When Sun Yongli bought his sleek Mercedes Benz C200 five years ago, he never expected the limo would give him such a headache.

The 33-year-old, who works for an Internet company in Shenzhen, told Xinhua that he chose the model partly due to the brand's reputation for safety, but Sun believes the smell of the vehicle's interior may have hurt his health.

He is among many Chinese affected by the same issue, with suspicions that the whiff indicates the presence of noxious chemicals. Now, ahead of the International Day for Protecting Consumers' Rights on March 15, pressure is being placed on the vehicle's manufacturers to do something about the issue.

Sun says the typical "new car smell" went away about a year after he bought the Mercedes in 2009, but another "melted crayon smell" became more and more conspicuous since 2011.

"I often felt dizzy and nauseous after driving for a little while," Sun claims. Besides short-term effects, he has found his immune system may have been damaged. "Since 2010, my physical examination results showed abnormally high levels of white blood cells and lymph indicators."

His C200 is a member of the Mercedes Benz C-class series of compact cars. It was produced by Beijing Benz Automotive Co., a joint venture by BAIC Motor Co., Daimler AG and Daimler Greater China Ltd.

The C-class vehicles, especially domestically manufactured ones, have been frequent targets of complaint in China for their smell.

On QQ Group, a group discussion function on popular instant-messager QQ, a search for "Benz," "peculiar smell" and "safeguarding legitimate rights" returns 33 groups attracting more than 2,000 users.

Their most common claimed symptoms include dizziness, headache, nausea, decreasing immunity and problems with eyes and respiratory organs.

"The Benz is poisonous. I have to open the windows when I'm driving it," according to a QQ user with the screen name "Sanloulouzhang," who like Sun owns a 2008-model C200.

His solution -- opening the window -- was also taken by most other owners in a QQ group interviewed by Xinhua. Of course, they have tried other more substantive measures too.

In mid 2012, Sun reported the smell to his after-service 4S store, which blamed the car's carpet and agreed to replace the offending floor covering free of charge.

The odor was gone for about six months, but than gradually returned to the same pungency by June 2013.

"It smells like melted crayon," he says.

Some Benz owners have had the air inside the vehicle tested and found high levels of benzene and formaldehyde, both cancer-causing chemicals.

Not in Sun's case. Tests showed the air inside his car contained safe levels of benzene and formaldehyde. But his doubts remained.

"The smell is so strong. There must be something else in the air," Sun believes.

Sanloulouzhang claimed both he and his daughter got pneumonia after he bought the car. "I was hospitalized last year for pneumonia. After that, I have seldom driven the car," he wrote.

The response from Mercedes offered little comfort.

A statement released last July by the company gave "sincere apologies" to affected C-class owners.

"We reconfirm that this condition (referring to the unpleasant smell) will not harm drivers' health, neither will it have negative impacts on vehicle performance and safety," the statement said.

The company attributed the odor to carpets, which it vowed to replace free of charge.

At present, the company offers to replace the car's carpets and clean the interior for affected C-class owners, a customer assistance employee at Mercedes-Benz (China) Limited told Xinhua when answering a telephone call on Wednesday.

The company confirmed the employee's answer in a written reply to Xinhua's interview questions Thursday, saying it have contacted affected users to replace carpets and clean cars since mid-May 2013.

"We have sent technicians and after-sales service personnel several times to conduct face-to-face communication with customers concerning the peculiar smell's source and solution," the reply statement said.

Not everyone is convinced by the company's blaming the carpets, as the replacement in both Sun and Sanloulouzhang's cases failed to eradicate the smell.

"We believe our service measures can eliminate the unpleasant smell and rule out the possibility of peculiar smell from the carpets," the statement said.

If customers complain again after changing carpets, "we will take necessary measures based on specific conditions of each vehicle to solve the problems", according to the statement.

There is much discussion online about measures that could be taken to reduce or cover the stink. However, no one knows for sure where the smell comes from or what harmful elements it indicates.

The current dilemma for Chinese car owners is complicated by the lack of effective legislation concerning the air quality inside an automobile.

A guideline on evaluating air quality in cars has been in effect since March 2012. But it lacks legally binding force as it is only an industry standard, and meanwhile it targets mainly new vehicles.

Now, Sun's C200 spends most of the time in his garage.

He has his fingers crossed that media exposure around the International Day for Protecting Consumers' Rights can help.

"I hope the car can be recalled and given back to me as a really safe one," he says.