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Here's my question.... With a 02 heater code is the pcm monitoring the signal line for time to activity test or is it directly measuring the amperage draw of the heater circuit when it commands it on pwm? I was thinking before pwm it would monitor signal line and after pwm it monitors amperage directly on the circuit. All the data pids on scan tool for amperage. Is this correct? I am talking about gm vehicles.
After re reading your question I see your asking of pre pwm system as well and on that old system I'm not sure it monitored anything other than complete circuit of heater someone may have a better answer there though.
