This evening, while I was on my way to home, check engine light came on and stayed on thereafter. Here is the list recently done on the car: transmission fluid flushed two weeks ago by dealer, replaced air filter by myself one week ago, and two weeks ago, I had used 87 octane gas, but emptied that and refilled with premium twice. I am pretty sure gas cap is tight. what could trigger that light on?

Your adivce will be greatly appreciated.

 

A check engine light could go on as a result of many things, most commonly knock sensor or oxygen sensors, but there are many others as well. Your best bet is to check the codes, either manually or by having a mechanic check it for you with a machine (some will do it for free, as do some stores like AutoZone, but others charge around $40).

If you want to do it yourself, go to the 4th Gen forum and look for the sticky that says, "Help! Check engine light is on!" or something to that effect. That will give you a pretty good idea of how to go about it.

Good luck!

 

Today, I used OBD scanner and got codes p0136 and p0325 (2O sensor and knocker sensor). Could that be caused by low octane gas used two weeks ago? I'd like to wait for a couple weeks before taking further action. Any suggestion?

1. reset ECU

2. if CEL comes back replace o2 sensor first, sometimes a knock sensor code comes along with other codes even if the knock sensor is fine

3. If knock sensor returns after o2 sensors are replaced then replace knock sensor

 

It is possible that the knock sensor code came with another code, in this case the O2 sensor. The O2 sensor is cheaper anyway so you might as well replace that first and make sure the knock sensor code is really there. My car saw only low octane gas for a long time and the check engine light never came on. In fact, it only came on a couple of months ago even when I was using higher octane gas (same codes as you) but I didn't have them replaced and it hasn't been on for a few weeks now.

Edit: The only thing your O2 sensor is really needed for is your emissions test when you get an inspection, and at least in MA (don't know about your state) the emissions test is only every 2 years so unless you're coming up on a test or you reregister the vehicle, it's not necessary to replace it right away.

 

You're right to just reset the codes and wait. That happened to me on cheap gas and the light never came back on after I reset.

 

If it comes back, you need replace the o2 sensors and that knock sensor. Leaving this alone will KILL your gas milage and horse power. Even a bad KS by itself will result in 20 less whp and horrible fuel economy!