Hello!!
I bought a used 2004 SLS in jan 2010 w/30k miles. The car was great up until 44k. Check engine light came on. Took to dealer and they said a bad cat. They replaced the cat under warranty..but said it was "ok to drive,but you will get a little decreased efficiency". I drove it that weekend....and had to turn off ac as temp started to climb. Not close to the "H" mark. Mileage decreased from 24 to 17. Caddy dealer replaced cat and entire exhaust free. Mileage is now 19. They checked out everything they could with computer..all parameters are within spec. Check coolant for head gasket problem and that checked out good also. I took it to a recommended knock off dealer yesterday, and he told me the parameters were perfect. I love this car, but do not want 19, especially after getting 24. The car is not missing. There is a reason the cat went out, and i do not think it is a coincidence that the mileage decreased same time as bad cat. I asked dealer and knock off about a "rich condition" dumping too much fuel, stuck open injector or bad pump (good pressure, but low flow) and they discounted these due to mapping, etc. Sensors are responding ok. I asked about putting an ammeter on pump to check current draw and he said he didnt have that tool, and caddy dealer though it was a waste of time. Besides the 19 mpg, it may be a bit sluggish..but that is difficult to tell. I was getting 395 miles each tank it is now 325. I hope that we can solve this. THANKS!! and have an enjoyable weekend. Walt

 

Unfortuneately it is not simple, seeing what scanner reads is important. 24 mpg is highway or combined? Generally replacing cat should not have negetive effect on economy.

 

Th mileage dropped when the 1st cat was bad....and never recovered. The 24 is combined,,,,,but I drive 90% on I 10 with no stopping. THANKS!!....i just re calked...23 not 24 combined.