Exhaust, fuel & emissions

Car fails emissions test because monitors are 'not ready'

An emissions test can fail simply because the car's internal self-test monitors haven't finished running yet, usually after the battery was disconnected, a code was cleared, or the check engine light was recently reset.

Difficulty
Easy
Time
A few days of normal driving
DIY cost
$0
Pro cost
$50–$150 if a shop is needed to diagnose a stuck monitor

Symptoms

  • Emissions test result says 'not ready' rather than pass or fail
  • No warning lights or driveability problems
  • Often follows a recent battery disconnect, repair, or code clear
  • May need a specific drive pattern to complete

Likely causes

  • CommonRecent battery disconnect or dead batteryDisconnecting power resets the monitors, which then need specific driving conditions to run and complete.
  • CommonRecently cleared check engine light or codeClearing codes, whether by a shop or a scan tool, also resets the readiness monitors.
  • CommonNot enough varied driving since the resetMonitors need a mix of highway and city driving, idle time, and drive cycles to complete.
  • PossibleAn underlying fault preventing a monitor from completingIn rare cases, a monitor won't complete because of an actual, unresolved problem.

What to do

  1. Ask when the battery was last disconnected or the check engine light was last cleared, since this explains most 'not ready' results.
  2. Look up a generic or vehicle-specific drive cycle for your car, which usually involves a mix of city and highway driving, idle, and steady speeds.
  3. Drive normally for several days to a week, including a range of trip types, before retesting.
  4. Get the car scanned to see how many monitors are ready versus not, since most tests allow one or two to be incomplete.
  5. If monitors still won't complete after a lot of varied driving, have a shop check for an underlying fault preventing completion.
  6. Retest at the emissions station once most or all monitors show ready.

Tools and parts

  • OBD-II code reader

When to call a pro

If monitors won't complete after extended, varied driving, a shop can check for an underlying issue preventing the self-test from finishing.

Safety

Generally low risk

Generally low risk for a careful person. Stop if anything looks different from what this guide describes.

Related guides

Operator provides AI-generated troubleshooting information. It can be wrong. For safety-critical, electrical, gas, structural, medical, automotive safety, or other high-risk problems, consult a qualified professional.