OBD2 Readiness Monitors on a Used Car: Spot a Cleared Code

If a used car shows several incomplete OBD2 readiness monitors, someone likely cleared its trouble codes or disconnected the battery recently, and the check engine light may come back after you buy it. Readiness monitors are self-tests the engine computer runs on systems like the catalytic converter and the evaporative system. Clearing codes resets them to "not ready," and they only complete again after the car is driven through the right conditions.

What readiness monitors are

Cars and light trucks sold in the U.S. from the 1996 model year on use OBD-II. The computer runs a set of monitors, and each one reports either ready (complete) or not ready (incomplete). Common ones include catalyst, oxygen sensor, oxygen sensor heater, evaporative system, EGR and, on some cars, secondary air. Diesels have their own set. The EPA inspection and maintenance page explains how states use emissions programs, many of which read these monitors.

Some monitors, like misfire and fuel system, run all the time. Others need a specific mix of cold starts, steady highway speed and idle time. The evaporative monitor is often the last one to finish and can take days of normal driving.

Why incomplete monitors matter when buying

A seller who knows a problem exists can clear the code with a cheap scan tool, and the light stays off for a while. The monitors give that away. A car that has been driven normally for a week or two should show most or all monitors ready. If you see several marked not ready, ask why. Honest answers exist: a new battery, a recent repair or a dealer that just did service. But you should hear that answer and see a receipt.

Our guide on whether CARFAX shows a check engine light explains why a history report rarely catches this, since a report records events like service visits, not live warning lights.

How to check monitors

• Plug a basic OBD2 scanner into the port under the dash, usually left of the steering column. Many cost less than a tank of gas.
• Turn the key to on, or start the engine, and open the readiness or I/M screen.
• Write down each monitor and whether it is ready, plus any stored or pending codes.
• Look at the "distance since codes cleared" value if your tool shows it. A small number is a red flag.

Pending codes are worth noting too. They show a fault the computer has seen once but not confirmed, which often means the light is about to come on.

What emissions programs allow

Many state inspection programs follow EPA guidance that allows a limited number of incomplete monitors: commonly one for 2001 and newer vehicles and two for 1996-2000 vehicles. Rules vary by state, so check your state's program. A car with more incomplete monitors than that will usually be turned away from an emissions test until it is driven more, which can delay registration after you buy it.

Signs of tampering beyond cleared codes

Look at the OBD port itself. Scratches, a loose port or extra wiring can point to a tuning device, a theft attempt or a dash that was opened. Our guide to a theft recovery with a tampered OBD port covers what that looks like. A mismatch between the odometer and the mileage stored in other modules is another reason to stop.

What to do with what you find

• All monitors ready and no codes: good sign, but still get an inspection.
• One or two not ready with a believable reason: ask for receipts and consider a second scan after a drive.
• Several not ready and a short distance since clearing: assume a problem is being hidden. Ask the seller to drive the car for a week and let you scan it again, or walk.

A pre-purchase inspection shop will read monitors as a matter of course, and many will check for codes in other modules too, such as ABS and air bags.

Pair the scan with the history report

The scan tells you what the car is doing today. A history report tells you what happened before: accidents, title brands, odometer readings and service visits. Used together, they catch a lot. If the report shows repeated emissions failures or a recent repair at a shop, that may explain the monitors. Check recalls too through the NHTSA lookup, since some recalls involve engine software that can affect warning lights.

What the report costs

As of early November 2026 a CARFAX bought direct usually costs around $44.99, and AutoCheck sits a little lower. We are a licensed CARFAX and AutoCheck reseller, not affiliated with CARFAX, AutoCheck or Experian. Our guest price has been about $5.50 a report, about $4.50 for members, and about $6.75 for a CARFAX and AutoCheck bundle. Confirm the price at checkout, since it can change.

Sources

• EPA, vehicle inspection and maintenance
• NHTSA recalls
• NMVTIS