A leaky mass airflow sensor or it can be a leak in the air intake hose.
why does my vr commodore idle rough when at complete stop
go to your local Volvo dealer, there is a recall or fix for that at no charge!
need to get the security code from a licensed Volvo repair. code is by VIN.
Fix the problem and the light will go out. Normally on Volvo 850's the ABS sensor goes out (actually a wire comes loose inside the unit) this sensor is under the hood on the ABS unit.
IACV - Idle Air Control Valve Your car is probably experiencing rough idle.
rough idlingTry cleaning the throttle body with throttle cleaner and tooth brush. Remove the cover first. Soot builds up in this area causing the flap to stick resulting in rough idle. I have cleaned mine to new condition and I still have a rough idle. The dealer told me these motors never idled very smooth in gear.
You can start with the 02 sensor. if that does not fix the problem then you may have to consider the ECU.
Check for broken motor mounts.
it might be your egr valve. there is most likely a little piece of crud or carbon stopping the valve from closing all the way this causes a rough idle. 20 minute fix at a shop.
The only way to fix the Volvo-503 code is to take it to the dealership and have them fix it for you.
the relay needs to be replaced. you will find it behind the hazard switch (attached to the switch - a long skinny thing) I looked forever till someone told me where it was. Even the Volvo parts guy didn't know and Lawrence Volvo of Walnut Creek, CA would not tell me...
Rough idle can often be attributed to a vacuum leak. I would inspect the lines and check under the vehicle for any unattached hoses (or hose nipples) Perhaps all the vacuum lines werent connected or a hose cut inadvertently. Hope this helps.