Shares of General Motors  (GM - Get Report) closed out last week with an ugly drop. The stock opened Friday with a news-inspired downside gap and remained under fairly heavy selling pressure all day despite the broader market's big upside reversal. GM's 3% fall drove the stock back down to a very key support zone. If the stock fails to hold the $30 area as April gets underway, a deep pullback may be ahead.

GM's powerful rebound off the February lows lifted the stock back up to its declining 200-day moving average. The stock was rejected here after surging nearly 20% from its Feb. 11 bottom. GM faded from this level, but the damage so far has been minimal while support near the February high has held. Following Friday's heavy selling wave, the March high is beginning to look more ominous. If GM bulls fail to make a stand near the $30 area, overhead pressure from its declining 200-day moving average will increase.

In the near term, GM investors should keep a close eye on the support zone near the $30 area. This key level includes the stock's February high, 50-day moving average and late March lows. A close below $29.90 would be a clear warning sign that more downside is on the way.

For patient GM bulls, a breakdown will drive the stock down to a very low-risk buy area. A drift down to the $28.50 area will retest important support near the September and January lows. Until this level is reached, GM may prove to be a frustrating long.

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This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.