What Does P0042 Mean?
The ECM has detected a malfunction in the heater control circuit of the Bank 1, Sensor 3 heated oxygen sensor. Sensor 3 is located downstream of a secondary catalytic converter (on vehicles with multiple cats per bank). This heater circuit fault prevents the sensor from reaching operating temperature quickly. The driver is unlikely to notice any drivability symptoms, as this sensor provides supplemental emissions monitoring data.
Common Causes
40%
Failed HO2S heater element (open or shorted internally)
20%
Blown O2 heater fuse or failed relay shared with other heater circuits
20%
Damaged wiring or connector from road debris (underbody location)
10%
Corroded ground connection at the sensor or ECM
10%
ECM heater driver circuit failure for Sensor 3
Diagnostic Steps
1
Check for companion O2 heater codes (P0030, P0036). If multiple heater circuits are affected, check the shared O2 heater fuse and relay first, as a common power supply issue is likely.
2
Locate the Bank 1 Sensor 3 O2 sensor (downstream of the secondary catalytic converter, typically near the rear of the vehicle). Disconnect the connector and measure heater resistance — expect 2–15 ohms. An OL reading confirms a failed heater.
3
Verify battery voltage is present on the heater power supply wire with key on. If no voltage, trace the circuit through the fuse and relay back to the power source.
4
Check the ground control wire for continuity to the ECM — should be under 1 ohm. The long wire run to Sensor 3 (often routed along the entire underbody) is more prone to damage than sensors closer to the engine.
5
Inspect the wiring harness along the underbody for physical damage from road debris, corrosion from road salt, or rodent damage. The Sensor 3 harness routing is the longest and most vulnerable of all O2 sensor circuits.
Estimated Repair Cost
$80 - $350
Parts + labor, varies by vehicle and location
The ECM has detected a malfunction in the heater control circuit of the Bank 1, Sensor 3 heated oxygen sensor. Sensor 3 is located downstream of a secondary catalytic converter (on vehicles with multiple cats per bank). This heater circuit fault prevents the sensor from reaching operating temperatur...
The most common cause of P0042 (HO2S Heater Control Circuit (Bank 1, Sensor 3)) is: Failed HO2S heater element (open or shorted internally)
Typical repair costs for P0042 range from $80 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Completely safe to drive. Sensor 3 provides supplemental catalyst monitoring and has minimal influence on fuel control. No drivability impact is expected. The only concern is emissions compliance — repair before your next emissions inspection.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0042 to identify the root cause.
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Quick Info
Category
Powertrain
System
Fuel System / Emissions
Difficulty
Type
Generic (SAE)
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