My son has a 2001 S40 and says that the "check engine" light will periodically come on and then go off. He says that it drives OK, with the exception that once it took a while to crank over. He also says that it seems to happen more frequently on hot days. He brought the car over and I connected the OBDII reader to it, but no codes were there. I left the reader with him, but the check engine light only came on once since, and it wasn't on long enough for him to do a scan. What can cause the "check engine" light to come on and off like this?

 

It could be misfiring or have a coolant temp sensor going bad.

 

I think the coolant temp sensor or thermostat may be acting up. Tech knows better than me but I would definetelybe ready to change those parts soon.

 

Another reason for the light to come on is due to a LOOSE GAS CAP.
My daughter purchased one of those "gas caps for dummies" which self-closes.
Well, after 2 months it kind of broke (without us realizing it) that's when the "check engine" light
came on, took it to Advantage Auto track and they pulled a code related to emissions sensor.
I just had it at the Volvo dealer a jef days earlier so I knew it was not a serious error.
In fueling up a few days later, I've noticed the broken cap. Bought a new one, filled the tank up,
but it took about a day or two for the light to out!

 

Thanks for the response. Can you tell me where the coolent temp sensor is located, and is it difficult to replace? Would an error code be generated for this fault? My son has the car away at school, so I can't look for it.

 

The sensor is in the thermostat housing right under the thermostat.
It is easiest to replace the thermostat and sensor at the same time.

 

Can you tell me if I have to drain any of the coolant before I replace both parts? Also, can you be a little more specific where the thermostat housing is physically located. I don't have the car here - I have to relay the message to my son at school. Thanks!

 

Here is a picture of an engine with the intake off and the thermostat and housing is on the left side.
If he follows the upper radiator hose on his left side(standing with the hood open looking at the engine) That leads to the thermostat housing.
He will need a T-40 torxes I believe it is to remove the screws.

 

Tech, thanks for all the help.

 

I went up to my son's school this weekend to replace the thermostat and coolant sensor. Of course they sent me the wrong sensor so I couldn't replace that. I did replace the thermostat, but the engine light still stays on. It never goes out. When I connect the OBDII reader to it, though, there are no error codes, and I can't reset the "check engine" light. If the sensor was bad, would I get error codes, and are there conditions when the "check engine" light is on and no error codes would be recorded?

 

If the Check Engine light is on, there should be a code stored. Even if the light goes off after the light comes on, a code is stored. Maybe the reader you are using has an incompatibility issue. If you can, try another reader.

B

 

I would think it is more of an scanner problem.
Hard start? check the regulator.

 

You sure it's not the Service light? The Check Engine light is close to it and might get confused. The Service light comes on every 15k miles and stays on for a couple minutes at every start. If the CEL is on there should be a code. (The Service light can be turned off very easy)