If your OBD-II scanner displays P0038, your vehicle’s Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected that the heater control circuit for the Bank 1 Sensor 2 heated oxygen sensor (HO2S) is operating above its expected voltage range.
The generic SAE definition for P0038 is:
HO2S Heater Control Circuit High Bank 1 Sensor 2
Unlike P0036, which indicates a general heater circuit malfunction, or P0037, which indicates a low-voltage condition, P0038 specifically means the ECM has detected excessive voltage or insufficient current flow in the heater circuit for the downstream oxygen sensor.
Depending on the manufacturer, the affected sensor may also be referred to as the:
- Downstream oxygen sensor
- Rear oxygen sensor
- Heated Oxygen Sensor (HO2S)
- Post-catalytic converter oxygen sensor
- Rear lambda sensor
- Heated lambda sensor
- Catalyst monitor oxygen sensor
- Bank 1 Sensor 2 oxygen sensor
The internal heater allows the oxygen sensor to reach operating temperature quickly after startup. When the ECM detects an open circuit, excessive resistance, or another condition causing higher-than-expected voltage in the heater circuit, it stores P0038.
Common causes include:
- Open heater element
- Open wiring
- Broken heater power wire
- Corroded connector terminals
- Disconnected oxygen sensor
- Failed heater relay
- Blown heater fuse
- Incorrect replacement oxygen sensor
- High-resistance wiring
- ECM driver failure
Unlike upstream oxygen sensor heater codes, P0038 typically has little effect on engine performance but can prevent emissions monitors from completing and cause an emissions inspection failure.
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P0038 Quick Facts
| Item | Information |
|---|---|
| OBD-II Code | P0038 |
| Description | HO2S Heater Control Circuit High Bank 1 Sensor 2 |
| Category | Powertrain |
| System | Heated Oxygen Sensor |
| Sensor | Bank 1 Sensor 2 |
| Sensor Location | Downstream (After Catalytic Converter) |
| Severity | Low to Moderate |
| Safe to Drive? | Yes |
| Typical Repair Cost | $100–700+ |
What Does P0038 Mean?
P0038 indicates that the ECM has detected a high-voltage condition in the heater circuit for the downstream oxygen sensor located on Bank 1.
Most manufacturers interpret a Circuit High condition as:
- Open heater circuit
- Open heater element
- Broken wiring
- Excessive circuit resistance
- Low current flow
- Disconnected sensor
When current flow becomes too low, voltage remains higher than expected and the ECM stores P0038.
Unlike upstream oxygen sensors, Bank 1 Sensor 2 is primarily responsible for monitoring catalytic converter efficiency, not controlling fuel mixture.
What Is Bank 1 Sensor 2?
Understanding oxygen sensor locations is essential.
Bank 1
Bank 1 is the side of the engine containing cylinder number one.
Inline engines have only Bank 1.
V-type engines have:
- Bank 1
- Bank 2
Sensor 2
Sensor 2 is always the downstream oxygen sensor.
It is mounted:
- After the catalytic converter
- In the rear exhaust pipe
- Downstream of the catalyst
Its primary responsibility is monitoring catalytic converter performance rather than adjusting air-fuel ratio.
What Does the Oxygen Sensor Heater Do?
The downstream oxygen sensor contains an electric heater that quickly raises the sensing element to operating temperature.
This allows the ECM to:
- Run catalyst efficiency tests sooner
- Complete readiness monitors
- Reduce cold-start emissions
- Verify emissions system performance
Without the heater, the sensor may require several minutes before becoming active.
Why Does the Downstream Sensor Need a Heater?
Although the downstream sensor doesn’t actively control fuel delivery, it must still become operational quickly to:
- Monitor catalyst efficiency
- Complete emissions diagnostics
- Detect catalyst deterioration
- Satisfy OBD-II monitor requirements
The heater dramatically shortens warm-up time.
How the ECM Detects P0038
The ECM continuously monitors:
- Heater voltage
- Heater current
- Heater resistance
- Driver feedback
- Warm-up time
- Sensor operation
P0038 may be stored if:
- Heater current is too low
- Heater voltage remains high
- Heater element opens
- Circuit resistance becomes excessive
- Power circuit opens
- ECM driver cannot energize the heater
P0036 vs P0037 vs P0038
These three codes diagnose the same downstream oxygen sensor heater circuit.
| Code | Description |
| P0036 | HO2S Heater Control Circuit Bank 1 Sensor 2 |
| P0037 | HO2S Heater Control Circuit Low Bank 1 Sensor 2 |
| P0038 | HO2S Heater Control Circuit High Bank 1 Sensor 2 |
Generally:
- P0036 = General heater circuit fault
- P0037 = Low voltage / excessive current
- P0038 = High voltage / insufficient current
Symptoms of P0038
Common symptoms include:
- Check Engine Light
- Failed emissions inspection
- Incomplete readiness monitors
- Increased cold-start emissions
- Delayed catalyst monitoring
- Slight decrease in fuel economy
Most drivers experience little or no noticeable drivability symptoms.
Common Causes of P0038
1. Open Heater Element
The heater inside the oxygen sensor burns open over time.
This is the most common cause.
2. Open Wiring
Broken wiring may result from:
- Engine movement
- Heat damage
- Previous repairs
- Rodent damage
3. Broken Power Wire
The heater loses battery voltage due to:
- Broken wiring
- Open splice
- Loose connector
- Damaged harness
4. Corroded Connector
Corrosion increases circuit resistance.
Inspect for:
- Green corrosion
- White oxidation
- Moisture
- Oil contamination
5. Loose Connector
Inspect for:
- Broken lock tabs
- Loose terminals
- Partially disconnected connector
6. Melted Wiring
The downstream oxygen sensor operates near:
- Catalytic converter
- Exhaust pipe
- Muffler
- Heat shields
High temperatures may damage the wiring harness.
7. Blown Heater Fuse
Some manufacturers use shared oxygen sensor heater fuses.
Always test the fuse electrically.
8. Failed Heater Relay
A faulty relay may interrupt power to the heater.
9. Incorrect Replacement Oxygen Sensor
Aftermarket sensors with incorrect heater resistance may trigger P0038.
Always verify the correct part number.
10. High Resistance
High resistance may develop because of:
- Loose terminals
- Corrosion
- Damaged wiring
- Poor splice repairs
11. Damaged Wiring Harness
Inspect for:
- Chafing
- Pinched wiring
- Previous repairs
- Heat damage
12. ECM Driver Failure
Although rare, the ECM output driver may fail internally.
Always eliminate wiring faults before replacing the ECM.
Is P0038 Serious?
Generally Low to Moderate.
Unlike upstream heater faults, P0038 usually has minimal effect on engine operation.
However, it may:
- Prevent readiness monitors from completing
- Cause emissions inspection failure
- Illuminate the Check Engine Light
- Delay catalyst diagnostics
Can You Drive With P0038?
Yes.
Most vehicles remain fully driveable.
Repair should still be completed before emissions testing.
How to Diagnose P0038
Step 1
Scan for additional trouble codes.
Related codes include:
- P0030 – HO2S Heater Control Circuit Bank 1 Sensor 1
- P0031 – HO2S Heater Control Circuit Low Bank 1 Sensor 1
- P0032 – HO2S Heater Control Circuit High Bank 1 Sensor 1
- P0036 – HO2S Heater Control Circuit Bank 1 Sensor 2
- P0037 – HO2S Heater Control Circuit Low Bank 1 Sensor 2
- P0136 – O2 Sensor Circuit Malfunction Bank 1 Sensor 2
- P0140 – O2 Sensor No Activity Bank 1 Sensor 2
- P0141 – O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 2
- P0420 – Catalyst System Efficiency Below Threshold Bank 1
Step 2
Confirm Bank 1 Sensor 2 location.
Verify the sensor is installed after the catalytic converter.
Step 3
Inspect the connector.
Check for:
- Loose terminals
- Broken locks
- Heat damage
- Corrosion
Step 4
Inspect wiring.
Pay particular attention to wiring near:
- Catalytic converter
- Exhaust pipe
- Heat shields
Step 5
Check the heater fuse.
Test using a multimeter.
Never rely solely on visual inspection.
Step 6
Measure heater resistance.
Infinite resistance usually indicates an open heater element.
Step 7
Verify power supply.
Battery voltage should be present when the heater is commanded on.
Step 8
Check the control circuit.
Inspect for:
- Open circuit
- High resistance
- Loose connectors
Step 9
Perform voltage-drop testing.
Inspect:
- Grounds
- Connectors
- Fuse terminals
- Wiring splices
Step 10
Command the heater with a bidirectional scan tool.
Verify:
- Heater activation
- Current draw
- ECM response
Step 11
Check Technical Service Bulletins.
Some manufacturers have released updated oxygen sensors or revised harness routing.
Step 12
Verify repair.
After repairs:
- Clear codes.
- Cold start the engine.
- Complete a drive cycle.
- Verify catalyst readiness completes.
- Confirm P0038 does not return.
Common Repairs
| Repair | Estimated Cost |
| Heater fuse replacement | $10–50 |
| Wiring repair | $100–400 |
| Connector replacement | $75–250 |
| Downstream 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 catalytic converter
- Replacing the wrong oxygen sensor
- Ignoring open wiring
- Skipping heater resistance testing
- Overlooking connector corrosion
- Failing to test heater voltage
- Installing incorrect aftermarket sensors
- Replacing the ECM before completing circuit tests
Vehicles Commonly Affected
P0038 may appear on nearly any OBD-II vehicle equipped with heated oxygen sensors, including:
- Acura
- Audi
- BMW
- Chevrolet
- Chrysler
- Dodge
- Ford
- GMC
- Honda
- Hyundai
- Infiniti
- Jeep
- Kia
- Lexus
- Mazda
- Mercedes-Benz
- Nissan
- Subaru
- Toyota
- Volkswagen
- Volvo
How to Prevent P0038
Reduce the likelihood of P0038 by:
- Inspecting oxygen sensor wiring during exhaust service
- Replacing damaged heat shields
- Repairing exhaust leaks promptly
- Using OEM-quality oxygen sensors
- Keeping connectors free from moisture
- Avoiding poor-quality splice repairs
- Repairing oil leaks before connectors become contaminated
- Securing wiring away from exhaust heat
Frequently Asked Questions
What does P0038 mean?
P0038 indicates a high-voltage condition in the heater circuit for the Bank 1 Sensor 2 oxygen sensor.
Where is Bank 1 Sensor 2 located?
After the catalytic converter on the engine bank containing cylinder number one.
Can a bad oxygen sensor cause P0038?
Yes.
An internally open heater element is one of the most common causes.
Can wiring cause P0038?
Absolutely.
Broken wiring and excessive resistance frequently trigger this code.
Will P0038 affect engine performance?
Usually not.
Most vehicles continue to run normally.
Can I drive with P0038?
Yes.
However, emissions readiness monitors may not complete.
Will P0038 fail emissions?
Yes.
An active Check Engine Light or incomplete readiness monitors generally result in an emissions inspection failure.
Will replacing the catalytic converter fix P0038?
No.
P0038 is a heater circuit fault, not a catalytic converter failure.
Final Thoughts
The P0038 code indicates the ECM has detected a high-voltage condition in the heater circuit for the downstream oxygen sensor (Bank 1 Sensor 2). In most cases, the problem is caused by an open heater element, broken wiring, excessive circuit resistance, or poor electrical connections rather than a faulty catalytic converter.
Begin diagnosis by verifying the correct oxygen sensor location, inspecting the wiring near the catalytic converter, measuring heater resistance, checking power and ground, and testing the heater circuit before replacing expensive components.
Related Articles
Continue diagnosing oxygen sensor and emissions-related problems with these Pro Street guides:
Upstream Oxygen Sensor Heater Codes
- P0030 Code Explained: HO2S Heater Control Circuit Bank 1 Sensor 1
- P0031 Code Explained: HO2S Heater Control Circuit Low Bank 1 Sensor 1
- P0032 Code Explained: HO2S Heater Control Circuit High Bank 1 Sensor 1
Downstream 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
Browse our complete diagnostic libraries:
- Complete OBD-II Code Guide: https://my.prostreetonline.com/dtc-obdii/
- B Code OBD-II Library: https://my.prostreetonline.com/dtc-doctor/b-code-obd-ii-codes/
- C Code OBD-II Library: https://my.prostreetonline.com/dtc-doctor/c-code-obd-ii-codes/
