HO2S Heater Control Circuit Low (Bank 2 Sensor 1)
The P0051 code means the vehicle's onboard diagnostic system has detected: HO2S Heater Control Circuit Low (Bank 2 Sensor 1). This is a moderate severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- moderate
- Estimated cost
- $100 - $350 (O2 sensor $80-$250; labor $50-$150)
Best-selling code reader on Amazon. Reads and clears check engine codes on any 1996+ vehicle. Under $30.
Check Price on AmazonAffiliate link -- we earn a commission at no extra cost to you.
Essential for diagnosing sensor codes, checking voltage, and testing circuits. Under $15 and pays for itself on the first job.
Check Price on AmazonAffiliate link -- we earn a commission at no extra cost to you.
Common Symptoms
- Check Engine Light on
- Poor fuel economy on cold starts
- Engine may run rich during warm-up
- Possible rough idle when cold
- Increased emissions
Probable Causes (Ranked by Likelihood)
- Open heater element in the bank 2 upstream O2 sensor Very High
- Blown O2 heater fuse High
- Broken wire in the heater circuit Moderate
- Corroded or loose connector pins Moderate
- Failed PCM heater ground driver Low
Step-by-Step Diagnostic Procedure
-
Check and replace the O2 heater fuse if blown.
-
Disconnect the bank 2 sensor 1 O2 sensor and measure heater resistance (2-15 ohms typical). Infinite resistance confirms a failed heater element.
-
Verify battery voltage at the heater power wire with the key on.
-
Check the ground circuit continuity from the sensor connector to the PCM (near 0 ohms).
-
Inspect the wiring harness for breaks or heat damage near the exhaust manifold on bank 2.
Common Fixes by Vehicle
What techs usually find when diagnosing P0051 on specific vehicles — tap your vehicle for the fix and the exact part:
2005-2015 Subaru 2.5L EJ25 (Outback, Forester, Legacy) easy DIY
Replace Bank 2 Sensor 1 upstream O2 sensor. Subaru horizontally-opposed engines run hot on the passenger side -- B2S1 fails first. Use Denso or NGK direct-fit. Check the 20A REAR WIPER / O2 fuse in the under-hood box; Subaru shares O2 heater power with accessory circuits.
Labor: 30 min2003-2014 Nissan 3.5L VQ35 (Murano, Maxima, Altima, 350Z) easy DIY
Replace Bank 2 (passenger side, firewall-side on FWD) upstream AFR sensor. VQ35 pre-cat failure contaminates B2S1 -- check pre-cat honeycomb condition; if broken, replace cats before wasting another sensor.
Labor: 45 min2007-2017 Ford 3.5L/3.7L V6 (Edge, Flex, Taurus, Explorer) easy DIY
Check the O2 heater fuse in the engine compartment box. Ford 3.5/3.7 commonly pops the B2 heater fuse when the heater circuit shorts. If fuse blows immediately on re-install, the sensor itself shorted -- replace Motorcraft DY-1180.
Labor: 30 minFrequently Asked Questions
Can I drive with code P0051?
You can drive short distances. The upstream O2 sensor on bank 2 will eventually warm up from exhaust heat, but it takes several minutes. During that time, the engine runs in open loop for that bank, wasting fuel.
What does low circuit mean?
Low circuit means insufficient current is flowing through the heater. This is usually caused by an open circuit -- a burned-out heater element, broken wire, or blown fuse that prevents current from reaching the heater.
How do I find bank 2 sensor 1?
Bank 2 sensor 1 is the upstream O2 sensor on the bank opposite cylinder 1. On most V6 and V8 engines, it is located on the exhaust manifold or downpipe before the catalytic converter on the passenger side.
Sources
This page is built from documented references. Verify against your own service info before repair work.
Explore More
Was this helpful?
Share carcodefinder.com with a friend who needs to decode their check engine light.