# The Pre-Purchase Inspection: What Our Evidence Says to Check > The checks that actually appear in our documented failure record, the walk-away signals mechanics state flatly on camera, and the parts of a truck inspection we cannot put a price on. Updated: 2026-08-21. Source: Big Rig Risk — https://bigrigrisk.com/guides/pre-purchase-inspection-truck/ (claims cited to primary sources; see /methodology/). Two numbers frame this entire page, and both of them are real bills somebody read out on camera. A documented aftertreatment breakdown on a Mack Anthem MP8 came to $9,136.54 all in, hotel and rental car included. A documented full dealer-performed overhaul on a Volvo D13 came to $34,090.58. Those are single cases, not averages, and we are not going to pretend they are averages. But they are the scale of thing a pre-purchase inspection exists to catch before it becomes yours. What follows is not a generic checklist. It is the set of checks that show up over and over in our own failure record, drawn from the buying logic on our chassis and engine pages. Where our evidence stops, we say so. ## Start with the fault codes, because they are the cheapest information you will buy A full code pull comes first, before the walkaround and before the test drive, because it tells you where to spend the rest of the inspection. A code read is the cheapest insurance in trucking at around $150. What you are asking for is not just "any lights on." You want active codes, stored codes, and derate history. On a truck we have on video, the dash readout showed zero active codes and eighteen stored codes. That is a truck that looks clean on the dash and has been talking to itself for a long time. Heavy trucks report faults as SPN and FMI pairs rather than the automotive-style P-codes most people expect, so ask for the readout in the form the ECM actually produces it. And do not accept "I cleared those last week" as an answer. A code that was cleared without the underlying repair is a code that comes back. ## The blow-by check, and the case behind it Have the shop check for blow-by at the crankcase breather while the engine runs. This is not a formality. In our record, an owner-operator's Detroit DD16 "started blowing oil out the breather," and that symptom led directly to a full in-frame rebuild. During that teardown a cracked turbo housing turned up as an unplanned addition to the job. That is the honest shape of blow-by as a buying signal. It is one observable symptom that opened into a rebuild plus a surprise. It does not mean every engine showing blow-by is finished. It means the number in your head for that truck is now a rebuild number, not a maintenance number. ## Oil leaks are not cosmetic on these engines Our record treats leaks as a timeline indicator rather than a nuisance. On a Detroit DD15, oil leaks are described as the engine's signature issue rather than head gaskets. On the DD16, gaskets "usually leak around 500K" miles with significant oil loss past that mileage, and the rocker box lower gasket is called out as one of the major leaks shops see. On the Cummins side, an owner weighing a full ISX rebuild at roughly 800,000 miles describes his truck as having no major engine issues yet, with one exception he states plainly: the oil pan is leaking real bad. That is the pattern. The leak shows up first, the rebuild decision shows up behind it. Get under the truck with a light and find out where the oil is coming from, not just that it is there. Oil pan, rear main, rocker box and valve cover are all different conversations. ## Verify what engine is actually in the truck This sounds absurd until you see how often it matters in our own data, and it is free. A Kenworth T680 comes with either a PACCAR MX-13 or a Cummins X15, and the two trucks are identical from the curb. Our T680 records exist as two separate pages for exactly that reason: the failure evidence is thick on one and nearly empty on the other. A Volvo VNL 760 ships natively with the Volvo D13, so a VNL listed with a Cummins X15 is a fleet special order or a repower, and our own record flags that listings on that chassis get misidentified. Read the engine tag. Pull the VIN and get the build sheet. Budgeting a rebuild off the wrong engine family is the single most expensive avoidable mistake in this whole process, and the seller may genuinely not know. ## The engine-specific checks that our record supports On a Cummins ISX15 or X15, ask for proof of fuel-pump service. Cummins built the ISX15 fuel pump with ceramic plungers starting in 2010 and addressed the problem around the 2016 to 2017 window, and the pump remains the number-one complaint on this engine across the production run. Listen for rough idling, excessive smoke and extended cranking, which are the three symptoms our video sources name as the tells. On any late-model diesel, ask for aftertreatment service history and check DPF ash cleaning against the 250,000 to 400,000 mile interval. Ask what the truck did for a living too, because SCR doser valve and NOx sensor faults track excessive idling and PTO use. On a Volvo D13, our record names injector cups, fuel filter housing check valves failing mostly past 500,000 miles, turbo actuator faults, and crankcase pressure sensor diaphragm failures that throw off the oil-level reading. If the truck has an I-Shift, know that clutch service after 500,000 miles is a bundled job covering clutch, actuator and air module, not a simple clutch swap. On a Mack MP8, our buying logic says to inspect the turbo actuator, DPF condition, injector prime-loss symptoms, coolant-pipe brackets for rust cracking, starter mount hardware, and oil pan and rear main sealing. And confirm it is an MP8 and not an MP7, because our own record caveats one video specifically on that mismatch. ## The walk-away signals, stated as flatly as our sources state them Some findings are not negotiating points. Our records put these in the walk column without hedging, and the mechanic testimony behind them is specific. - Broken cylinder head bolts. One mechanic documents these recurring across multiple engines he has serviced, and says there is no way to know when one will break unless you pull it. - Spun main bearings, which our source ties directly to bearings not being replaced on schedule, and from there to broken crankshafts and head bolts. - Visible metal in the oil. - Dropped cylinder liner symptoms or flattened camshaft lobes on an ISX. - Recurring injector leak-down with no root-cause repair behind it. - An active turbo or DPF failure with no repair receipts. Our Mack record's own instruction is to walk or negotiate toward a documented repair credit of $9,000 or more, based on the sourced real-world breakdown. - A seller who will not allow an independent inspection. ## What an inspection cannot tell you, and where our evidence runs out This is the section other inspection articles do not have, and it is the reason to trust the rest of this one. Our evidence set is built from engine and drivetrain failure documentation. It contains no sourced figures for brake service, tire replacement, frame repair, or transmission overhaul on a Class 8 truck. So we are not giving you dollar thresholds for those systems. Have them inspected, absolutely, and get a written quote from the shop for anything they flag. Just do not price them off a range you found online, ours included, because we do not have one that we checked. The other thing an inspection cannot do is diagnose from a dash fault alone. A 2012 Coronado owner in our record paid what he described as almost a thousand dollars for a dealer-level diagnostic visit on a controller fault just to get everything checked out, and the transcript never states what the root cause turned out to be. Diagnosis is its own cost, separate from repair, and on an ambiguous electronic fault it is not small. ## The one question underneath all of it Every record we have written lands in the same place. Maintenance history beats odometer. A 700,000-mile truck with oil analysis, documented pump work and aftertreatment receipts is a better buy than a 450,000-mile truck with silence in the folder. So the inspection is really two inspections running in parallel. One shop looks at the truck. You look at the paperwork. If the paperwork does not exist, price the truck as though every deferred item on this page is coming due, because on an unknown-history truck that is the honest base case. Model-by-model failure records with sources: [browse the database](/brands/). Full purchase math: [the used ISX15 and X15 buying guide](/guides/used-isx15-x15-truck-buying-math/).