P1151 is a diagnostic trouble code that applies to vehicles manufactured by Ford, in this case the C-Max. This code occurs when the upstream O2 sensor on bank 2 fails a diagnostic test twice in a row.
DTC P1151 is usually caused by a bad oxygen sensor, fuel injector, or vacuum leak. But, there can be other causes as well.
….it’s most likely the oxygen sensor itself that has caused P1151.
P1151 Definition
Ford chose P1151 to indicate “Lack of Upstream Heated Oxygen Sensor Switch | Lean Bank 2“
Upstream– The upstream oxygen sensor is the sensor before your C-Max’s catalytic converter. It’s usually bolted to the exhaust manifold or right after it.
Bank 2– Bank 2 is the part of the engine with the second cylinder. It is only applicable to engines with a “V” configuration. Bank 2 is easy enough to find. Here’s a short article on how to find which side of the engine has bank 2. It’ll open in another tab.
Once you find it come back here and we’ll try and figure out what’s causing P1151 in your C-Max.
P1151 Symptoms: Ford C-Max
Typically, there are not going to be any symptoms associated with P1151 (other than the check engine light).
P1151 Causes
- Bad O2 Sensor (Bank 2 Sensor 1): See above if you need help locating it.
- Vacuum Leak
- Fuel System Issue: The fuel pressure could be too high or low. Or there could be a clogged fuel injector.
- Bad Mass Airflow Sensor
- PCM (The vehicle’s computer)
Ford C-Max P1151 Diagnosis and Fix
There are some things that you can do at home to detect what is causing P1151 and repair it. Here they are in order of likelihood/ease to check on your C-Max. Certainly engine and model year are going to change that a little bit, so take the order with a grain of salt.
Bad O2 Sensor Wiring
The upstream oxygen sensor on your C-Max is easy enough to find. Once you know where bank 2 is, just follow the exhaust from the manifold until you find the oxygen sensor.
Inspect the wiring harness to see if it has been damaged in any way. Look at the contacts where they plug into the sensor itself. Due to the location of an oxygen sensor on hot exhaust components, the harness fails more than a typical wiring harness would.
Bad Oxygen Sensor
P1151 is triggered because the oxygen sensor failed a test. Specifically, a test is designed to make sure that it has the proper output voltage and response rate. It must fail this test twice in order for the check engine light to come on.
With that in mind, knowing that the oxygen sensor has failed this test, it’s most likely the oxygen sensor itself that is at fault. Here’s more on bad oxygen sensors in the C-Max.
Vacuum Leak
The system could have a vacuum leak. Determining where a vacuum leak is can be easy. Here’s a great video on exactly how to do that.
Fuel System
The fuel system itself can cause P1151.
The fuel pressure could be too low. This would be caused by a clogged fuel filter, weak fuel pump, or bad fuel pressure regulator. But, If the fuel pressure is too low, you’d likely get P1131 (same code, just the other side of the engine) as well.
With the above in mind, if the fuel system is to blame, it’s likely an issue with a fuel injector being clogged.
A clogged fuel injector can lean the mix out on bank 2, without affecting bank 1.
Ford C-Max P1151 Wrap Up
As you can see, P1151 can be caused by a myriad of issues. It’ll more then likely be a bad oxygen sensor, fuel injector, or vacuum leak that is causing it.
Good luck diagnosing your C-Max. If there’s anything you’d like to add, please drop a comment.