Closed Loop vs Open Loop

Two fueling modes: closed loop corrects fuel from O2 feedback, open loop runs the commanded value blind.

Closed loop is when the ECU reads the oxygen sensor and actively trims fueling to hold stoich, used at idle and cruise for emissions and economy. Open loop is when the ECU ignores O2 feedback and simply delivers the commanded AFR from its tables, used during warm-up, hard acceleration, and Power Enrichment where stoich is neither wanted nor safe. The distinction matters because your tuning approach differs by mode: in closed loop you read fuel trims to correct the VE/MAF table, but in open loop there is no feedback safety net, so the commanded fuel had better be right. This is exactly why you tune WOT (open-loop) fueling with a wideband, since the ECU will not catch a lean condition for you. In HP Tuners VCM Scanner, log the closed/open-loop status flag to know which mode a data point came from. In VCM Editor, the open-loop enable conditions live in the fuel control and PE tables.