Fix the exhaust leak and upstream O2 sensor first — then the converter, then the brakes
If your GMC Sierra has a leaky exhaust, a P0420 code, and a brake job that's past due, don't order the brake parts first, and don't replace the catalytic converter until you've fixed the leak and tested the upstream oxygen sensor. On a typical 2014–2023 Sierra, getting that sequence right is worth roughly $500–$1,200 in avoided repeat labor (based on independent-shop quotes from January 2025; verify current pricing before you commit).
The logic is simple. A pre-catalyst exhaust leak pulls outside air into the exhaust stream. The upstream O2 sensor reads that extra air as a lean condition, the ECU adds fuel to compensate, and the unburned fuel cooks the catalytic converter. You end up replacing a $900–$1,800 converter because of an $80–$150 sensor and a $40 exhaust gasket. The cheapest part on the truck is the one that destroys the most expensive one.
Upstream vs. downstream: they do very different jobs
The two sensors on a Sierra exhaust are not interchangeable in function. The upstream sensor — the one before the converter — measures raw exhaust oxygen and drives fuel trim. Per OBD-II standards (SAE J1979), it should switch rapidly between roughly 0.1V and 0.9V once the engine is at operating temperature. The downstream sensor, sitting after the converter, is a catalyst monitor. Its job is to be boring. A healthy downstream sensor holds a relatively steady voltage, usually around 0.4–0.5V or a gentle waveform, because the converter has already cleaned up the exhaust.
When there's a leak before the converter, the upstream sensor gets fooled into thinking the engine is running lean. Fuel trims go positive. The downstream sensor then watches the converter work overtime — or fail — and eventually you get P0420. Slap a new converter on without fixing the leak and the replacement gets cooked the same way. I've watched this happen twice in a single month with two different shops that tried to save time by skipping the smoke test.
So glad the second shop I consulted pushed back on replacing the converter. I almost approved a $1,400 parts order that would've been toast inside six weeks.
When the catalytic converter actually needs replacing
Not every P0420 means a dead cat. If the upstream sensor is lazy, the leak is unmetered, or the engine has a misfire, the converter may still be fine. Check fuel trims, check for pending misfire codes, and verify downstream sensor activity before you order a converter. On a 2019–2023 Sierra, a direct-fit converter runs about $700–$1,400 in parts, plus $200–$400 in labor at most independent shops (January 2025 quotes; verify locally). If you're in California, confirm CARB compliance for the specific part number before install — a non-compliant converter is a guaranteed smog failure, and the return shipping is on you.
Then, and only then, the brake parts
Once the exhaust side is verified, the brake work is straightforward. For rear drum and hose replacements on a Sierra, Centric 123.40014 (brake drum) and Centric 150.37303 (rear brake hose) are common catalog numbers — but confirm fitment against your specific trim and model year before ordering. I've seen more wasted trips from a mismatched hose fitting than from any other single part on these trucks. It's not difficult; it's just easy to assume the part number covers every trim. It usually doesn't.
Here's the efficiency angle, and it matters more than most people admit: the reason to do exhaust first isn't just about cost. It's that a two-minute scan and a five-minute smoke test replace an hour of guessing later. Shops that run the diagnostic sequence up front finish the job in one visit. Shops that guess order parts twice — and the second time, they pay rush shipping.
Where this order doesn't apply
My experience is based on roughly 200 mid-range repair orders, mostly half-ton Sierras in the 2014–2023 range. If you're working with a 2500HD diesel, a lifted truck running aftermarket exhaust, or a vehicle that's already had a converter replaced once, the diagnostic order is similar but the specific part numbers and price ranges are not. Also, if there's any active misfire, stop — fix the misfire first. A converter installed behind a misfiring engine is a $1,200 donation.
One honest limitation: I skipped a smoke test once to save a $60 diagnostic fee. The leak was at the flange, not the gasket I'd assumed, and I ended up replacing the converter unnecessarily. Net loss, about $400 after returning the good converter and redoing the flange. That's when our shop wrote the "smoke test first, every time" rule. Not ideal, but the rule has paid for itself many times over.