does help an older diesel pass:

• A full warm-up before the test
• Repairs done ahead of time, not the morning of
• A clean air filter and no exhaust leaks
• A working, intact particulate filter if the truck has one
• All original emissions parts in place for the visual inspection