Question about 2008 Chevrolet Cobalt

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System monitor - clearing DTCs -

How many mile or starts before all sensors are ready for Smog Check.

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  • avenger09546 Apr 20, 2009

    As a clarification, I must pass smog, and I am constantly resetting my check engine lights because my thermostat is stuck and allows coolant to flow constantly, giving me a check engine light but causes no problems other than a lower coolant operating temperature which is nice.

    Like I said I must pass smog, however because I reset my DTC's, none of my vital system sensors....o2, catalytic, purge and so on are ready, and I want to know how many startup cycles or how many miles I must drive until they're all ready for smog check again....

  • gerry bissi May 11, 2010

    If you have your check engine light on, just disconnect the battery for an hour. Then drive the car so it relearns your car again. It might run rough for a while.

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It's not a matter of how many miles, it's more "how" you drive. If you do not have an engine light on now, whatever you do, DON'T disconnect the battery. That will trip all your readiness monitors and make it more difficult to complete the drive cycle. If you have a code set now and the engine light is on, it would be best to get the car scanned and find out how to repair the problem or you are going to waste a lot of time and money driving around and going to the test to get rejected. If there is no light on but you have made a repair and cleared the codes, this is how to drive the car to reset the readiness monitors.

Performing a GM OBDII Driving cycle:

  1. Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 122°F (50°C) and within 11°F (6°C) of the ambient air temperature at startup.
    Do not leave the key in prior to the cold start or the heated oxygen sensor diagnostic may not run.
  2. Idle. The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge "No Flow", Misfire, and - if closed loop is achieved- Fuel Trim.
  3. Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 55mph (88km/hr) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
  4. Hold Steady Speed. Hold a steady speed of 55mph (88km/hr) for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.
  5. Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch. It is important to let the vehicle coast along gradually slowing down to 20 mph (32km/hr). During this time the EGR, Purge and Fuel Trim diagnostics will be performed.
  6. Accelerate. Accelerate at 3/4 throttle until 55-60mph (88-96 km/hr). This will perform the same diagnostics as in step 3.
  7. Hold Steady Speed. Hold a steady speed of 55mph (88km/hr) for five minutes. During this time, in addition to the diagnostics performed in step 4, the catalyst monitor diagnostics will be performed. If the catalyst is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst.
  8. Decelerate. This will perform the same diagnostics as in step 5. Again, don't press the clutch or brakes or shift gears.

Posted on Apr 20, 2009

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