The car is a 03 MCS at 69 Kmiles and the Service Engine Soon (SES) light came on this afternoon. First thing that comes to my mind is the gas cap. In fact I put some gas yesterday morning, drove to work and came back in the evening (65 miles total). This afternoon, before even leaving the driveway the SES light came on. Could that be the reason? Does it take that long for the SES light to come on if the gas cap was somehow loose?

I drove from home to work with the SES light on, and put some more gas, cleaned the gas cap closed it tightly. From the gas station to parking lot, I drove it about less than a mile, and the light is still on. Should the light go off once the cap is tightly closed? How long does that take (few miles)?

If the reason is not the gas cap, is it safe to drive the car with SES light on for a while (few days)? What is the most common reason for the SES light to come on besides the gas cap?

An error code needs to be read from the computer to begin the troubleshooting, right?

Thanks for any input you may have...

 

If it truly was the cap, the situation was corrected, you wouldn't see the lamp. O2 sensor failure is common...

You do need to scan to see what code was thrown. Your SA will probably agree, but if the vehicle appears to run normally, your safe to drive until you can diagnose the error. Keep in mind mileage may drop significantly as most OBD errors are related to emission sensor failiures.

 

It turned out to be the gas cap (luckily). It took 5 ignition cycles for the light to go off. Whew! I was going through multiple scenarios in my head starting with O2 sensor and other parts of the emission.
Thanks for the input.
Cheers!

 

Well... I spoke too soon; the SES light came back on again. While driving back home, I re-started the car at every red light. After about 5-6 re-starts, the light went off. Let's see if it will come back on again tomorrow.

Does this pattern fit with the SES light coming from the gas cap? I am very sure that I put it back tightly after a re-fill earlier.

Any input would be appreciated.
Cheers!

 

You would really need to hook up a OBD2 scanner to the OBD2 connector to pull the code to see what is going on.

 

I'm not a hundred percent sure but I believe that when your computer detects and error and generates a code there is a number of turn key ignition cycles that will turn off the SES light if the OBC does not detect the same error.

 

Many auto parts stores will scan you for no charge. I get mine checked at Auto Zone.

 

Hi All,

Light came on this morning on my 05 MCSa. Searching for information when I saw this post.

I stopped and checked the gas cap when the light came on. Restarted engine- no change. Drove to work- light still on. Tryed two more restarts (I think) after arriving at work- no change. Light still on when I started car to drive home. Pulled into garage at home and decided to try turning off AC, then cycled ignition, and on the final restart the light went off. Thought AC or compensator might be part of problem until I read the 5 ignition cycle discussion and realized that I probably did exactly that by accident.

Already have an appointment at dealer for next week. It will be interesting to see what happens tomorrow. I'll try and update when I the see if the light returns and/ or I get scan results.

Regards

 

Thanks for the replies. I am going to try to get a hold of an OBDII reader from somewhere (e.g., Autozone). The consensus seems to be the O2 sensor. Let's see what the code reader will come up with...

 

I had the same trouble. It came on, went off etc. and then I changed my air filter and it hasn't came on again. I bought the car with 44,000 miles and have put 10,000 on it since Oct. I bought a new filter a while back but have procrastinated about changing it. The obll reading I got was the 02 sensor so I decided to change the filter because if was bad it could cause the sensor to go off. The filter was nasty dirty and had probably never been changed. My car runs better and is getting better milege as well. This could be your problem.

 

Himmmm.... Since you mentioned the air filter, I dont think I have replaced that yet (almost 70K on the clock). I will pick one up today and "try" to do it myself. I will get a code reader shortly today and see what number will come up from the screen.
Thanks!

 

I got the code reader from Autozone, and it read 2 codes (the light came on twice). Both are the same: P0130 - O2 Sensor Circuit Malfunction (Bank1, Sensor 1).
The light went off again this morning and has not come on after ~20 miles. I checked the Bentley manual to see where these sensors are located. Unfortunately, I wont be able to do it myself :( How much is "reasonable" for O2 sensor replacements before I start calling around?
Cheers!

 

It turned out to be the O2 sensor. I picked up an air filter from the dealer. Now I have to figure out how to change it Are there any DIY instructions somewhere? I looked at the Bentley manual and have a "coarse" idea about what needs to be done. I would appreciate any tips on this? Do I need any special tools?
Thanks in advance!

 

I installed a new air filter and reset the codes. Let's see what happens now. I'll keep the code reader with me for a while and keep reading it if/when it comes on again. In the mean time, I'll call around for O2 sensor replacement.

By the way, after reading Smokey's beautiful DIY instructions on air filter replacement, I thought the most difficult part was going to be the clamp. Rather, putting the cover on without moving the new filter from its place took the longest. Actually, I don't think the original filter was put in right either. It was bent at the back edge and was cracked from few places. Needless to say, the air filter was in a very bad situation. I was going to take a picture, but I was embarrassed to do so... From now on, a new air filter every 10K for me.

I'll keep you guys/pals updated on the SES situation.
Cheers!

 

I have an '03 MC and the light was coming on and off after refueling. Brought it to dealer and they said "bad gas" and cleared the code. Used other fuels for awhile and light returned. Returned to dealer and they again suggested bad gas and possible misfire on third cylinder. Car, by the way, was running perfectly throughout. I mentioned oxygen sensors and the dealer said it was not that.

While I was at the dealer I checked the new cars and noticed the gas cap on the new cars seemed tighter than mine. Went to the parts counter and bought a new cap for $20. Service Advisor did not think it would solve the problem. One month and 2500 miles later no SES light and I am using the same gas stations I used previously.

I recommend this $20 fix for out-of-warrenty cars !!!

 

After a new air filter, 1 week, 1 tank and 340 miles later, the SES light is nowhere to be seen
I am going to return the code reader tomorrow to AutoZone.
I guess the badly clogged air filter was the reason in my case.
Thanks again for the replies/suggestions.
Cheers!