Every buying guide in this niche tells you to pull the codes. Almost none of them tell you what comes back, and the ones that do usually get it wrong, because they are describing a car.
A $150 code read is the cheapest insurance in trucking. It is worth understanding what you are buying for that money.
Truck codes are not car codes
If you have ever had a check engine light on a passenger vehicle, you know the format: a P followed by four digits, P0401 and the like. Heavy trucks running on the J1939 network report faults differently. What comes back is a pair of numbers, an SPN and an FMI.
SPN stands for suspect parameter number, which identifies what the ECM is complaining about: a particular sensor, circuit or subsystem. FMI stands for failure mode identifier, which describes how it is failing: reading high, reading low, no signal, out of range. The pair together is the fault. One number tells you where, the other tells you what kind.
This matters practically, not just semantically. If you go looking up a truck's fault as though it were an automotive P-code, you will find nothing or you will find the wrong thing. Ask the shop for the readout in the form the ECM produced it.
What a real readout looked like
Our video record includes an owner walking through the code check on an International ProStar dash, no scanner involved. On that truck the procedure runs through the cruise control buttons: key on, press the on and resume buttons once, and the gauges sweep before the codes come up in the small display.
What his truck reported: zero active codes, and a total of eighteen stored codes, which then read out one at a time as SPN and FMI pairs. His advice on what to do with them is exactly right and exactly as unglamorous as it sounds. Write them down and look them up.
Note that this button sequence is specific to that dash. Other makes have their own procedure or need a scanner. But the shape of the answer is the same across the class: a count of active faults, a count of stored faults, and a list.
Zero active and eighteen stored is the whole lesson
Sit with that readout for a second, because it is the single most useful thing on this page.
That truck's check engine light was off. By the only test most buyers apply, it passed. And it was carrying eighteen stored faults, which is the ECM's memory of things that went wrong and were never resolved, or were resolved by somebody who never cleared the log, or resolved themselves and came back.
Active codes tell you what is failing now. Stored codes tell you what this truck has been doing with its life. The second list is the one the seller is not showing you, and it is the one you are paying $150 to see.
A truck with 400,000 miles and no codes at all in either column is not necessarily good news either. Either the maintenance history is genuinely spotless, or the log was cleared shortly before you arrived. Ask for the documentation that would distinguish those two, and notice whether it exists.
The case where the code was worse than the problem
Here is the documented example that should keep you from over-reading a readout, and it is one of the strongest single pieces of evidence in our set.
An International LT with an A26 came into a shop with a multi-cylinder misfire. That is exactly the fault description that makes a buyer walk. Multiple cylinders sounds like the bottom end, or the head, or something with a five-figure number attached.
Diagnosing it required pulling the valve cover and disconnecting the EGR valve just for access, and the technician noted that removing the doghouse for full access would add another couple hours of work on top. The root cause, once they got there, was a single failed number one injector shorting out the entire injector circuit. One part, presenting across multiple cylinders.
Our own record's buying logic draws the conclusion directly: a multi-cylinder fault code alone does not justify a walk-away without a proper diagnostic. The code told the truth about the symptom and lied about the scale.
What the codes should send you looking for
Codes are a direction, not a diagnosis. Here is where our failure record says the common ones point.
Aftertreatment and derate history. This is the first thing to ask for by name. EGR, DPF and SCR faults dominate the complaint record on every late-model diesel we have documented. A history of derating on aftertreatment codes tells you the stack has been struggling. What it costs to fix varies enormously, and the one complete aftertreatment repair invoice in our record, a turbo, actuator and DPF event on a Mack MP8, came to $9,136.54 including four nights of hotel.
EGR. The valve sticks from carbon buildup, which our sources tie directly to idling. A Detroit DD15 EGR valve, part A4701402108, listed at $1,350.00 on a 2026-08-19 price check, and a mechanic on camera puts the job at roughly $1,000 or more in parts plus two or more hours of labor. We could not source an EGR cooler price from any source in the set, so if a cooler is implicated, get a written quote rather than trusting a number.
DPF. Ask when the filter was last ash-cleaned against the 250,000 to 400,000 mile interval. The only DPF figure we own is $312.50 to send a filter out for cleaning and get it back. We do not have a sourced replacement price and are not going to invent one.
Fuel system on a Cummins ISX15 or X15. The fuel pump is the number-one complaint on this engine across its production run. Ceramic plungers started in 2010 and the problem was addressed around the 2016 to 2017 window, though a mechanic in our record says on camera that pump complaints kept coming through 2021, which is the period that is supposed to be the fixed one. We have not confirmed the recall scope, so we are not telling you a post-2017 build is safe. Rough idling, excessive smoke and extended cranking are the symptoms our sources name.
Injectors. Around $1,000 each on the Cummins ISX and X15, quoted on camera. One mechanic's framing is worth repeating: he does not call injector failure a defect, he calls it the price you pay for doing business. Expensive wear, not a design flaw. Multiple injector codes still change your number.
Turbo actuator faults on a Volvo D13. Called out in our record as a frequent fault on these trucks, alongside injector cups and fuel filter housing check valves that mostly fail past 500,000 miles.
What a code can never tell you
A DPF code could be a clogged filter, a failed regeneration, or a bad pressure sensor. A fuel pressure code could be the pump, the regulator, a filter or a sensor. The A26 case above is the same lesson with a bigger number attached.
So the code pull is step one and it is diagnostic triage, not diagnosis. It narrows where the rest of the inspection looks. And be aware that the narrowing itself has a price: a 2012 Coronado owner in our record describes paying almost a thousand dollars for a dealer-level diagnostic visit on a dash controller fault just to get everything checked out, and the transcript never says what the cause turned out to be.
How to use what you find
You will hear that codes do not mean anything, or that they were cleared last week. Neither is a reason to skip this. A code cleared without the repair behind it comes back.
The useful move is not a percentage discount off the asking price. It is to take the specific systems the codes flag, get the shop to quote those specific repairs in writing, and negotiate against the quote. That is why the sourced figures above matter more than a made-up range: $1,350 for a documented EGR valve, $312.50 for a DPF cleaning, roughly $1,000 an injector, $9,136.54 for one complete real-world aftertreatment breakdown. Those are numbers you can hold up in a conversation.
And when the codes point at something our record cannot price, say so out loud in the negotiation. "Neither of us knows what this costs" is a legitimate position, and it usually favors the person who is willing to walk.
Engine-specific failure records and the evidence behind these figures: the Cascadia ISX15, T680 X15 and Mack Anthem MP8 records. Full purchase math: the used ISX15 and X15 buying guide.