That's what Mitsubishi Motors Australia has argued after a Canberra tradesman launched a lawsuit against the motoring giant over a Triton ute he claims was defective and not fit for purpose as a work vehicle for his fencing business.

Paul Knowles – who fitted the 2015 ute with aftermarket products such as bash plates, roof brackets, light bar, spot and flood lights, and a bullbar - demanded a refund for vehicle after it continually overheated and made a "squealing noise".

Mitsubishi and its Canberra dealer dispute the allegations, arguing the unapproved "aftermarket modifications" had compromised the ute's engineering design and cooling system.

In the ACT Magistrates Court, Mr Knowles has sought payment of more than $56,334.99 for the vehicle's purchase price, interest from the finance agreement, after-market additions, loss of business, and costs.

Papers – filed by Mr Knowles legal team, Kamy Saeedi Law – said the car had been bought in June 2015 and collected the following month.

In April 2016, the car made squealing noises under the bonnet so he returned it to Commonwealth Motors.

The dealer repaired the towbar, replaced a cracked exhaust manifold, and fixed a window.

The turbocharger was replaced in July and the exhaust manifold changed in August.

The manifold was replaced again the following month due to warping.

In October, further squealing was noticed and Commonwealth Motors identified a number of issues, including loosened and missing bolts, leaking coolant, warped engine parts, and inconsistent data collected by the car's engine control unit.

The bullbar and bash plates were removed, but it did not resolve the issues and further repair work was performed in November.

In January and February 2017, the car overheated a number of times and Mitsubishi was contacted.

The motoring giant conducted a number of tests on the vehicle during a test drive.

In April, a warning light was activated and the Triton went "into limp home mode".

Commonwealth Motors said the engine warning light had been owing to a factory recall, but the light reactivated within one hour of servicing.

Court papers said Mr Knowles also raised complaints about fading paint and premature cracking and peeling of internal leather finishes.

The lawsuit says the purchase agreement included terms that the car be free from defects, safe, durable, acceptable, fit for purpose, and be under warranty until 2020.

It is alleged that Mr Knowles told Commonwealth Motors the Triton would be a work vehicle for his fencing company and expected to transport fencing material, tow heavy trailers, and drive on unsealed and rough roads.

Court papers allege the car had been substantially unfit for purpose and had not been remedied within a reasonable time.

"The plaintiff would not have acquired the car had they known of the failures," the statement of claim reads.

But both Mitsubishi and Commonwealth Motors argued Mr Knowles' aftermarket modifications to the Triton had extremely restricted the airflow to the cooling systems.

"The plaintiff fitted aftermarket modifications to the vehicle, which at no time were approved by [Mitsubishi Motors Australia], as such modifications diverged from the vehicle's original engineering design, and in particular, had the effect of causing an extreme restriction in airflow to the vehicle's cooling system," the motoring giant's defence said.

"The aftermarket modifications to the vehicle lead to air intake, coolant and exhaust temperatures to be above normal which consequentially caused damage to the engine, including the cylinder head and warping, cracking and abnormal operation of the exhaust manifold, all of which were replaced by [Commonwealth Motors] under the manufacturer warranty."

Mitsubishi said the lawsuit had not provided evidence to show the vehicle had been unsafe, could not be used for work, and had not demonstrated that the fencing company had incurred damages to the business.

Commonwealth Motors defence also alleged Mr Knowles had been told to remove the aftermarket parts, but did not want to because the car would then look "terrible".

It also said Mr Knowles had been provided with a loan vehicle on each occasion he had brought in his Triton for work.

"The second defendant is not liable to make any payment … because the plaintiff has caused any loss or damage associated with the condition fo the car, by reason of fitting the aftermarket parts," documents said.

"The second defendant is not liable … because … the car has been damaged by the plaintiff for reasons not relating to the state or condition of the car as the time of supply."

Commonwealth Motors also filed a notice claiming contribution or indemnity against Mitsubishi in the event the court found the dealer liable.

The matter is listed to next come before court in October.