P0051 Code: O2 Sensor Heater Control Circuit Low (Bank 2 Sensor 1)
The bank 2 upstream sensor heater draws no current: open element, lost feed or broken wire.
At a glance
Severity: Low
- Meaning
- O2 Sensor Heater Control Circuit Low (Bank 2 Sensor 1)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $0–$600
- DIY difficulty
- Moderate
P0051 means the engine computer commanded the heater of the bank 2 upstream sensor on and found an open circuit: no current, or a control-wire voltage showing nothing is connected. Because bank 2 exists only on V, flat and some twin-manifold inline engines, this sensor is often on the side of the engine that is hardest to reach, and that shapes the diagnosis. Instead of removing parts to reach the sensor first, it pays to start where access is easy – the fuse box, the scan tool and the engine computer connector – and work toward the sensor only once the open has been narrowed down. The usual end result is still a burned-out heater element, but on this bank connectors disturbed during earlier work at the back of the engine, and wiring chewed by rodents or cracked by heat, turn up more often than on bank 1.
Can I keep driving? Driving is fine. Bank 2 gets no mixture correction until the engine is warm, so fuel use on short trips rises slightly.
How to read P0051
- P Powertrain
- 0 Generic (SAE)
- 0 Fuel/air & emissions (aux.)
- 51 Specific fault
Symptoms
- Check engine light soon after every cold start, only the bank 2 heater flagged
- Bank 2 upstream sensor never reports active on short trips
- Code appeared after rear spark plugs, intake plenum gaskets or valve cover work
- Chewed insulation, nest material or droppings under the hood
- Bank 2 sensor circuit code (P0150) logged at the same time
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Heater element burned open in the bank 2 upstream sensor
- Connector unplugged or damaged during work at the back of the engine
- Wiring chewed by rodents or heat-cracked near the rear exhaust manifold
- Blown bank 2 heater fuse on engines with one fuse per bank
- Terminal backed out at the engine computer connector
- Heater driver in the engine computer failed open
Diagnosis step by step
Start with the scan tool and fuse box
Confirm in live data that only the bank 2 heater is off or faulted and that bank 1 works. Then check the heater fuses; some V engines use a separate fuse for each bank, so a good bank 1 doesn't clear the bank 2 fuse.
Tools: OBD-II scanner, Multimeter
Test at the computer before digging for the sensor
Back-probe the bank 2 heater control pin at the engine computer with the engine idling cold. A healthy circuit shows battery voltage that drops to near zero whenever the heater is switched on. If the pin shows no voltage at all, the break is somewhere between the fuse and the computer, heater included.
Tools: Multimeter
Measure the whole loop from one end
Disconnect the battery, unplug the engine computer and pull the heater fuse. Measure from the load side of the fuse cavity to the control pin: you should see the heater resistance plus a fraction of an ohm. OL means an open somewhere in the loop, which you can now split at the sensor connector.
Tools: Multimeter
Reach the sensor connector and split the circuit
At the connector, measure the heater on the sensor side. A few ohms means the sensor is fine and the break is in the harness; OL means the element is gone. On transverse engines the connector may be reachable from under the car even when the sensor itself isn't.
Tools: Multimeter, Flashlight
Look for rodent and heat damage
Along the harness at the back of the engine, look for chewed insulation, nesting material and brittle, cracked sections near the manifold. Repair with sealed splices and heat-rated wire, and remove any nest so it doesn't happen again.
Tools: Flashlight, Pliers
Replace and reassemble carefully
If the sensor is open, replace it and make sure every connector disturbed on the way in is fully latched, including coil and injector plugs. Clear codes, cold-start and check that bank 2 goes active as quickly as bank 1.
Tools: O2 sensor socket, Torque wrench, OBD-II scanner
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace bank 2 heater fuse | Yes | $1–$15 |
| Reconnect or repair a connector disturbed during earlier work | Yes | $0–$150 |
| Repair rodent-damaged engine harness | No | $150–$450 |
| Bank 2 upstream sensor, part only | Yes | $80–$270 |
| Bank 2 upstream sensor replaced at a shop | No | $250–$600 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
A check engine light for P0051 means an automatic fail on an OBD-II emissions test. After the fix, allow several cold starts so the bank 2 sensor and heater monitors report complete before the test.
Frequently asked questions
Why test at the engine computer before going to the sensor?
On many V engines the bank 2 sensor takes real work to reach. A few measurements at the fuse box and computer connector tell you whether the loop is open at all and roughly where, so you only dig in once you know the sensor or its wiring is the problem.
P0051 appeared after my rear spark plugs were changed. What should I check?
On many transverse V6 engines the intake plenum comes off for that job, and the bank 2 sensor connector and harness sit right underneath. Check that the connector is latched and that the harness wasn't pinched when the plenum was bolted back on.
Can rodents really cause P0051?
Yes. Rodents like the sheltered, warm space at the back of an engine and chew wiring insulation. A chewed heater wire gives exactly this open-circuit code. Look for droppings or nest material near the firewall.
I have P0051 and P0031 together. Two bad sensors?
Unlikely. Two heaters failing at the same moment usually means they lost a shared supply: a common fuse, heater relay or engine computer power feed. Check that first.