UPDATE 1-General Motors China sales go into reverse as market stalls

* General Motors Q3 China sales drop 14.9 pct y/y

* First quarterly sales drop since Q1 2017

* Carmaker sells 835,934 vehicles in the quarter

* Economic slowdown, trade war hitting auto market

* Dealers say Buick engine shift hurting sales (Adds spokeswoman comment, details, pictures)

SHANGHAI, Oct 8 (Reuters) - General Motors Co's quarterly sales in China fell for the first time in over a year, hit by faltering economic growth and a wider slowdown in the world's biggest auto market amid a whipsawing trade war with the United States.

The U.S. carmaker sold 835,934 vehicles in the third quarter ended September, down a sharp 14.9 percent from a year earlier, which the firm said was due to a "softening" vehicle market and issues shifting to a new engine system with its Buick brand.

"The major reasons are a softening market, slowing lower-tier cities, Buick's engine change-over and a strong Q3 last year," a Shanghai-based GM spokeswoman said. She added the fall was not linked to trade tensions.

The fall marks the maiden drop since the first quarter of 2017, when GM's China sales fell 5.2 percent. GM switched to reporting only quarterly China sales earlier this year, scrapping monthly sales figures it had previously revealed.

China's automobile sales have been falling more broadly in recent months, with a slowing economy and trade frictions making consumers cautious about spending, an industry body said last month.

GM has been shifting its Buick cars to a new type of more efficient three-cylinder engine to meet emissions targets, which Chinese dealers told Reuters had hit sales because consumers were not yet convinced by the smaller engines.

"Many consumers still have concerns because they read negative comments about three-cylinder technology online, which aren't really fair," said a sales manager surnamed Hu at a Buick dealer in Zhejiang province.

China, the world's largest auto market, is critical for the U.S. carmaker. It sold over 4 million vehicles in the country last year, even more than it sold in the North Americas market.

GM's sales in China inched up 0.7 percent in the second quarter of the year, slowing from an 8 percent rise in the January-March quarter.

GM's joint venture in China, Shanghai GM, will also recall over 3.3 million Buick, Chevrolet and Cadillac vehicles from Oct. 20 due to a defect with the suspension system, a Chinese regulator said earlier this month.

China's main auto industry body, the China Association of Automobile Manufacturers (CAAM), is set to reveal September auto sales later this week. (Reporting by Adam Jourdan, Norihiko Shirouzu and Yilei Sun; Editing by Muralikumar Anantharaman and Himani Sarkar)