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JFN1, ASE Certified Technician
Category: GMC
Satisfied Customers: 4508
Experience:  GM Master Technician
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I have a 1999 GMC Sierra. I have driven it over 300 miles

Customer Question

I have a 1999 GMC Sierra. I have driven it over 300 miles yet the Sec Air system and Evap system Mon - monitors read incomplete. there are NO hard codes. I did recently change the knock sensors. How can I get these cleared so I can get a California smog cert
Submitted: 1 year ago.
Category: GMC
Expert:  JFN1 replied 1 year ago.

EVAP monitor will only run after a cold start. The vehicle has to be completely cold, such as sitting overnight, before the EVAP monitor will even attempt to run. Then, it only runs under certain circumstances.

The secondary air monitor also only runs under very specific conditions. I'll list them here:


  • No MAP DTCs
  • No IAT DTCs
  • No ECT DTCs
  • No TP Sensor DTCs
  • No HO2S DTCs
  • No VSS DTCs
  • No system voltage DTCs
  • No Fuel Trim DTCs
  • No Misfire DTCs
  • No CCP DTCs
  • MAF is less than 25 grams per second
  • Engine load is less than 34 percent
  • Power Enrichment Mode is not active
  • DFCO (Decel Fuel Cutoff Mode) is not active
  • Convertor Over Temperature not active
  • Engine run time after Closed Loop is more than 20 seconds
  • Air/Fuel Ratio is 14.7:1
  • Fuel Trim counts between 124–132
  • Engine speed above 550 RPM
  • ECT is between 80–107°C (176–225°F)
  • System voltage is more than 11.7 volts
  • IAT is more than 2°C (36°F)

Once all these criteria are met, the PCM software is programmed to pick an appropriate time to run the AIR pump. While GM does not tell us exactly when this occurs, I have noted it happening at low speeds, around 35 mph, at steady throttle. When the AIR pump comes on, the PCM is expecting to see oxygen sensor readings drop to near zero.

EVAP is tougher. Cold start, and normal (not aggressive) driving. The test will run about ten minutes into the drive cycle. If you shut the engine off and restart it, it won't try to test again until you have a cold soak.

It's not a matter of miles, it's a matter of getting just the right conditions. And while I don't know the CA requirements, I know that in my state and many others, you can pass a smog test with two readiness monitors that are not ready on a 1999 car or truck. Do you happen to know what the threshold is in CA?

Customer: replied 1 year ago.
Thank you for the reply. unfortunately, it does not move me forward. I have done about 50 cold starts in the morning driven the car and still nothing. And just an CA I can have a smog test on a 1999 or older vehicle with only the evap monitor inc.. if a car is 2000 or newer no incompletes. this was a new rule implemented at the beginning of 2016.Again I appreciate your response, but if you have no other thoughts please put this question back into the open form so hopefully someone has had a similar issue and can guide me home.
Expert:  JFN1 replied 1 year ago.

I understand. That's good info to know about CA emissions, I didn't know they had tightened the standards.

I have seen conditions where the PCM will not perform readiness testing due to a logic lockup. It's worth trying to resolve: disconnect both battery cables from the battery, and hold them together for several seconds. This clears volatile memory, and will cause the PCM to restart with a baseline strategy.

Other times, I have performed PCM programming, to update the software, and that seems to get the PCM going again, too.

GM has a book that outlines various readiness testing procedures for different models. I am trying to put my hands on it, but have so far been unsuccessful. Let me work on this a bit before you bail on me. I like a challenge.