What Is a Precheck in Smog Testing, and When Should You Avoid It?

A "precheck"—also known as a pre-test, courtesy test, or diagnostic smog preview—is an inspection service offered by some Smog Check stations.1 The idea is to evaluate whether your vehicle is likely to pass California’s strict emissions test before running the official, recorded test that reports results directly to the state.2

While a precheck can be genuinely helpful in specific, uncertain situations, it is often unnecessary, occasionally overpriced, and sometimes used by shops as a needless upsell.

If you want to understand exactly what a precheck involves and, more importantly, when it’s smartest to skip it, here is the complete breakdown.

What Exactly Is a Smog Precheck?

A precheck is an unofficial, optional inspection that mimics the procedures of a real Smog Check without electronically submitting the results to the DMV or the Bureau of Automotive Repair (BAR) database.

Using the same equipment and scan tools as the official test, the technician will:

Read Readiness Flags: Check the status of your On-Board Diagnostics (OBD-II) readiness monitors (for 1996+ cars).3

Scan for Codes: Look for any pending or stored fault codes that haven't yet triggered the Check Engine Light (CEL).

Evaluate Emissions Status: For older cars (pre-2000), they may run a quick tailpipe emissions sample to assess pollution levels.4

Visual Inspection: Note any obvious visual issues, such as missing catalytic converters or damaged vacuum lines.

The key difference is the reporting: if your car is heading toward failure, a precheck gives you the information needed to make repairs first, avoiding an official, publicly logged fail record.5

When a Precheck Might Actually Be a Smart Investment

The vast majority of well-maintained, modern cars do not need this service. However, there are legitimate scenarios where a precheck provides valuable peace of mind or saves you from an automatic fail:

After Recent Repairs or Battery Work: Disconnecting the battery or replacing emissions sensors (like an O2 sensor or catalytic converter) resets the onboard computer. The vehicle requires a full "drive cycle" to set the readiness monitors again. A precheck confirms all monitors are "Ready" before you pay for a failed official test.

When the Check Engine Light Just Cleared: Just because the light turned off doesn't mean your systems have been verified by the computer. A precheck confirms the monitors are set and that no new codes are pending.

Older or Borderline Vehicles: If you drive a pre-2000 car or a high-mileage vehicle that usually passes by a narrow margin, a preview can help you identify a weak catalytic converter or high $\text{NO}_{\text{x}}$ levels that might be improved with a short tune-up before the official inspection.

Avoiding a Recorded Fail: A smog fail is logged by the DMV. If avoiding an official fail record is important for business reasons (e.g., selling a high-value used car), the precheck provides a safe preview.

The Big Problem: When You Should Absolutely Avoid a Precheck

For many drivers, a precheck is an unnecessary fee that only serves to waste time and money.

If Your Car is 2000 or Newer and Has No Check Engine Light: For modern OBD-II cars, the pass/fail determination is based almost entirely on electronic data: no codes, all monitors set. If your car is running well and has no warning lights, a precheck is paying a technician to confirm what your dashboard already tells you.

When It’s Used as an Upsell: A station should never insist that a precheck is mandatory or required. If a shop pressures you or implies the official test cannot start without paying for a precheck first, politely decline the service and leave. They are trying to create an unnecessary revenue stream.

When the Price is Absurd: Some stations charge $\text{\$40–\text{\$60}}$ for a precheck. Since the full, official test often costs $\text{\$50–\text{\$70}}$ (plus the certificate fee), you are essentially paying for the inspection twice. Only consider a precheck if the fee is minimal and purely diagnostic.

If You're In a Hurry: A precheck adds an extra step to the process. If your car is mechanically sound and ready, you are better off skipping straight to the official inspection to save time.

The Technical Reality: Why Smog Checks Are Binary

For 95% of the cars on California roads today, the Smog Check is a simple computer handshake. The official testing machine asks the car: "Are your emissions systems functional?" And the car returns one of three clear answers: Pass, Fail, or Not Ready.

A precheck cannot magically alter this reality. It uses the exact same technology as the official test. If your car is ready, a precheck is redundant. If your car is not ready, you need a proper drive cycle on the road to set the monitors, not a pre-test in the bay.

In 2026, with the BAR tightening rules on readiness monitors, prechecks may become more common among drivers who have recently done DIY repairs or disconnected their battery. But even then, remember: a precheck should never replace good maintenance and a proper drive cycle—it only confirms the readiness.

Bottom Line: A smog precheck is an optional tool, not a mandatory step. It is useful when you have genuine doubt about your car's electronic readiness, but usually a waste of time and money when your vehicle is well-maintained and free of warning lights.