Truck unit diagnostics before purchase: which faults are critical

A report with a list of codes does not yet show how much preparing a truck for operation will cost. Proper truck diagnostics before purchase must distinguish an old record in memory from an active fault, confirm it with parameters and conclude with a re-scan after a test drive.

What should be included in the report

For a complete inspection, you need to obtain a report with the following data:

* VIN, mileage, engine hours, inspection date and battery voltage; * list of all scanned modules: engine, transmission, EBS/ABS, SCR, DPF, retarder, cabin; * full code, description text, status — active or stored, number of occurrences; * mileage or engine hours of the first and last occurrence, if the module records them; * freeze frame and actual parameters: pressure, temperature, corrections, actuator positions; * results of actuator tests, calibrations and re-scanning.

The software must be able to read the manufacturer’s proprietary parameters. A universal scanner can read the code but will not show clutch life, regeneration history or the conditions under which the fault occurred.

Before connecting the scanner, check the condition of the vehicle using the 30-minute truck inspection checklist: electronics will not reveal a frame crack, kingpin play, oil leaks or uneven tire wear.

Engine and fuel system

On a cold engine, shaft synchronization, actual and target fuel pressure, and starting performance are checked. After warm-up, injector corrections, boost pressure, fluid temperatures, EGR position and response under load are checked.

Critical signals include active oil pressure, overheating, synchronization or fuel pressure faults; significant deviation of actual values; and codes that return after driving. An injector code can also indicate a problem with the wiring, compression or fuel line. The repair budget is determined after testing, not by the fault name alone.

Transmission and clutch

For automated transmissions, read actuator, sensor, control unit and CAN errors. Check clutch life or position, calibration, air pressure and temperature. During a test drive, check forward and reverse movement, all gears and hill starts.

Deal-breaker signs include incomplete calibration, spontaneous transmission disengagement, clutch slippage, or an active actuator error that returns after testing. Features of AS-Tronic, TipMatic and other gearboxes are detailed in the comparison truck with automatic vs. manual.

EBS/ABS and pneumatics

A wheel speed sensor code can indicate a problem with the sensor, cable, connector or pulse ring, so the signals must be checked while driving. For EBS, check the modulators, pressure, power supply, CAN communication, air build-up time and pressure drop after the engine is switched off.

Active errors in the modulator, EBS unit, pressure system or communication with the semi-trailer pose a safety risk. EU vehicle safety requirements also apply to heavy trucks. Scanning should be supplemented by checking the system for leaks and testing braking forces.

SCR, AdBlue, DPF

For the SCR system, check NOx levels before and after the catalyst, AdBlue pressure and dosing, temperature sensors and neutralization efficiency. For the DPF, check differential pressure at idle and under load, estimated soot and ash levels, and the history and results of recent regenerations.

Critical signs include power limitation, failed regeneration, recurring SCR codes, critical DPF loading or abnormal sensor readings. DAF notes that delayed maintenance of the DPF soot filter increases fuel consumption and accelerates the ageing of the exhaust aftertreatment system. Erasing an AdBlue code is not a repair.

How to detect recently cleared errors

Cleared fault memory may be indicated by incomplete monitors and low mileage since the codes were erased. Save the report before starting the engine, warm up the engine, perform a test drive and scan the system again. If the code returns, the fault is confirmed. If the seller does not allow a repeat scan, it is better to walk away from the deal.

When to negotiate and when to decline

Negotiate if the fault code is stored and does not return after the test, the live parameters are normal, the cause has been identified, and the service centre has provided a written estimate for the specific VIN. Additional inspection is required when there is a group of related faults, borderline adaptation values or a mileage discrepancy between control units.

Refuse the purchase or demand repairs if active faults concern oil pressure, overheating, EBS, transmission calibration or power limitation; if the codes return after a test drive; or if the seller does not allow a cold start or repeat scan. First establish the cause and obtain a repair estimate, then negotiate.

At TIRkomis, you can select a specific truck in the catalog, immediately perform computer diagnostics at TIR-service Renamax and test the vehicle on the road. The manager will organize an inspection of the selected truck and provide information about typical faults for the corresponding model.

Book a diagnostic inspection and test drive. Inform the manager in advance which truck you would like to inspect—it will be prepared for a cold start, service diagnostics and a test drive.