Those conditions will trigger a check engine light. Most complains are due to high expectations.
One Grand Caravan customer complained of poor fuel economy without explanation. I asked the service writer for details because a 4400# box will not get excellent fuel economy. The owner claimed in the past he could drive from Las Vegas to San Diego (350 miles) and arrive with half a tank. Nothing can be done to satisfy the customer in some cases.
In my environment a 3.3L or 3.8L Grand Caravan will achieve 13 MPG in the city, 25 MPG on the highway. If you are getting 6 MPG you have a problem but a 7 MPG change from one driving condition to the next may be normal.