How Short FT Works

Short FT is a temporary correction to fuel delivery which changes with every cycle of the oxygen sensor. Under normal conditions, it fluctuates rapidly around its ideal value of 0% correction and is only functional during closed loop operation.

Short FT is a parameter on the OBD-Il data stream, that can be displayed on the Diagnostic Tester. Its normal range is ± 20%, but under normal operating conditions, rarely goes beyond ± 10%.

Short FT responds to changes in 02S output. If basic injection duration results in a lean air/fuel ratio, Short FT responds with positive corrections (+1% to +20%) to add fuel or enrich the mixture. If basic injection is too rich, Short FT responds with negative corrections (-I% to -20%) to subtract fuel or enlean the mixture.

When Short FT is varying close to ± 0%, this indicates a neutral condition where the basic injection duration calculation is very close to stoichiometry, without any significant correction for 02S.

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