P0051 means the engine control module found the heater control circuit of heated oxygen sensor bank 2 sensor 1 at a lower voltage than expected. The heater family runs P0030 through P0064 by sensor position: P0050 is the general circuit code for bank 2 sensor 1, P0051 low, P0052 high. Bank 2 is the cylinder bank that does not contain cylinder 1; sensor 1 is the upstream sensor before the catalytic converter. Volkswagen's specification for the Touareg gives the threshold as heater voltage below 3.26 V while the heater is commanded.
Severity: low. Can you keep driving? Yes. The heater exists to bring the sensor to operating temperature quickly after a cold start. Without it the sensor still works once exhaust heat warms it, so the engine runs in open loop on that bank a little longer, with a small economy and emissions penalty. The check engine light stays on and the vehicle fails an emissions inspection until fixed.
Which sensor is bank 2 sensor 1
On a V6 or V8, bank 1 holds cylinder 1 and bank 2 is the other side; the manufacturer's numbering decides which. On GM V engines bank 1 is the driver side; on many Ford V8s bank 1 is the passenger side; on transverse V6 engines bank 1 is usually the rear bank against the firewall. Sensor 1 sits in the exhaust manifold or the down-pipe before the converter. An inline four-cylinder has no bank 2, so P0051 on a four-cylinder is a translation error unless the engine is a rare split-bank design.
What the module is measuring
The heater is a resistor of a few ohms fed with battery voltage through a fuse or relay and switched to ground by a driver in the module. With the driver on, the module expects the circuit voltage to sit near zero; with it off, near battery voltage. "Low" means the module saw near zero when it expected battery voltage: an open heater, an open feed, a blown fuse, or a heater wire shorted to ground.
FCA's bulletin 18-059-16 (May 2016) lists P0051, O2 sensor 2/1 heater circuit low, among the codes addressed by a powertrain flash on the affected vehicles, which is a reminder that on some calibrations the heater diagnostic itself was too sensitive and the fix was software.
Causes, in the order they are found
An open heater element inside the sensor. Heaters fail more often than sensing elements, and an open heater reads infinite resistance at the sensor's heater pins.
A blown fuse or a failed relay on the heater feed, often shared by several sensors; if two or more heater codes set at once, the feed is the fault.
A heater wire chafed to ground where the sensor harness passes near the exhaust manifold or a bracket.
A connector burned or corroded by exhaust heat and road spray.
A calibration that flags the heater too readily, corrected by a flash, as in FCA's bulletin.
The module driver, last.
A diagnostic order
- Record companion codes. Several heater codes together mean a shared fuse or feed; P0051 alone means this sensor or its wiring.
- Check the heater fuse and relay for the bank 2 sensors.
- Unplug the sensor and measure heater resistance across its heater pins: a few ohms per the specification. Infinite is an open heater and the sensor is replaced.
- Key on, sensor unplugged: measure the feed pin for battery voltage. None means the fuse, relay or feed wire.
- Check the control wire to the module for a short to ground with the module disconnected.
- Check for a software update if the sensor and circuit pass.
- Replace the sensor only if its heater measured open, clear the code, and let the readiness monitors complete.
Frequently asked questions
Can I drive with a P0051 code?
Yes. The oxygen sensor still works once exhaust heat warms it; the engine runs slightly longer in open loop after a cold start. Expect a small economy penalty and a check engine light, and a failed emissions test until it is fixed.
Is P0051 the oxygen sensor?
Often, because the heater inside the sensor is the part that fails open most. But a blown heater fuse, a shared feed, or a heater wire grounded near the manifold sets the same code. Measure the heater's resistance and check the fuse before replacing the sensor.
Which sensor is bank 2 sensor 1?
The upstream sensor, before the catalytic converter, on the cylinder bank that does not contain cylinder 1. Which side that is depends on the engine's numbering; the service manual's diagram is the only reliable answer.
How much does it cost to fix P0051?
A fuse costs almost nothing. An upstream oxygen sensor is a moderate part with a short replacement if it comes out cleanly; seized sensors add time. A wire repair is under an hour.
Related codes
- P0050 / P0052: bank 2 sensor 1 heater circuit and circuit high.
- P0030 / P0031 / P0032: bank 1 sensor 1 heater, the same fault on the other bank.
- P0056 to P0058: bank 2 sensor 2 heater, the downstream sensor.
- P0151 to P0155: bank 2 sensor 1 signal circuit, the sensing element rather than the heater.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.