Owners of 2500 and 3500 Dodge Ram trucks have filed a class-action lawsuit against Fiat Chrysler and engine manufacturer Cummins, claiming that the two companies “conspired to knowingly deceive consumers and regulators of illegally high levels of diesel emissions in their vehicles”.

According to the complaint, the affected Cummins diesel engines serve to conceal true emissions output, and in doing so cause the catalytic converter to wear out more quickly, resulting in the vehicle burning fuel at a higher rate, and often requiring customers to replace the converter after the warranty has expired at a cost of US$3,000-$5,000.

The lawsuit claims that, in order to produce a diesel engine that had desirable power and fuel economy, yet emissions levels low enough to meet government standards, Chrysler and Cummins developed the 6.7-litre diesel engine with sophisticated NOx adsorber technology.

The primary emission control after-treatment technologies include a diesel particulate filter (DPF) and a NOx adsorber catalyst system to capture and reduce NOx into less harmful substances, such as nitrogen and oxygen. Testing shows that the catalysts are not durable and do not meet emission standards. Furthermore, injection of fuel to regenerate the DPF occurs with excessive frequency. NOx emissions during these regeneration events are nearly 10 times the emission standards, according to the lawsuit.

The legal limit of NOx emissions for stop-and-go driving is 200 mg/mile. When tested, Dodge Ram 2500s emitted 702 mg/mile on average, and 2,826 mg/mile at maximum emissions. The California (and other Section 177 states that have adopted the California standard) NOx limit for highway conditions is 400 mg/mile. Testing for the 2500 shows an average of 756 and max of 2,252 mg/mile.  During these periods of excessive emissions, the vehicle’s on-board diagnostic (OBD) system does not capture fault codes nor provide any indication of excessive emissions to the operator.

Fiat Chrysler said in a statement cited by Reuters it “does not believe that the claims brought against it are meritorious” and the company “will contest this lawsuit vigorously”.

Cummins spokesman Jon Mills told the news agency the lawsuit “has no merit. We are obviously disappointed in the effort to tarnish our image and we plan to vigorously defend ourselves.”

Reuters reported last month Fiat Chrysler and Cummins have been fighting over the $200m estimated cost for a recall of 130,000 newer 2500 Ram pickup trucks equipped with Cummins diesel engines that could exceed US pollution limits.

The US Environmental Protection Agency and California Air Resources Board have demanded a recall of 2013-2015 model year Ram 2500 pickup trucks with 6.7-litre Cummins diesel engines because moisture can lead to the deactivation of the selective catalyst reduction system, causing excess nitrogen oxide emissions, Cummins said.

Fiat Chrysler has sued Cummins to recover the $60m it has spent to date repairing 42,000 trucks at its own expense, a company lawyer said in court documents cited by Reuters. Settlement talks are ongoing.

Cummins counter-sued, saying Fiat Chrysler would not cooperate in the recall “for one reason – money” and said the automaker was “holding both Cummins and its own customers hostage”.

When the emissions system fails, the warning light goes on and if the vehicle isn’t fixed soon the vehicles go into “limp mode” that allow them to only be driven very slowly, Reuters said.