What Are Readiness Monitors?
OBD2 readiness monitors are self-tests that your vehicle's ECU runs continuously while you drive. Each monitor checks a specific emissions-related system — the catalytic converter, evaporative emissions system, oxygen sensors, and so on — and marks itself Ready once it has run and passed.
When a monitor hasn't had the chance to run yet (or was reset after a battery disconnect or code clear), it shows as Not Ready. Emissions testing stations check these statuses: too many "Not Ready" monitors and you'll fail the test before they even plug in their scanner.
Key point: Clearing DTC codes (or disconnecting the battery) resets all readiness monitors back to "Not Ready." You must complete enough normal driving cycles to let them run before an emissions test.
The Common Readiness Monitors
| Monitor | What it tests | Type |
|---|---|---|
| Catalyst | Catalytic converter efficiency (compares upstream/downstream O2 sensors) | Drive-cycle |
| Heated Catalyst | Secondary catalytic converter warm-up time | Drive-cycle |
| Evaporative System (EVAP) | Fuel vapor leak test — checks gas cap seal and charcoal canister | Drive-cycle |
| Secondary Air System | Air injection for cold-start emissions reduction | Drive-cycle |
| Fuel System | Long-term and short-term fuel trim health | Continuous |
| Misfire | Engine misfires that could damage the catalytic converter | Continuous |
| Oxygen Sensor | O2 sensor response time and switching frequency | Drive-cycle |
| Oxygen Sensor Heater | O2 sensor heater element function | Drive-cycle |
| EGR System | Exhaust gas recirculation flow and valve operation | Drive-cycle |
Continuous monitors (Misfire, Fuel System, Comprehensive Components) run at all times while the engine is on. Drive-cycle monitors only run under specific conditions — certain speed ranges, temperature windows, and engine load levels — which is why they can take several days of normal driving to complete after a reset.
What Is Mode 06?
Mode 06 is an OBD2 diagnostic mode that returns raw test results from the non-continuous monitors. Instead of just "Ready / Not Ready," Mode 06 shows the actual measured value alongside the minimum and maximum threshold values the ECU requires for a pass.
This lets you spot a monitor that is technically "Ready" but barely passing — a useful early
warning that a system is degrading before it sets a fault code and turns on the check engine light.
Mechanics often use Mode 06 data to catch a failing catalytic converter weeks before a
P0420 code appears.
Using the Readiness Check Screen in Wrench Buddy
Step 1 — Connect your OBD2 adapter
Ensure your Bluetooth adapter is connected. From the main menu, look for the green "Connected" status indicator. If not connected, tap Connect Scanner first.
Step 2 — Open Readiness Check
From the main menu, tap Readiness Check. Wrench Buddy queries the vehicle using OBD2 Mode 01 (for monitor status) and Mode 06 (for raw test data) simultaneously.
Step 3 — Read the results
Each monitor is listed with one of three statuses:
- Ready — The monitor has run and passed. Good to go for testing.
- Not Ready — The monitor hasn't completed its self-test cycle yet. More driving needed.
- Not Supported — Your vehicle doesn't use this monitor. This is normal and does not affect your test.
Emissions test threshold: Most U.S. states allow 1 "Not Ready" monitor for 1996–2000 vehicles and zero "Not Ready" monitors for 2001 and newer vehicles. Check your state's specific rules — they vary.
Why Are My Monitors Showing "Not Ready"?
The three most common reasons:
- You recently cleared codes or disconnected the battery. Resetting clears all monitor status. You need to complete a full drive cycle (see below).
- A fault is preventing the monitor from running. If there's an active DTC, the ECU may not allow certain monitors to complete. Fix the underlying fault first, then drive to reset monitors.
- Driving conditions haven't been met. Some monitors require highway speeds, specific ambient temperatures, or a cold start. Short city-only trips may not trigger them.
The OBD2 Drive Cycle
A drive cycle is a specific sequence of driving conditions designed to satisfy all monitor prerequisites at once. The EPA-defined generic drive cycle is a good starting point:
- Cold start (engine coolant below 50°C / 122°F, ideally after sitting overnight).
- Idle for 2–3 minutes with accessories off.
- Moderate acceleration to 55 mph (88 km/h) — no wide-open throttle.
- Steady cruise at 55 mph for 3 minutes.
- Decelerate without braking (lift off throttle) to 20 mph (32 km/h).
- Accelerate back to 55–60 mph and cruise for 5 minutes.
- Decelerate smoothly to a stop.
Repeat this sequence once or twice, then check Wrench Buddy's Readiness screen again. EVAP monitors often require an additional cold soak (let the car sit overnight) between attempts.
Tip: Some manufacturers publish vehicle-specific drive cycles that complete monitors faster than the generic EPA cycle. Search your vehicle's year/make/model + "OBD2 drive cycle" for the manufacturer's procedure.