P0793

P0793 indicates the PCM/TCM is seeing no signal from the Intermediate Shaft Speed Sensor (ISS), the sensor that monitors a mid-transmission shaft (usually input clutch drum or intermediate planetary) to calculate slip and time shifts. Without ISS data the transmission cannot compute ratio across all gears and typically defaults to a limp or harsh-shift strategy. Cheapest checks: visually inspect the sensor connector for fluid intrusion and bent pins, then back-probe the sensor while cranking or driving and look for a clean AC waveform on a scope (typical Hall-effect ISS sensors produce a 0 to 5V square wave, while variable-reluctance types produce an AC sine wave that scales with shaft RPM, often 100 mV peak-to-peak at idle climbing to several volts at highway speed). Measure resistance on VR-type sensors against spec (commonly 270 to 360 ohms on GM 6L80, 250 to 400 ohms on Ford 6R80 intermediate sensors); Hall-effect sensors require 5V reference, ground, and signal verification at the connector. Inspect the tone ring on the affected shaft through the pan or speed-sensor port for missing or damaged teeth. Caveat: P0793 can also be triggered by a hardware fault inside the TCM that drops the sensor input, so if the sensor, harness, and tone ring are all confirmed good, the TCM is the next suspect (rare but documented).

Vehicle-specific patterns: 2006 to 2010 Cadillac Escalade/Chevrolet Tahoe with 6L80 trips P0793 from internal harness ISS wire chafing on the case casting, addressed by GM TSB 10-07-30-005 with revised internal harness. 2009 to 2014 Ford F-150 with 6R80 sets P0793 from the ISS sensor itself failing internally, a known wear item that Ford addressed with updated part number. 2007 to 2012 Dodge Caliber/Jeep Compass with CVT (JF011E) sets the equivalent secondary-pulley speed code from CVT step motor or input speed sensor failure. 2004 to 2009 Nissan Titan with RE5R05A trips P0793 from input/turbine speed sensor failure, Nissan TSB NTB04-073 covers updated sensor. Estimated repair: $180 to $750.