I'm up in NJ on another trip from FL, put about 1250 miles on the car since I left, averaged 30.5 mpg on the last leg from NC to NJ. The car runs perfectly, but today all of a sudden the Check Engine Light goes on. I checked the gas cap and it just spins and clicks as usual but doesn't lock on to the filler neck, so I decided to stop into a large Chevy dealership that I used to know near my daughter's house and buy a new gas cap. $20, and it says on the cap Made in Germany. So much for US products.
Anyway, the nice guy at the parts dept. tells me I need to get the Service Dept to reset the computer to get rid of that Check Engine Light message on the dashboard. I stopped there on the way out and the service guy says they have to interrogate the car to see why that message came on and to reset to remove the message if it doesn't go away by itself after a few days with the new cap. So when I get back in the car, I pull the codes, remove/reset them, (only one popped up) and guess what?, the Check Engine Light is now out. That's exactly what they were going to do, that's how you turn off that message, except they probably charge $80-100 for the "diagnostic scan" and to reset the computer. Do they really think all of us are that stupid?
If you get that message, pull the codes to see what's going on, reset them and the message is also cleared. It's usually from some emissions control and most likely from a loose gas cap or one that wasn't put back on. I had the same issue with a 2005 Saturn Vue, replaced the cap and all was good. In the present case,my cap is probably original, is 16 years old and the rubber seal inside is dried out and shrunken and a little bit cracked so it probably wasn't sealing tight enough and that sets the alarms off.
Always remember, before panicking and paying some stealership big bucks, pull the codes and fix it yourself.

 

Good for you for correcting this yourself!

But understand that the ability to read codes and reset them went away on the C6 and C7. You need to have a code reader like an MDS tool or on the older cars, a TechII tool. Many other cars never offered the ability for the knowledgeable owner to address this situation on his own.

Cut them a little slack - some of the techs working in the dealerships today were in diapers when the C5 first came out!

 

I don't think its an uncommon problem with a lot of modern cars, had the exact same issue with a minivan, changed cap at a buddy who owns a repair shop suggested, problem gone. Best wishes!

 

Nice... no stealing from the stealership with you...thanks for sharing your experience and solution...

 

Good that you fixed it yourself and saved money in the process.

 

To be honest with you, it didn't take much discovery to figure out the gas cap was bad - it would turn and click but not lock on to the filler neck and was loose, so it was worn out and needed replacement because it wouldn't seal properly. That was a no-brainer. After I replaced it and started the car the message did not spontaneously go away so that's when I pulled the codes, found one and reset it. I really didn't look at what it was because resetting that code solved the problem and the message was gone. Had resetting that code not taken care of the issue, I would have pulled it again and researched what it was when I got home.
My point in starting this thread was to give some info on what that Check Engine light refers to, usually something in the engine emissions system and most often is related to the gas cap. Also, READ THE MANUAL that comes with the car. Or, you can just give the stealership your money if you think that's easier.

 

BTW, that dealership had 5 brand new C7's right out front ranging from the lowest to the highest in price and power, and they are truly gorgeous when you see them in person standing right next to them. Very tempting, but big bucks.

 

My 05 blazer actually has a light in the dash for gas cap.

 

Theoretically you are absolutely right but if resetting the code didn't work to reset the Check Engine light and there was some kind of other problem, the code would have popped up again and then I could have dealt with it. Since I have been driving the car since then and no codes appear, I am certain the problem was resolved with resetting whatever code that was.

 

I have a handheld code scanner. While the C5s have superb internal diagnostic capabilities, it's pretty much one of the ONLY cars that does.

Between my GF and myself, we have 7 cars. The scanner is worth it's weight in gold to me. I keep the ODB-II reader in my DD Benz, as I tend to go through cars quite often. My GF is looking for a used Benz SUV, and I'm going to look at one tonight on the way home. Guess what I'll be hooking up as part of my pre-sales inspection. I don't buy an (OBD-II capable) vehicle without doing so.

FWIW, I have a Mercury Racing scanner for my boat, and even my Harley has inherent code reading like the C5 does. Those computers are in there to communicate with us.

Also, the DiabloSport tuner; is also a code scanner and resetting tool. I keep that in my DD truck.

 

I was going to get my 99 C5 smogged in California. I previously had a check engine light so I cleared all codes before arriving at the shop and the tech did me a favor to check DTC's with his OBD scanner. He said that there were still some codes appearing and that I would have to drive the car to have them cycled through. I would fail the smog test. I assume that the car detects certain conditions and either throws a code or then resolves them. Could I reset them with an ODB scanner and associated app to avoid driving and "cycling" the codes? I see that there are inexpensive models with an app that provide some capabilities. Thanks in advance

 

Get a c-7, they don't have a gas cap.,,,,,LOL

 

I had to go through this for my inspection last weekend. After clearing codes you can put it through a drive cycle yourself by doing the following:


Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 50°C (122°F) and within 6°C (11°F) of the ambient air temperature at startup. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.

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.

Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 88km/hr (55mph) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.

Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.

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 32km/hr (20 mph). During this time the EGR, Purge and Fuel Trim diagnostics will be performed.

Accelerate. Accelerate at 3/4 throttle until 88-96 km/hr (55-60mph). This will perform the same diagnostics as in step 3.
Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) 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.

Decelerate. This will perform the same diagnostics as in step 5. Again, don't press the clutch or brakes or shift gears.

 

I just had a check engine light on my DD with no performance issues. Thinking it must the the legendary gas cap too. I bought a scanner on the way home from work. It was a P0128 code translating into a coolant thermostat failure. Surprise surprise!!! Buy a OBD scanner and be confident you have the correct diagnosis IMHO! Best $pent. Good luck on your suspected diagnosis...till next time.

 

The scanner I bought (OBD2 Innova 3100i) will upload vehicle specific codes to memory and display if triggered by problems. It also indicates that generic or manufactured specific codes are displayed, So will work with a C5.