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Car failed emissions test

Why is my car failing emissions test


This is Part II of a series on Readiness Monitors. If you haven’t read Part I, find it here

If your state requires emissions testing and the technician failed your car right away, you’re wondering “Why is my car failing emissions test?” Well, when a car fails right away it’s because the technician noticed that the readiness monitors were not set to “complete” status.


Every modern car has a computer that runs tests on each emissions related system in the car. These tests are called readiness monitors. If the tested system doesn’t pass the test, the computer will store a trouble code and turn on the check engine light. However, in many cases the computer can see a problem but NOT store a code right away. In those cases the computer stores a “pending code” and starts the test over. After repeating the test the programmed number of times the computer will either pass the system or store a hard trouble code. Unfortunately, in the mean time, the readiness monitor will be set to “not ready.” If you take your car in for emissions testing during that time period, they will fail your vehicle on the spot.

Now every car maker has different specifications for how to conduct the testing. Most require you to drive the car at X speeds, for X amount of time, with X number of cold starts, and X amount of time before and after a gas refueling. A technician can look up the exact test sequence for your particular year, make, model, and engine combination.

Avoid failed emissions test

Most inexpensive code readers will tell you if your car is ready to be tested. It checks all the readiness monitors to see if they’re in the complete stage. Some car makers make it a bit easier to check readiness status without a code reader or scan tool. Honda, for example let’s you do it yourself.


Readiness status on Honda Civic

I’ll use a 2006-11 Honda Civic for this example. As you read through the enabling and resetting steps, understand that if even on criteria is off, you’ll wind up with a failed emissions testing:

First, be aware of what extends the test times:

1) Disconnecting the battery. All readiness monitors start from scratch

2) Low outside temps dramatically increase monitor time

3) Driving in stop and go traffic will increase monitor times.

4) All “enabling” criteria must be met. If any of the enabling criteria don’t meet the minimum specification, the test won’t complete

Honda self test readiness status

Turn the IGN switch to ON, but do not start the engine. The check engine light should come on for 15-20 seconds. It should then go OUT to tell you that all monitors are in the complete stage. However, if the check engine light flashes five times, one or more of the readiness monitors are not complete. Honda gives directions on how to manually change the status from incomplete to complete. In some cases you don’t need a factory scan tool

Honda catalytic converter monitor and readiness code


Enabling criteria:

The engine coolant must be at least 158 degrees F

The intake air temperature sensor must read above 20 degrees F

The vehicle must be driven at speeds above 25-mph.

Do the test

1) To reset with a factory scan tool: Connect the factory scan tool and go to readiness codes

2) Start the engine

3) Test drive the vehicle under stop and go conditions with short periods of steady cruising above 25-mph. The test should complete in about 5-miles.

4) If the monitor does not change from incomplete to complete, check for a pending trouble code and recheck the enabling criteria.

Evaporative Emission (EVAP) Control System Monitor and Readiness Code

Enable Criteria

1) Battery voltage must be more than 10.5 V.

2) Engine is at idle.

3) Engine coolant temperature sensors 1 & 2 are between 176°F (80°C) and 212°F (100°C).

4) The MAP sensor shows less than 46.6 kPa (14 inHg, 350 mmHg).

5) The vehicle speed is 0 mph (0 km/h).

6) Intake air temperature sensor shows between 32°F (0°C) and 212°F (100°C).

Do the test

  1. Connect the factory scan too to the DLC.
  2. Start the engine.
  3. Select EVAP TEST in the INSPECTION MENU with the scan tool, then select the FUNCTION TEST in the EVAP TEST MENU.

If the result is normal, readiness is complete. If the result is not normal, go to the next step.

  1. Check for a Pending DTC. If there is no DTC, one or more of the enable criteria were probably not met; repeat the procedure.

Air/Fuel ratio sensor Monitor and Readiness Code Enable Criteria

Enable Criteria

1) Engine coolant temperature sensor 1 must show 140°F (60°C) or more.

Do the test

  1. Start the engine.
  2. Test-drive the vehicle under stop-and-go conditions with short periods of steady cruise. During the drive, decelerate (with the throttle fully closed) for 5 seconds. After about 3.5 miles (5.6 km), the readiness code should switch from incomplete to complete.
  3. Check the readiness codes screen for the AIR FUEL RATIO (A/F) SENSOR in the DTCs MENU with the scan tool.
  • If the screen shows complete, readiness is complete.
  • If the screen shows not complete, go to the next step.
  1. Check for a Pending DTC. If there is no DTC, the enable criteria was probably not met. Select the DATA LIST MENU. Check the ECT SENSOR 1 in the ALL DATA LIST with the HDS. If the ECT SENSOR 1 is less than 140°F (60°C), run the engine until it is more than 140°F (60°C), then repeat the procedure.


The air/fuel sensor heater test has no enabling criteria. After running for 1-minute the air/fuel heater must be up to operating temperature or it will fail the test

Do the test

  1. Start the engine, and let it idle for 1 minute. The readiness code should switch from incomplete to complete.
  2. If the readiness code is still not set to complete, check for a Pending DTC. If there is no DTC, repeat the procedure.


This readiness code is always set to available because misfiring is continuously monitored. So there is no specific test. However, there are certain conditions that can mess up the constant monitoring

Since driving over a rough road can cause the monitor to mistake road surface for misfire, the monitoring will pause and the misfire counter will reset. The misfire monitor pauses, and the misfire counter holds at its current value, if the throttle position changes more than a predetermined value, or if driving conditions fall outside the range of any related enable criteria.


©, 2015 Rick Muscoplat

Posted on by Rick Muscoplat

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