My Pro Street

P0042 Code Explained: HO2S Heater Control Circuit Bank 1 Sensor 3 Causes, Symptoms & Fixes

OBD-II scanner displaying P0042 beside a third downstream oxygen sensor located after a secondary catalytic converter with the heater wiring highlighted.

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:

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:

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:

P0042 Quick Facts

ItemInformation
OBD-II CodeP0042
DescriptionHO2S Heater Control Circuit Bank 1 Sensor 3
CategoryPowertrain
SystemHeated Oxygen Sensor
SensorBank 1 Sensor 3
Sensor LocationDownstream of the secondary catalytic converter (where equipped)
SeverityLow 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:

The heater enables the oxygen sensor to:

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:

Many vehicles do not use a Sensor 3 at all.

How the Heater Circuit Works

Each heated oxygen sensor contains:

When the engine starts cold, the heater receives battery voltage through:

The ECM controls the heater by switching either:

The ECM continuously monitors:

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:

How the ECM Detects P0042

The ECM monitors:

The code may set if:

Symptoms of P0042

Common symptoms include:

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:

5. Corroded Connector

Inspect for:

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:

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:

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:

Step 2

Verify the vehicle actually uses a Bank 1 Sensor 3.

Many vehicles only have:

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:

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:

Step 9

Command the heater using a bidirectional scan tool.

Verify:

Step 10

Clear codes.

Perform a complete drive cycle.

Verify P0042 does not return.

Common Repairs

RepairEstimated 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

Vehicles Commonly Affected

P0042 is most commonly found on vehicles equipped with multiple catalytic converters, including:

(Some models only.)

How to Prevent P0042

Reduce the likelihood of P0042 by:

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

Related Emissions Codes

Browse our complete diagnostic library:

Exit mobile version