If your OBD-II scanner displays P0042, your vehicle’s Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a malfunction in the heater control circuit for the Bank 1 Sensor 3 heated oxygen sensor (HO2S).
The generic SAE definition for P0042 is:
HO2S Heater Control Circuit Bank 1 Sensor 3
Unlike P0036, which affects the first downstream oxygen sensor (Bank 1 Sensor 2), P0042 applies to Bank 1 Sensor 3. This sensor is found on some vehicles equipped with multiple catalytic converters or dual-stage emissions systems, where a third oxygen sensor is used to monitor additional catalyst efficiency.
Depending on the manufacturer, the affected component may also be referred to as the:
- Heated Oxygen Sensor (HO2S)
- Oxygen Sensor 3
- Rear oxygen sensor
- Post-catalyst oxygen sensor
- Catalyst monitor sensor
- Lambda sensor
- Heated lambda sensor
The oxygen sensor contains an internal electric heater that quickly raises the sensing element to operating temperature. When the ECM detects an electrical problem in the heater circuit, it stores diagnostic trouble code P0042.
Common causes include:
- Failed oxygen sensor heater
- Open heater circuit
- Shorted heater wiring
- Melted wiring harness
- Corroded connector
- Blown heater fuse
- Failed heater relay
- Poor ground
- Damaged wiring
- ECM driver failure
Although P0042 generally has little effect on engine performance, it can delay catalyst monitoring, prevent emissions readiness monitors from completing, and illuminate the Check Engine Light.
For additional diagnostic information, visit:
- Complete OBD-II Code Library: https://my.prostreetonline.com/dtc-obdii/
- Body (B) Code Library: https://my.prostreetonline.com/dtc-doctor/b-code-obd-ii-codes/
- Chassis (C) Code Library: https://my.prostreetonline.com/dtc-doctor/c-code-obd-ii-codes/
P0042 Quick Facts
| Item | Information |
|---|---|
| OBD-II Code | P0042 |
| Description | HO2S Heater Control Circuit Bank 1 Sensor 3 |
| Category | Powertrain |
| System | Heated Oxygen Sensor |
| Sensor | Bank 1 Sensor 3 |
| Sensor Location | Downstream of the secondary catalytic converter (where equipped) |
| Severity | Low to Moderate |
| Safe to Drive? | Yes |
| Typical Repair Cost | $100–700+ |
What Does P0042 Mean?
P0042 indicates the ECM has detected a malfunction in the heater control circuit of the Bank 1 Sensor 3 oxygen sensor.
Bank 1 Sensor 3 is not present on every vehicle. It is typically found on:
- Vehicles with dual catalytic converters
- Multi-stage catalyst systems
- Some luxury vehicles
- Certain diesel engines
- Select European and Japanese models
The heater enables the oxygen sensor to:
- Reach operating temperature quickly
- Monitor catalyst efficiency
- Reduce cold-start emissions
- Complete emissions readiness monitors
When the ECM detects abnormal heater voltage, current, or resistance, it stores P0042.
What Is Bank 1 Sensor 3?
Understanding oxygen sensor locations helps prevent unnecessary parts replacement.
Bank 1
Bank 1 is the engine bank containing cylinder number one.
Sensor 3
Sensor 3 is located behind the second catalytic converter, or behind an additional catalyst used in advanced emissions systems.
Unlike Sensor 1:
- It does not control fuel delivery.
- It primarily monitors emissions system performance.
Many vehicles do not use a Sensor 3 at all.
How the Heater Circuit Works
Each heated oxygen sensor contains:
- Oxygen sensing element
- Electric heater
- Signal circuit
- Ground circuit
When the engine starts cold, the heater receives battery voltage through:
- Oxygen sensor fuse
- Ignition circuit
- Relay (manufacturer dependent)
The ECM controls the heater by switching either:
- Ground
- Power
The ECM continuously monitors:
- Heater voltage
- Heater current
- Heater resistance
- Warm-up time
If the circuit falls outside its calibrated limits, P0042 is stored.
Why Does Sensor 3 Need a Heater?
Even though Sensor 3 is used mainly for emissions monitoring, it still must reach operating temperature quickly.
The heater allows the ECM to:
- Monitor catalyst efficiency sooner
- Complete readiness monitors
- Reduce cold-start emissions
- Detect catalyst deterioration
How the ECM Detects P0042
The ECM monitors:
- Heater voltage
- Heater current
- Resistance
- Driver feedback
- Sensor warm-up time
The code may set if:
- Heater circuit opens
- Heater shorts
- Current becomes excessive
- Voltage falls outside specifications
- Resistance becomes abnormal
Symptoms of P0042
Common symptoms include:
- Check Engine Light
- Failed emissions inspection
- Incomplete readiness monitors
- Increased cold-start emissions
- Delayed catalyst monitoring
Most vehicles experience no noticeable drivability symptoms.
Common Causes of P0042
1. Failed Oxygen Sensor Heater
The heater element eventually burns out.
This is the most common cause.
2. Open Heater Circuit
Broken wiring interrupts heater current.
3. Shorted Heater Wiring
Heat damage or wiring contact with the exhaust may short the heater circuit.
4. Melted Wiring Harness
Sensor 3 is located near extremely hot exhaust components.
Inspect for:
- Melted insulation
- Burned wiring
- Chafed harness
5. Corroded Connector
Inspect for:
- Green corrosion
- White oxidation
- Water intrusion
- Loose terminals
6. Blown Heater Fuse
Some manufacturers power multiple oxygen sensor heaters through one fuse.
7. Failed Heater Relay
Certain vehicles use dedicated heater relays.
8. Incorrect Replacement Sensor
Incorrect heater resistance may trigger P0042.
Always install the correct direct-fit oxygen sensor.
9. Damaged Wiring Harness
Inspect for:
- Rodent damage
- Chafed wiring
- Pinched harnesses
- Previous repairs
10. ECM Driver Failure
Although uncommon, the ECM heater driver can fail internally.
Is P0042 Serious?
Generally Low to Moderate.
The vehicle usually remains fully driveable.
However, prolonged operation may:
- Prevent readiness monitors from completing
- Cause emissions inspection failure
- Delay catalyst diagnostics
- Keep the Check Engine Light illuminated
Can You Drive With P0042?
Yes.
Most vehicles continue to operate normally.
Repairs should still be completed before emissions testing.
How to Diagnose P0042
Step 1
Scan for additional codes.
Related codes include:
- P0036 – HO2S Heater Control Circuit Bank 1 Sensor 2
- P0043 – HO2S Heater Control Circuit Low Bank 1 Sensor 3
- P0044 – HO2S Heater Control Circuit High Bank 1 Sensor 3
- P0141 – Heater Circuit Bank 1 Sensor 2
- P0161 – Heater Circuit Bank 2 Sensor 2
- P0420 – Catalyst Efficiency Below Threshold Bank 1
Step 2
Verify the vehicle actually uses a Bank 1 Sensor 3.
Many vehicles only have:
- Sensor 1
- Sensor 2
Consult the factory service manual if necessary.
Step 3
Locate Bank 1 Sensor 3.
Inspect the sensor and connector.
Step 4
Inspect wiring.
Check for:
- Heat damage
- Chafed insulation
- Broken wiring
- Previous repairs
Step 5
Test the heater fuse.
Use a multimeter.
Step 6
Measure heater resistance.
Compare with manufacturer specifications.
Step 7
Verify power and ground.
Confirm battery voltage reaches the heater.
Step 8
Perform voltage-drop testing.
Inspect:
- Grounds
- Connectors
- Wiring splices
Step 9
Command the heater using a bidirectional scan tool.
Verify:
- Heater activation
- Current draw
- ECM response
Step 10
Clear codes.
Perform a complete drive cycle.
Verify P0042 does not return.
Common Repairs
| Repair | Estimated Cost |
| Heater fuse replacement | $10–50 |
| Wiring repair | $100–400 |
| Connector replacement | $75–250 |
| Oxygen sensor replacement | $180–550 |
| Heater relay replacement | $80–200 |
| ECM programming | $100–300 |
| ECM replacement | $1,000–2,500 |
Common Diagnostic Mistakes
- Replacing the wrong oxygen sensor
- Assuming every vehicle has a Sensor 3
- Ignoring heater fuse failures
- Overlooking heat-damaged wiring
- Using universal oxygen sensors
- Replacing the ECM before testing the circuit
Vehicles Commonly Affected
P0042 is most commonly found on vehicles equipped with multiple catalytic converters, including:
- Audi
- BMW
- Lexus
- Mercedes-Benz
- Nissan
- Porsche
- Subaru
- Toyota
- Volkswagen
- Volvo
(Some models only.)
How to Prevent P0042
Reduce the likelihood of P0042 by:
- Inspecting oxygen sensor wiring during exhaust repairs
- Replacing damaged heat shields
- Repairing exhaust leaks promptly
- Using OEM-quality oxygen sensors
- Keeping connectors clean and dry
- Securing wiring away from hot exhaust components
Frequently Asked Questions
What does P0042 mean?
P0042 indicates a malfunction in the heater circuit for Bank 1 Sensor 3.
Where is Bank 1 Sensor 3 located?
It is typically installed after a secondary catalytic converter on vehicles equipped with multiple catalyst systems.
Does every vehicle have a Sensor 3?
No.
Most vehicles only have Sensor 1 and Sensor 2.
Can a bad oxygen sensor cause P0042?
Yes.
A failed internal heater element is the most common cause.
Can I drive with P0042?
Yes.
Most vehicles remain fully driveable.
Will P0042 fail emissions?
Yes.
Incomplete readiness monitors or a Check Engine Light usually result in an emissions inspection failure.
Will replacing the catalytic converter fix P0042?
No.
P0042 is an electrical heater circuit fault, not a catalyst efficiency problem.
Final Thoughts
The P0042 code indicates the ECM has detected a malfunction in the Bank 1 Sensor 3 heated oxygen sensor heater circuit. While not every vehicle is equipped with a third oxygen sensor, those that are rely on it to monitor secondary catalyst efficiency and ensure emissions compliance.
Begin diagnosis by confirming your vehicle actually uses a Bank 1 Sensor 3, then inspect the wiring, connectors, heater resistance, fuse, and power supply before replacing components. In most cases, the repair involves replacing the oxygen sensor or repairing heat-damaged wiring rather than replacing expensive catalytic converters.
Related Articles
Continue diagnosing oxygen sensor and emissions-related faults with these Pro Street guides:
Oxygen Sensor Heater Codes
- P0036 Code Explained: HO2S Heater Control Circuit Bank 1 Sensor 2
- P0037 Code Explained: HO2S Heater Control Circuit Low Bank 1 Sensor 2
- P0038 Code Explained: HO2S Heater Control Circuit High Bank 1 Sensor 2
- P0043 Code Explained: HO2S Heater Control Circuit Low Bank 1 Sensor 3
- P0044 Code Explained: HO2S Heater Control Circuit High Bank 1 Sensor 3
Related Emissions Codes
- P0141 Code Explained: O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 2
- P0161 Code Explained: O2 Sensor Heater Circuit Malfunction Bank 2 Sensor 2
- P0420 Code Explained: Catalyst System Efficiency Below Threshold Bank 1
- P0430 Code Explained: Catalyst System Efficiency Below Threshold Bank 2
Browse our complete diagnostic library:
- Complete OBD-II Code Guide: https://my.prostreetonline.com/dtc-obdii/
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