P0042 — O2 Sensor Heater Control Circuit (Bank 1 Sensor 3).

🔧 DTC P0042 – O2 Sensor Heater Control Circuit (Bank 1 Sensor 3)

Meaning:
The ECM/ECU has detected a fault in the heater circuit of the Bank 1 Sensor 3 (S3) oxygen sensor.

📌 What is Bank 1 Sensor 3?

Bank 1 = side of engine with cylinder #1.

Sensor 3 = Downstream of the catalytic converter (after the second catalyst, if the vehicle has multiple cats).

This sensor monitors catalyst efficiency and helps in fuel trim adjustments during certain driving conditions.

🔍 Common Causes

✔️ Open heater circuit inside the O2 sensor
✔️ Short to ground or power in the heater wiring
✔️ Blown O2 sensor fuse
✔️ Corroded or damaged O2 sensor connector
✔️ High resistance in wiring or bad ground
✔️ Faulty ECM/PCM driver (less common)

🧰 Diagnostic Procedure (Step-by-Step)

1. Confirm the Code

Scan the vehicle for DTCs.

Make sure P0042 is present.

Check if other O2 sensor or heater codes exist (P0036, P0136, P0420, etc.).

2. Visual Inspection

Inspect the Bank1 Sensor 3 harness for:

Melted wires (often due to exhaust heat)

Frayed or cut wires

Loose or corroded connector pins

Oil or water intrusion

👉 Fix any visible wiring issues first. Clear codes and retest.

3. Check O2 Sensor Heater Fuse

Many cars have a dedicated fuse for O2 heater circuits.

Look for fuses labeled:

O2 Heater

A/F Heater

ETCS

EFI No. 1 / IG2 (varies by manufacturer)

👉 If blown, replace and test again.
If it blows again — wiring short or bad sensor heater.

4. Test the Heater Element Resistance

Unplug Bank 1 Sensor 3 connector.

Using a multimeter, measure resistance across the 2 heater pins.

Typical range: 3 – 20 ohms (varies by manufacturer).

If:

Resistance Reading Meaning

Open circuit (OL) Heater burnt out → Replace O2 sensor
0–1 ohm Shorted heater → Replace O2 sensor
Very high (>30 ohms) Weak heater → Replace O2 sensor
Within spec Heater is OK → Check power/ground

5. Check Power Supply to Heater

Turn key ON, engine OFF.

One pin on the vehicle harness should read battery voltage (12V).

If no power → blown fuse, relay issue, or open wire.

6. Check Ground Control

The ECU usually controls O2 heater ground.

With sensor unplugged, check continuity from the ground pin to chassis ground.

If no continuity, there is an open in the ground circuit.

If ground is OK → suspect ECM driver or intermittent wiring fault.

🔧 Repair Options

✔️ Replace Bank 1 Sensor 3 O2 Sensor

This is the most common fix (burnt heater).

Use OEM or quality aftermarket (Denso/NGK/NTK).

Avoid cheap local sensors—they often cause repeated faults.

✔️ Repair/Replace damaged wiring

Fix melted insulation

Repair broken wires or pins

Ensure proper heat shielding and routing away from exhaust

✔️ Replace blown fuse

Find the root cause → don’t just replace the fuse.

✔️ PCM/ECM Fault (rare)

Replace only after all other options fail and wiring is confirmed good.

🧪 After Repair

1. Clear DTCs.

2. Road test for 10–15 min including:

Idle

Acceleration

Steady cruise

3. Re-scan for codes.

4. Confirm O2 sensor S3 voltage switches slowly (0.6–0.9V steady for downstream .

Welcome to madurai car service 💐🙏🏻.