This paper presents an improved MAP decoder to be used for joint source-channel arithmetic decoding for H.264 symbols. The proposed decoder uses not only the intentional redundancy inserted via a forbidden symbol but also exploits residual redundancy by a syntax checker. A breadth-first suboptimal sequential MAP decoder is employed. The decoder eliminates paths in the decoding tree that result in invalid syntax or that decode a forbidden symbol. In contrast to previous methods, this is done as each channel bit is decoded. Simulations using intra prediction modes show improvements in error rates, for example, syntax element error rate reduction by an order of magnitude for channel SNR of 7.33dB. The cost of this improvement is more computational complexity spent on the syntax checking.