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Old 09-26-2019, 02:05 PM   #1
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Smog with Check Engine Light

I just had my clutch replaced and driving home I stalled it a couple of times-guess I'm not used to the feel of a silky smooth clutch. When I got home I noticed the check engine light was on so I immediately drove back to the shop and had him check it out. He put it on the diagnostic machine and it showed lots of misfires, but after checking all the sensors, he could find nothing else wrong. He told me the engine light would go out after a few starts and sent me on my way.

That leads me up to today. I need to get a smog test to renew my registration and when I took it over to the test station, the guy said that cannot test with the check engine light on, its an instant fail. He also told me I cannot just have the shop reset the light. It will still fail. So now I want to know what is the criteria that lets the computer reset the light. Is it number of starts? If so how many? Is it drive time? If so how long? or what?

I have a few weeks to get the smog test, but I don't want this to become an issue Someone must have faced this before. Please help!

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Old 09-26-2019, 02:37 PM   #2
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Your shop is full of it.
The light needs to be reset. See if the misfire codes come back.
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Old 09-26-2019, 02:54 PM   #3
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I had a really similar experience! Driving home after having the clutch replaced, the CEL came on. It was a bad O2 sensor. I always wondered if it had taken a hard knock during the removal of the transaxle.

In most states, if they do the smog check by looking at the OBD, you'll fail if a code has been recently reset. You have to drive the car a certain number if cycles and miles before the OBD will check out good.
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Old 09-26-2019, 04:57 PM   #4
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Originally Posted by Zibodo View Post
I just had my clutch replaced and driving home I stalled it a couple of times-guess I'm not used to the feel of a silky smooth clutch. When I got home I noticed the check engine light was on so I immediately drove back to the shop and had him check it out. He put it on the diagnostic machine and it showed lots of misfires, but after checking all the sensors, he could find nothing else wrong. He told me the engine light would go out after a few starts and sent me on my way.

That leads me up to today. I need to get a smog test to renew my registration and when I took it over to the test station, the guy said that cannot test with the check engine light on, its an instant fail. He also told me I cannot just have the shop reset the light. It will still fail. So now I want to know what is the criteria that lets the computer reset the light. Is it number of starts? If so how many? Is it drive time? If so how long? or what?

I have a few weeks to get the smog test, but I don't want this to become an issue Someone must have faced this before. Please help!
Maybe stalling your car caused the ECM to think there's a misfire. If there were actual misfires you'd most likely notice it while driving, especially putting the engine under load.

Not sure about Porsches, but on most cars if the problem doesn't persist the check engine light will go out after about 4 drive cycles that the ECM does not detect any problems. If your mechanic said there's nothing wrong, the light should go out by itself.

The bad thing about clearing the code is that the computer will then have to run all it's diagnostics, i.e. Catalyst, EVAP, etc. before it's ready for the smog test. Usually that takes a longer time than just letting the light go out by itself. Again, if the initial problem does not persist.

Last edited by piper6909; 09-26-2019 at 05:00 PM.
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Old 09-27-2019, 08:40 AM   #5
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It would help to know the actual code numbers. Take it to Autozone and have them read and then clear/reset the CEL. They should do it for free. You then need to follow the attached drive cycle to ensure that the vehicle is ready. My OBDII reader shows whether all of the monitors are set and ready for testing. I don't remember where I found these instructions. I have followed it several times successfully.


Do you know what monitors are showing as not ready? If you have an OBDII reader you can plug it in and it will show you what monitors are not ready. I had a problem with my Oxygen sensor monitor and followed this drive cycle and it cleared..GOOD LUCK!


Below we outline the driving pattern you'll need to perform in order to get your Porsche's OBD-II emission monitors to become READY or COMPLETE. Perform the entire drive cycle. Completing only one portion of the drive cycle will not make the particular monitor which you're trying to make READY complete. The entire drive cycle must be performed every time you attempt the set any of the monitors.

Porsche 911 Drive Cycle - OBDII Emission Monitor Reset Procedure

Prior to starting the driving cycle ensure your Porsche's fuel tank is between 1/4 and 3/4 full. Also do not exceed 3,000 rpm or 60 mph during any portion of the drive cycle. Take extra caution when perform the Porsche drive cycle on public roadways. if you think you will be unable to safely perform the drive cycle please seek assistance from an experienced Porsche mechanic or smog check repair center.

A. Start your Porsche 911 and let it idle for approximately 2 1/2 minutes. This will allow the ECU to diagnose oxygen sensor aging; meaning the computer will be monitoring the oxygen sensor warm-up period and reaction to rising exhaust temperatures. During this period the ECU is also testing the Secondary Air Injection system.

B. Begin driving. Accelerate to 20-30 MPH and maintain your speed for 3 1/2 minutes. The ECU will be testing catalytic converter efficiency.

C. Next, accelerate to 40-60 MPH. Once again maintain steady speed, this time for approximately 15 minutes. The EVAP system is now being tested along with adaptation range (2) and the oxygen sensor (switching).

D. Decelerate and come to a complete stop. If equipped with a manual transmission, idle in neutral for 3 minutes. If equipped with an automatic Tiptronic transmission, idle in drive (D) for 3 minutes. Adaptation range (2) is now being tested.

E. Repeat steps B through D. Drive cycle complete.

I followed the instructions provided by the San Diego PCA Tech Advisor and it worked

Drive in 10 minute cycles 1400-2800 Rpm in 5-6 gear as slow as you can drive in light traffic.
If you stop or go out of RPM range the cycle resets the timer.
After 5-6 cycles you will probably have it.
For evap you must have 1/3-2/3 of a tank of fuel.
Start the car cold and let it idle for 10 minutes with lots of accessories on. (lights, A/C or heater)
Do the same at the end of the drive cycle.

It took 3 cycles of slow freeway driving to get it reset
I drove in 6th at around 58 MPH
People were looking at me oddly driving a Porsche so slow!

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