Diesel mules are an SOB to burp out. It can take up to 45 minutes to truly burp one out. #2, The diesel mule hates to get overheated. If, as you say, you kept driving agter the light came on, then I would go ahead and plan on replacing head gasket. 9 out 10 diesel mules that come to our shop that have been driven after the light came on, we find the head gasket craked.