P0051 – O2 Sensor Heater Control Circuit Low (Bank 2 Sensor 1).

This fault means the upstream oxygen sensor (B2S1) heater is not drawing enough current — usually caused by high resistance, open circuit, or low supply voltage.

✅ Meaning of P0051

Bank 2 – Sensor 1 (upstream)
Heater circuit voltage is lower than expected → ECM detects low current flow.

✅ Common Causes

Most likely (top causes in garages)

1. Heater element open inside the O2 sensor
2. Blown O2 heater fuse
3. High-resistance wiring or corroded connector
4. Poor ground for O2 heater
5. Low battery voltage during warm-up
6. ECM driver failure (rare)

✅ Pinpoint Diagnostic Procedure (Step-by-Step)

Use a multimeter, test light, and wiring diagram.

1. Identify the correct sensor

📍 Bank 2 Sensor 1 = upstream O2 sensor on the cylinder bank without cylinder #1

2. Check heater fuse

Most cars use a fuse labeled:

O2 HTR, HTR A/B, A/F heater, or EFI fuse

If blown → replace fuse + check wiring for short.

3. Check heater power supply at the sensor connector

Unplug the O2 sensor, then:

Red/white or black wire = 12V heater feed

Key ON

Should see Battery voltage (11.5–14V)

❌ If no voltage:

Open fuse

Broken wire

Relay fault

Power supply issues

4. Check heater ground control from ECM

The ECM usually grounds the heater to control it.

Set multimeter to continuity check:

ECM control wire → chassis ground
Should NOT show direct ground (because ECM pulses it), but should show 1–10Ω to ECM.

❌ If open circuit → broken ground wire

5. Measure heater resistance of the O2 sensor

Typical O2 heater resistance:

Sensor Type Normal Resistance

Zirconia (Most cars) 4–12 Ω
A/F Ratio Sensor 1.5–4 Ω

🛠 Test:
Measure across the two heater pins (not signal wires).

❌ If resistance = Open circuit or >20Ω → sensor heater is bad → replace sensor

6️⃣ Check wiring for high resistance

Tug test along the harness

Inspect for burnt, melted, or oil-soaked cables

Check connectors for corrosion or loose pins

Fix any damaged section.

7. Clear codes & perform warm-up test

Start engine cold and watch live data:

Parameter: O2 Heater Current / O2 Heater Status
Should switch to ON within 10–20 seconds.

✅ Most Common Fixes

1. Replace O2 sensor (Bank 2 Sensor 1) – 70% of cases
2. Repair/replace wiring to sensor – 20%
3. Replace blown heater fuse – 8%
4. ECM issue – <2% (very rare)

Welcome to madurai car service 💐🙏.