Smog Checks for Older Cars and Trucks: What to Expect

A lot of the vehicles we smog at Walnut Smog & Repair are older: 1990s Toyota pickups, Nissan Frontiers, Camrys that have been in the family for decades. One customer has brought his 1996 Toyota truck in "about every two years" for its smog check. Older vehicles get a different kind of test than new ones, and they fail for different reasons. Here's what to expect.

A different test

Most newer vehicles get an OBD test, where the smog equipment reads the car's own computer. Older vehicles, generally 1999 and earlier, get a tailpipe emissions test instead. A probe goes in the tailpipe and measures what actually comes out. In much of California, including Pasadena, that test is often done on a dynamometer, a set of rollers that lets the car run at road speed while it sits still in the bay. You'll see ours in the shop.

What's measured

• HC (hydrocarbons): unburned fuel. High HC usually means a misfire, worn plugs or wires, or a vacuum leak.
• CO (carbon monoxide): a sign the engine is running rich, often from a bad oxygen sensor, leaking injectors or a sticking thermostat.
• NOx (oxides of nitrogen): created by very hot combustion. High NOx often points to a clogged EGR system, a lean condition or a tired catalytic converter.

Older cars also get the visual inspection and functional checks, including the gas cap and, where equipped, the check engine light.

Why older vehicles fail

Age wears out the parts that keep emissions in check: oxygen sensors get lazy, vacuum hoses crack, EGR passages clog with carbon, ignition parts wear, and catalytic converters lose efficiency. None of that means the car is finished. Most of it is fixable, often for much less than people expect. One customer's '99 Frontier failed and he assumed it needed a converter; it needed an O2 sensor. Another's 1996 Camry passed after real diagnosis and repair.

How to keep an older car passing

• Change the oil before the test. Old, fuel-diluted oil can raise HC readings. See oil change.
• Keep ignition parts fresh. Plugs, wires, cap and rotor matter a lot on older engines. See engine tune-up.
• Replace a clogged air filter. It affects the air-fuel mixture. See filters.
• Arrive warm. Drive at least 15 to 20 minutes on the way so the engine and converter are at full temperature.
• Fix leaks and smoke. Visible smoke is a failure on its own.
• Keep the original emissions parts. Missing or modified equipment fails the visual inspection.

If it fails

Bring the report. The numbers tell us a lot about where to look. We'll diagnose, repair and retest it here. See failed smog repair, or read what to do after a failed smog check.

Older cars welcome

Walnut Smog & Repair is a STAR Certified station at 1323 E Walnut St #2, Pasadena. Older cars and trucks are a big part of what we do. Call (626) 449-6252.

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