Hey guys, I'm having a bit of an issue with my S40 that I bought a couple weeks ago. So a little background, I bought the car with 135k miles on it and it runs pretty great. I put some fuel system cleaner through it and after a few days the check engine light came on.

I took it to AutoZone and they ran the code for me. It came up P0128 and the guy told me I probably just needed some coolant. I checked the coolant and it was at the minimum line on the reservoir so I filled it up. The engine light is still on after driving the car maybe 30-40 minutes after filling it up. My first question is how long should it take for the check engine light to turn off after I put coolant in?

After not seeing the engine light go off, I looked up the code myself and saw that it could be a number of things. One common issue, I've read, is a faulty thermostat that needs replacing. My second question is have any of you had experience with this particular engine code and some of the possible fixes?

Upon further reading, I discovered that this code could be an indicator of a much bigger problem. A blown head gasket. I really hope this isn't the issue but I'm sort of worried it might be. My friend, who is much better versed in car maintenance than I am, told me that sometimes the fuel system and component cleaners can dislodge some gunk that might be plugging a leak for you and if the gunk is suddenly cleaned out some bigger problems can crop up such as gasket issues.

I have noticed that there is some whitish smoke coming from my exhaust. Perhaps no more than normal? I don't know really know how much if at all you're supposed to see fumes coming from the exhaust. My third question is, should I be concerned if I see any fumes at all coming from my exhaust? Should fumes be invisible when the car is idling?

My fourth question is, are there any other things I should be aware of or is there anything else I should be doing? My car isn't overheating and it's idling smoothly. It seems to be accelerating just fine and I've heard that head gasket leaks can cut into engine performance because of a loss of compression. Any thoughts? Also, I should add, the car is not overheating which is usually a symptom of a gasket leak.

Thanks in advance for your responses.

 

For me it was a bad thermostat. Nothing more. The check engine light will eventually disappear after replacing the t-stat but it's much easier to simply erase the code with a code scanner tool.

I wouldn't worry about a blown head gasket given your description. Just replace the t-stat and erase the code.