Cause
Open heater supply circuit
A break in the power feed to a heating element rather than in the element itself, and it is worth separating because the module cannot tell the two apart from where it sits — it commands heat, sees no current, and reports the circuit. The element may be in perfect condition.
Heater circuits carry unusually high current, and that shapes where they fail. An intake air heater or a heated catalyst draws tens of amps, so the failure points are the ones that carry it: a corroded ring terminal, a relay with pitted contacts, a fusible link, a crimp that has annealed with heat cycling. These are joints rather than wires, and a continuity test made with a meter's tiny test current will happily pass a joint that collapses under load.
So test it under load or not at all. Measure voltage drop across the connection with the circuit commanded on; a joint that reads zero ohms cold and drops several volts hot is the classic finding here. Cold weather makes it appear and warm weather hides it, because these circuits only energise when the temperature is low enough to ask for them.
Typically a moderate job.
Codes this can cause (5)
Codes where this is a likely explanation first, rather than merely a possible one.
- P0424 Heated Catalyst Temperature Below Threshold — Bank 1 Likely for this code Moderate
- P0434 Heated Catalyst Temperature Below Threshold — Bank 2 Likely for this code Moderate
- P0542 Intake Air Heater A Circuit High Likely for this code Low
- P0543 Intake Air Heater A Circuit Open Likely for this code Low
- P0599 Thermostat Heater Control Circuit High Likely for this code Moderate
Information, not instruction
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