Getting my 1996 Ford Ranger ready to pass a state emission inspection, I had to diagnose Ford trouble codes P0155 and P1151.

P0155 O2 Sensor Heater Circuit Bank 2 Sensor 1

What are the common causes of code P0155?​

  • Problem on the ground side of the O2 sensor heater circuit​
  • Problem on the power side of the O2 sensor heater circuit​
  • O2 sensor heater element has high resistance​
  • O2 sensor heater element has an open circuit​

P1151 Lock of HO2S21 Switch – Sensor Indicates Lean

P1151 FORD Possible Causes​

  • Faulty Upstream Oxygen Sensor Bank 2 (P0155 above?)​
  • Fuel pressure high or low​
  • MAF sensor dirty/defective​
  • Vacuum leak on engine​
  • Leaking fuel injector or fuel pressure regulator​
  • Faulty Powertrain Control Module (PCM)​

O2 Sensor Bank Locations:

The trouble code indicated that the bad O2 sensor was in Bank 2 Sensor 1 which placed it on the drivers side.

Making The Repair:

Replacing an O2 sensor is a cheap and simple process. The O2 sensor takes a 22mm or 7/8-inch wrench. I actually had a 7/8-inch line wrench that worked perfectly. Some people use a regular wrench and just slide it over the plug, or cut the plug off since the new O2 sensor comes with a plug.

Your new O2 sensor should have some anti-seize on the threads. If not, make sure you add some.


Although the exhaust and O2 sensor was rusty looking, I was surprised at how easy it came out.

Here’s the new O2 sensor in after adding a dab of anti-seize to the threads.

It’s Fixed!

After replacing the O2 sensor and clearing the codes, I took the Ranger for a test drive and the code didn’t come back.

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