As far as your coupon question, we'd think the coupon is only valid on weekends. The smog station may be advertising a "weekend only" coupon in an effort to get customers in on the slow weekend days. Call the smog station to verify this.
As far as the fuel additive you are using in your Toyota Sienna, it all depends on the type of fuel additive you are using and whether the manufacturer of the additive says it is safe to use during a smog check. You might want to contact them or at least review their website for details.
Typically it is not suggested to smog test a vehicle with fuel additive in the gas tank. If you've added engine cleaner fuel additive to your Toyota Sienna we recommend you deplete the fuel, and fill 1/4 to 3/4 tank full of fresh fuel before the smog inspection. Also ensure your drive your Toyota Sienna at least 20 minutes prior to the smog test to ensure it is properly warmed up.
