How to Drive Your Car So It’s “Ready” for a California Smog Check – Midnight Smog

If you’ve ever gone in for a Smog Check and been told “your monitors aren’t ready”, you know how frustrating it is. The check-engine light is off, the car seems fine, but the machine still won’t pass it.

At Midnight Smog in San Marcos, we see this a lot—especially after:

• A battery replacement or disconnect

• A repair shop cleared codes with a scan tool

• A DIYer cleared a check-engine light at home

  
  

When codes are cleared, your car’s computer also clears its emissions readiness monitors. Until those monitors run and complete, the state’s system often sees the car as “Not Ready”, and you can fail or be rejected from the California Smog Check.

The good news? With the right kind of driving, most cars can set their monitors again without any special tools.

What Are Readiness Monitors?

Modern vehicles continuously test their own emissions systems. They track items like:

• Misfire

• Fuel system

• Oxygen sensors and heater

• Catalytic converter

• EVAP system (fuel vapor control)

• EGR and other components on some models

• Catalyst

  
  

Each of these is a monitor. After codes are cleared or the battery is disconnected, many of these monitors reset to “Not Ready.” The vehicle then needs a certain pattern of real-world driving—called a “drive cycle”—to let those tests run.

California’s Smog Check program reads these monitors. If too many are not ready, the car cannot pass, even if there’s no active check-engine light.

Before You Start a Drive Cycle

Here are a few tips that make a big difference:

• Fuel Level: Keep the tank roughly between 1/4 and 3/4 full. Many cars will not run EVAP tests if the tank is completely full or near empty.

• No Current Faults: If the check-engine light is on or a problem isn’t fixed, some monitors will never complete.

• Plan for Mixed Driving: You’ll need both city and highway conditions—short trips alone usually won’t do it.

  
  

Generic Drive Cycle Most Cars Respond Well To

Every manufacturer has its own “perfect” drive cycle, but that information can be hard to find and very specific. The following generic drive cycle works well for many vehicles and is a good starting point before your Smog Check at Midnight Smog.

1. Cold Start and Idle (2–3 Minutes)

• Start the car after it has been sitting at least 8 hours so it’s fully cold.

• Turn A/C and defroster OFF.

• Let the engine idle in Park/Neutral for 2–3 minutes without pressing the gas.

This helps the computer begin basic tests as the engine warms up.

2. City Driving – Low Speed (10–15 Minutes)

• Drive locally at 25–45 mph for about 10–15 minutes.

• Use smooth, light acceleration and braking.

• Try to include a few 1–2 minute periods at a steady 30–40 mph.

This segment helps with fuel trim, oxygen sensor, and sometimes EGR-related tests.

3. Highway Cruise (5–10 Minutes)

• Get on the freeway and gently accelerate to 55–65 mph.

• Hold a steady speed for 5–10 minutes (cruise control is fine if safe).

• Avoid big throttle changes or rapid speeding up/slowing down unless traffic demands it.

This is where many vehicles run catalyst and additional O2 sensor checks.

4. Deceleration / Coast-Down

• While still on a safe stretch of road, at about 55–60 mph, take your foot completely off the gas.

• Let the car coast down toward about 20 mph, braking lightly only as needed.

• Try to get 20–30 seconds of pure “off-throttle” decel.

• If traffic allows, do this 1–2 times.

  
  

A lot of vehicles use this closed-throttle deceleration to run specific internal tests.

5. Second Mixed Drive (10–20 Minutes)

• Do another 10–20 minutes of mixed driving:

  • 5–10 minutes city (25–45 mph, normal stop-and-go).

  • 5–10 minutes highway (55–65 mph) with at least one more gentle coast-down if possible.

  
  

At this point, many vehicles will have completed most, if not all, of their important readiness monitors.

6. Key-Off Soak

• Park the vehicle and turn it OFF for at least 4-6hrs

• Some systems (especially EVAP) need a cool-down or “hot soak” period before they finalize.

Often, doing this pattern over a day or two of normal driving will allow the rest of the monitors to complete.

What If Monitors Still Aren’t Ready?

Some vehicles are more picky, and a few can be stubborn:

• Certain makes/models may require a manufacturer-specific drive cycle.

• If the check-engine light turns back on or a code returns during your drive cycle, there is likely still a fault that needs to be repaired.

• If you’ve driven a couple of days with this pattern and monitors still won’t set, it may be time for more detailed diagnosis.

  
  

At Midnight Smog, we can check your readiness monitors before the official test so you don’t waste a visit.

Come See Us at Midnight Smog in San Marcos

If you’re preparing for a:

• Biennial DMV smog renewal

• Change of ownership smog

• Out-of-state vehicle registration

• Or just want peace of mind before registration

  
  

We’re here to help.

Midnight Smog – Test-Only Smog Station in San Marcos, CA

Serving San Marcos, Vista, Carlsbad, Oceanside, and North County

📍 1285 Stone Drive, Suite 101, San Marcos, CA 92078

📞 Call 760-304-0122

🕒 Open late to fit your busy schedule

Drive the pattern above for a day or two, keep your fuel between 1/4 and 3/4 tank, and then come see us. We’ll do our part to make your California Smog Check as fast and painless as possible.