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The eK Wagon is one of the Mitsubishi models that has higher emissions than shown in tests (Photo: Mitsubishi)

Mitsubish Motors said it had improperly conducted emission tests for four Japanese mini-cars, including two models it manufactures for Nissan.

The Japanese carmaker said in a statement that it used improper testing methods "to present better fuel consumption rates than the actual rates."

The cars involved are the eK Wagon and eK Space, which Mitsubishi sold under its own name, and the Dayz and Dayz Roox, which have been supplied to Nissan since 2013.

Mitsubishi said that, through the end of March, it had sold 157,000 units of the ekWagon and eK Space and had supplied 468,000 Dayz and Dayz Roox models to Nissan.

Production and sales of the four models have been halted and Mitsubishi said it will discuss compensation with Nissan.

Mitsubishi said it is appointing a committee of external experts, who will also investigate whether any cars built for international markets were affected.

Mitsubishi's announcement follows Volkswagen's admission last year that it had installed so-called "defeat devices" in several of its models. The software recognizes when the cars are being tested and then instructs the diesel engines to run cleaner than they would in normal road conditions.

Volkswagen, which faces lawsuits and high costs to fix the cars involved, still needs to reach agreement with US authorities on what action to take there.

The VW deceit has focused worldwide attention on testing regimes, which in most countries test cars in laboratory-like settings that have little in common with normal driving conditions.

Other car brands have denied that they have manipulated tests, but on Wednesday's Germany's Sueddeutsche Zeitung newspaper reported that regulators in the country have found that many diesel models fail to filter nitrogen oxide from their emissions at low temperatures.

According to the report, carmakers are allowed under EU rules to temporarily switch off  exhaust-cleaning systems in cold weather conditions to avoid engine damage.

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Following is the Mitsubishi statement, as it was issued early Wednesday:

Improper conduct in fuel consumption testing on products manufactured by Mitsubishi Motors Corporation (MMC)

In connection with the certification process for the mini-cars manufactured by MMC, we found that with respect to the fuel consumption testing data submitted to the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), MMC conducted testing improperly to present better fuel consumption rates than the actual rates; and that the testing method was also different from the one required by Japanese law. We express deep apologies to all of our customers and stakeholders for this issue.

The applicable cars are four mini-car models, two of which are the "eK Wagon" and "eK Space" which have been manufactured by MMC; and the other two are the "Dayz" and "Dayz Roox" which have been manufactured by MMC and supplied to Nissan Motors Corporation (NM) since June 2013. Up until the end of March 2016, MMC has sold 157 thousand units of the eK Wagon and eK Space and supplied 468 thousand units of the Dayz and Dayz Roox to NM.

Since MMC developed the applicable cars and was responsible for obtaining the relevant certifications, MMC conducted fuel consumption testing. In the process of the development for the next generation of mini-car products, NM examined the fuel consumption rates of the applicable cars for NM's reference and found deviations in the figures. NM requested MMC to review the running resistance(*) value set by MMC during tests by MMC. In the course of our internal investigation upon this request, MMC learned of the improper conduct that MMC used the running resistance value for testing which provided more advantageous fuel consumption rates than the actual rates. MMC will sincerely respond to our customers who own and use the applicable cars. (*) running resistance: rolling resistance (mainly generated by tires) and air resistance while vehicles are moving

We have decided to stop production and sales of the applicable cars. NM also has stopped sales of the applicable cars, and MMC and NM will discuss compensation regarding this issue.

During our internal investigation, we have found that the testing method which was different from the one required by Japanese law has been applied to other models manufactured by MMC for the Japanese domestic market.

Taking into account the seriousness of these issues, we will also conduct an investigation into products manufactured for overseas markets.

In order to conduct an investigation into these issues objectively and thoroughly, we plan to set up a committee consisting of only external experts. We will publish the results of our investigation as soon as it is complete.