P0147

P0147 sets when the PCM detects a fault in the heater control circuit for Bank 1 Sensor 3 -- either the heater is drawing too much current (shorted element), no current (open element or broken wire), or the control side cannot pull the circuit to ground. The post-cat heater is a simple ~6-14 ohm resistive element fed 12V on one side and pulsed to ground by the PCM on the other. Cheapest first: key-on engine-off, back-probe both heater pins -- one should sit at battery voltage, the other should pulse to near 0V at roughly 1-10 Hz once the PCM commands heater on (about 10-30 seconds after key-on cold). If both pins sit at 12V, the PCM driver is not pulling down or the heater is open. Pull the connector and ohm the heater element across the two heater pins: 4-15 ohms is healthy on most narrowband sensors at room temperature; open or above 30 ohms means a new sensor; under 2 ohms means a shorted element that will burn the PCM driver if you keep driving. Caveat: a fried PCM heater driver (internal MOSFET) will set this code with a perfectly good sensor installed -- if the new sensor sets the same code within 60 seconds and the harness ohms out clean, the driver inside the PCM is dead and replacing sensors becomes an expensive guessing game.

Vehicle-specific patterns: 2005-2010 Jeep Grand Cherokee WK 4.7L -- post-cat heater fuse (F26 in the underhood TIPM) shares circuit with bank-1 pre-cat and pops on a wet startup, Chrysler TSB 18-040-09 swaps the TIPM. 2004-2009 Toyota Prius Gen2 1NZ-FXE -- sensor 3 heater element fails internally from extended cat-warmup cycles, no harness fault present, sensor replacement only. 2006-2012 Ford Fusion 3.0L Duratec -- heater ground splice S402 behind the right strut tower corrodes and adds resistance, sensor heats but slowly enough to trip the monitor. 2007-2013 GMC Acadia / Buick Enclave 3.6L -- PCM heater driver fails internally, GM bulletin 09-06-04-026 covers PCM replacement after the harness checks clean. Estimated repair: $150 to $720.