Fuel Mileage Dropped

[gmtofd]

At about 123K miles the highway fuel mileage is dropping from 20-21 down to 16 mpg. There was a PO420 code, (bank #1 cat system efficiency below threshold). The CEL has since shut off. The garage that is used on occasion offers a cat converter solvent clean service. Going to have them go at it. When we first brought the Saab, I've done the spark plug and air filter change. Runs fine, as mentioned, no codes pertaining to cylinder misfiring or any other PO codes. Does anyone have any thoughts?

Thanks for any feedback.

 

[Mooseman]

I think your cat is clogged, which would go with the code and MPG loss. Not too sure any kind of solvent can clean it out as there have been attempts done on YouTube and such to no avail.

 

[Mounce]

Yeah I wouldn't trust a "cat cleaning" sold by a shop. Just an upsell and honestly the first time I've heard of it and I work on the industry for a big company in the South East. If it was a thing worthwhile we'd have it.

 

[Kelly@PCMofNC]

Yeah, what they said...the cat cleaning deal is not going to work, it likely just needs to be replaced. A clogged cat won't only hurt mileage but it can possibly cause other damage as well, so it is something I would attend to sooner than later (even if the light is still coming and going).

 

[shovenose]

Instead of paying the shop big bucks to pour some chemical in the tank just try Cataclean or CRC Guaranteed to Pass both of which are readily available at your local auto parts store.

 

[gmtofd]

Thank you all for your responses. There was a history on the truck's VIN of a air valve replacement and wondering if before it was changed, if it, (the air valve) was making the fuel curve excessively rich, tripping the PO420 code?

 

[Mooseman]

Depends on which valve. If it's one that could make it run rich, it could kill the cat. P0420 just tells you the cat isn't working properly. Before replacing it, I would try to check that the engine is running right, especially the thermostat and fuel trims.

 

[gmtofd]

Sorry, the vehicle history indicated a throttle body replacement.

 

[Kelly@PCMofNC]

Thank you all for your responses. There was a history on the truck's VIN of a air valve replacement and wondering if before it was changed, if it, (the air valve) was making the fuel curve excessively rich, tripping the PO420 code?

No, I would say that is very unlikely. P0420 almost always means a failing cat.

 

[gmtofd]

The other GMT360 in the family, a '04 TB I6 with 181 miles has never had any such P0420 problems. That mtr uses zero oil consumption between it's 6k miles oil changes with Mobil 1. The Saab 5.3 Saab will use about a 1qt between changes. The observation is that oil consumption is a factor in the cats getting clogged tripping the P0420 code. Anyway, the solvent cleaning service was used and the wife claimed an immediate increase in fuel mileage, (so she says). The guy at the shop says the solvent used comes from Reaction Chemical, it's part no is CR100, although, I'm unable to find anything on the web about them.