My post-Thanksgiving week began with the rude discovery that some schmuck had cut the catalytic converter out of my truck. I discovered this in a rather odd way: It wasn’t because the truck—the 2008 Chevy 3500HD Duramax diesel I’ve written about before, the one I bought for a song from my former engineering job a few years back when they closed the building—suddenly had a loud exhaust. It didn’t, a fact I attribute to the presence of the turbo. It probably would’ve been deafening had there been just an open pipe coming down from the exhaust manifold.

Instead, a few days before Thanksgiving, I heard what sounded like a blower fan running, the kind of sound you hear when either the viscous-fan clutch is seized or the electric fan is on high—except it was coming from directly beneath the truck. I crawled under, and the noise seemed to be coming from the transmission. With winter approaching and the air temperature dropping, I thought that maybe the transmission was just cold and loud.

I thought I’d use the truck to run a few errands, warm it up, and see if the noise changed. It didn’t. Again I crawled under it, and again I thought the noise—not a whine but a whirring—seemed to be centralized at the transmission. Although the truck has only 29,000 miles on it, its maintenance history is unclear, so I thought that the responsible thing to do was take it into the local transmission shop that used to look at the boxes on my Suburbans for a checkup. Maybe it just needed a fluid and filter change.

So after Thanksgiving, that’s what I did. I drove the truck the two miles to the transmission shop, parked it outside, and spoke with the proprietor. He asked, “A whirring sound, not a whining sound?” I nodded. He came out, stuck his head under it, and a few seconds later stunned me by saying, “Your cat’s been cut.” It was the last thing I ever expected him to say, and I was, frankly, embarrassed that I hadn’t noticed it. But when I crawled under, I understood why I didn’t see it: The cat is—or was—up against the left side of the right frame rail; you can see this in the cover photo. Even to see the cut pipes, I needed to skooch a bit under the truck. But there it was—or wasn’t, in this case.

No no no no no.

Well, crap.

Catalytic-converter theft isn’t anything new. The platinum, palladium, and rhodium inside, used to catalyze exhaust gases, are rare and valuable. Priuses have been a particular target for years due to the hybrid vehicles having a larger amount of these metals in their cats than most other cars, and thus being worth more in the scrap-recycling market; 45 seconds with a battery-powered Sawzall can reportedly net a thief between $300 and $1,500 at a disreputable metal recycler. Moreover, due reportedly to the slowdown in mining of these metals that happened during the pandemic, the metals became scarcer, the theft rate dramatically increased, and even diesels like mine whose cats don’t contain rhodium became targets due to their high ground clearance providing easy access.

In my case, the fact that when I bought Carter (the ’76 2002 with bad rod bearings) last spring, I completely filled my driveway, jumped at an offer from a friend to sublet me a $50-a-month parking space for the truck in a lot located a mile from my house and right next to (ironically) the town recycling depot, and parked the amply-high truck there for weeks at a time without moving it, probably painted a bullseye in the shape of a catalytic converter on the back.

The truck was very visible in a very public location, and leaving it there wasn’t the smartest thing I’ve done.

I filed a police report and learned that even here in my tony suburb of Newton, Massachusetts, police are receiving several reports a day of cat theft (“Mostly Priuses,” the officer said). I then called my insurance agent, who said that unfortunately I was carrying a $2,000 deductible. I immediately knew that this meant I’d be handling the repair myself.

On any modern fuel-injected gas-powered vehicle, the catalytic converter is flanked on both sides by an oxygen sensor, allowing the car’s ECU to measure the pre-cat and post-cat exhaust mixture. When the cat gets cut, these sensors are sometimes taken with it, leaving a mess of hanging wires. Fortunately, the truck doesn’t have these complications, and the removal was fairly clean. I knew very little about the diesel system in the truck, but needed to come up to speed. It turns out that 2008 is new enough that the truck has a diesel particulate filter (DPF) behind the cat to catch the particulates and burn them off, but not new enough to use the blue diesel exhaust fluid (DEF) that folks complain about. The catalytic converter on a diesel is more properly called a diesel oxidation converter (DOC), but for the sake of clarity, I’ll keep referring to the part that was cut out as “the cat,” and the entire replaceable section—the cat and the pipes it’s welded between—as “the cat assembly.”

The cat assembly on the truck has four-inch double-walled tubing that is attached at the front to the engine with a heavy-duty band clamp, and attached at the rear to the DPF with a four-bolt flange. Since both of these adjoining sections were intact, it looked like I could either buy a new or used cat assembly and bolt it in, or find someplace to weld a new cat into the remaining pipe.

This is the front cut of the cat…

…and this the rear. The diesel particulate filter (DPF) is behind it, looking like a big center resonator.

I was in store for a rude awakening on all of these options.

Not surprisingly, a new OE cat assembly for the truck is pricey. The GM dealer list price for what I needed—a cat assembly for a 2007 to 2010 Duramax LMM—is $1,837. The discounted price from dealer websites is as low as $1,190, but there’s a $400 core charge, and I don’t have one because someone stole mine. Combined with shipping and tax, the door-to-door would be nearly $1,750. Yikes.

So, look for one used, right? I mean, that’s what I do with anything pricey in my Hack Mechanic world, right?

Not so fast, cat boy. Anything regarding emissions is wrapped in federal laws. It turns out that it’s illegal to sell or install a used catalytic converter unless it’s been certified as functioning properly, which involves testing and labeling. This doesn’t mean that you can’t find someone selling one on eBay or Craigslist or Facebook Marketplace (like R12 Freon), but it does mean that if you’re looking to pay someone reputable to fix the car for you, they may be unlikely to take the risk of procuring and installing something inexpensive, used, and uncertified.

I looked on eBay and found a fellow selling a legal recertified cat assembly for my truck (the vendor has an EPA-certified business performing this kind of recertification). The price: $700, which, with shipping and taxes, would come to nearly $850. Man, that hurts. It was, however, the only click-and-buy legal used cat assembly I could find, and it was half of what I’d pay for a new one.

The recertified cat assembly was found on eBay.

I then went full Hack and spent hours combing Craigslist and Facebook Marketplace. The illegality of selling used catalytic converters means that Craigslist ignores “catalytic” if you specify it as a search term. You can, however, search for “cat,” and it’ll hit on the partial word.

But Craigslist is a ghost of its former self; most of the action these days is on Facebook Marketplace (FBM). However, FBM’s search engine is beyond horrible. Type in “Duramax catalytic converter” and it shows you dozens of pages of hits for Duramaxes and exhausts, a tiny fraction of which are an actual Duramax catalytic converter. After searching for hours for a cat assembly for a 2007 to 2010 Duramax DMM and sending out a hail of messages, I found exactly one correct part; the others had either already been sold or weren’t compatible.

A guy way up in northern Maine had it. He was selling a full low-mileage exhaust, taken off a 2010 Duramax, for $650. Unfortunately, he wouldn’t sell just the cat, and he wouldn’t ship, so the $200 price advantage over the eBay recertified cat wasn’t worth the all-day drive up and back.

But then I found a guy in Sandwich, Massachusetts (just over the bridge onto the Cape), selling a cat assembly on Craigslist from a 2004 diesel Sierra, removed at 70,000 miles. It looked the same as the eBay photo for mine, with the clamp flange at the engine end and the four-bolt flange at the DPF end. He was asking just $145.

Sure looks the same other than a slightly-differently-shaped catalytic converter, right?

However, when I did some searching in the dealer parts database, I found that the cat assembly in my truck and this one have different part numbers. With more research, I learned that 2007 to 2010 Duramax LLMs had the DPF filter, whereas the diesel Sierra the $145 cat was from didn’t have the DPF.

But it would still fit, right? I posted the question to the Duramax Diesel forum, and no one could say definitively whether it would or wouldn’t fit. I posted all this to Facebook, and Kenworth service manager and BMW guy Justin Tarleton explained to me that the purpose of the diesel oxidation converter is to oxidize the fuel that’s injected into the exhaust so that it can burn the particulates collected in the DPF and turn it into soot, and thus even if the earlier cat assembly looks the same and bolts up, it won’t do what it’s supposed to do.

Regarding paying someone to weld in a cat, I called the two shops who’ve done custom exhaust work for me in the past. The first shop declined to give me a quote, saying that they don’t really do diesel work and didn’t know which aftermarket cat would be compatible with the DPF. The second shop said that they’d need to see it first (which is understandable), then said that it’d likely cost around $1,500, but that they wouldn’t be able to get to it for two weeks because they needed to replace the cats on 56 city snow plows, all of which had had them cut out while sitting in the same lot. Boy, misery enjoys company.

Now, on the one hand, I’ve never removed a catalytic converter from a vehicle and operated without it, and I wasn’t about to start now, but on the other hand, it drives me crazy to be put in a situation where I need to spend big money to simply regain functionality, particularly functionality that’s divorced from you as the driver, such as paying a thousand bucks to get a repair shop to get your check-engine light to go off after it comes on due to some minor evaporative-control issue. In this case, this wasn’t money that was going to be spent on something fun like a five-speed or Recaro seats; I was only in this situation because some bastard had stolen my cat. As I say over and over, there are thirteen cars, none of them get everything they need, and I make zero apologies for the decisions I make regarding keeping them running.

So I made a decision that, for the moment, I was comfortable with: I ordered a $40 flexible exhaust pipe to connect the sections vacated by the stolen cat.

Let me be absolutely clear about this: This was not a cat-bypass pipe to be permanently installed. The idea was to temporarily band-clamp the flexible pipe in place so that I’d be able to safely start, move, and drive the truck and not have exhaust spewing out directly under the passenger compartment while I continued to search for a permanent but cost-effective option, whether that was welding a cat in place or buying the correct assembly. I had zero intent on leaving it that way.

But I never installed it. 

Here’s why. First, when I looked at the butchered exhaust more carefully, I realized that although there was a rubber hanger suspending the front of the DPF canister, it was really the rigidity of the cat assembly that held the front of the DPF up, and with the cat missing, the cut pipe coming out of the front of the DPF was simply resting on a frame cross-member. You can see this in the photo above. Plus, the DPF is plumbed with hoses and pipes in order to be able to burn off the particulates if it goes into an active regeneration cycle; a flimsy flexible exhaust pipe would do nothing to secure and support the front of the DPF. The idea of driving the truck this way with this big heavy assembly bouncing around and potentially ripping out the DPF plumbing was anathema to me.

Second, although I paid little for the truck, and although it was initially a mouse-infested mess, it’s a very nice truck with only 29,000 miles on it. It deserved better than a flexible exhaust pipe, even temporarily.

Third, with winter rapidly approaching, and with me being unable to fit the truck in my garage due to its height, any exhaust repair will need to occur in the driveway, and I wanted to get on with it before temperatures plummeted and snow coated the driveway.

Fourth, I only have so much bandwidth to put into things. You can’t make a career out of solving every problem. At some point you need to just pick an option and pay up. Otherwise the time you’re burning trying to save money becomes worth more than the money. There’s no sense in owning this truck if I can’t jump in it and use it to pick up a set of wheels or seats, or tow home a car or help a family member move, and I realized that I would be hesitant to do any of those things with a flexible exhaust pipe installed as a temporary solution while I’m waiting an indeterminate amount of time for a well-priced permanent one.

Put another way, yeah, I thought about driving down to Sandwich and picking up the earlier cat assembly. The guy sounded like it had been kicking around his garage forever. He’d probably take a hundred bucks for it. It looked like it would fit. If it didn’t, I’d be out only a hundred bucks and a few hours of driving. If it did fit, it might serve the dual purpose of sealing up the exhaust as well as providing the rigidity to support the front of the DPF up off the frame crossmember. But with its age, and the fact that it wasn’t designed for a vehicle with a DPF, in reality, it would be closer to a bypass pipe than it was to the correctly-functioning part. Was that the solution I wanted for the truck? No.

So I contacted the guy on eBay selling the recertified cat assembly and threw myself at his mercy. I even ‘fessed up that his was the only click-and-buy used solution I could find, so I realized that he had no incentive to lower his price. But I asked him if he would do so anyway.

To his enormous credit, he dropped the price by $100; shipped and taxed, it came to $727 delivered to my door. It should be here by December 9. One clamp in the front and four bolts in the back, and I should have the old one off and the new one on.

The truck will now live back at my house, not in the highly-visible parking lot next to the town recycling center. I’m hoping that this alone should buy some protection against it happening again. I may also look at fabricating some sort of a plate to make the cat less accessible.

But I admit that I can’t get the last verse of one of my favorite Bob Dylan songs out of my head:
“And here I sit so patiently
Waiting to find out what price
You have to pay to get out of
Going through all these things twice.”Rob Siegel


Rob’s newest book, The Best of The Hack Mechanic, is available here on Amazon, as are his seven other books. Signed copies can be ordered directly from Rob here.



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