Fuel induction service Thursday. Check engine 420 today. Detergent in fuel line?

2008, 160k miles, runs like a peach. Went in for oil change, replacement of failed TPMS system, and regular maintenance Thursday. Mechanic, who specializes in hybrids and who has taken care of this car in my 70k miles of ownership, recommended fuel induction service. My fuel efficiency lately has not been what I thought it should be based on the last few winters, I said let's do it.

I drove car about 40 miles, check engine light came on.

Went back to mechanic's sister location (local company, 3 locations) as they were open on Saturday and my regular location was not. The mechanic read the code, P0420, and asked if they had done a fuel induction service. When I said yes, he said that if not all of the detergent is flushed from the line, it can cause the car to pop this code and that the solution was to drive the car to vaporize the remaining detergent. I therefore drove the car. About 70 miles later, during which the car was running absolutely as normal (actually, substantially improved MPGs over what I had been getting- 50 vs. what had been 42-43, thank you winter- also, possibly thank you to the induction service) the check engine light came on again.

Now as I look up the code I'm worried about the catalytic converter.

A couple of questions here.
- Is that "detergent in the line" thing legit?
- Is that a plausible reason for the light to be on a second time?

I'm thinking that out of an abundance of caution, I need to be driving a different car to get where I need to go tomorrow. I'm taking the car in to my regular mechanic on Monday.

 

Can you expand on the monitor test? Is this looking at readings taken before and after the cleaning, or is there enough info stored that I'd be able to look at this with a post-test only?

Good to know, and thanks for reminding me how expensive it's going to be if it is the cat...

 

Yes, MD is a CARB state.

I don't check the oil as often as I should, but at least once in between changes (every 5000 mi, synthetic.) Habit from my old Civic, which wanted to drink oil like I drink coffee. I don't find it to be using more than it should.

The fuel induction service they perform is the actual "take out and clean throttle body," not just "feed it a bottle of Seafoam and call it a day."

I need the car to run effectively and pass emissions testing, which it's due for as soon as my county reopens the VEIP stations to their intended purpose instead of drive-through COVID testing sites. So... ETA unknown!

Shoot, @TMR-JWAP , you weren't kidding about the sale prices on eBay. Apparently $1400 in "not working or for parts" condition is not unreasonable.

 

In case anyone else needs this information later...

Car went back to the usual shop earlier this week and the GM looked at it this time. After additional diagnostics and an extensive road test, he agreed with the other location's assessment that it was likely detergent residue in the lines and said that in some cases he has seen it take up to 2 full fuel fill-ups to clear. He turned off the light and cleared the code. About 150 miles later it came back on. So it goes. I'll go back in if it doesn't click off after 2 more fuel cycles.

Cat's fine. O2s are fine. Drives happily. Apparently, to answer my first question, this is a thing!