PO171 and PO174 are very common codes on all types of vehicles. These codes set when the computer believes that the engine is running to lean on cylinder bank one or bank two. I say believes, because the engine may or may not actually be lean.
One of the engine sensors could be giving the wrong signal to the ECM ( engine computer ) or there can be broken or defective parts near a sensor causing it to give an incorrect reading.
Some problems that could cause a false PO171, PO174 code to set;
- Exhaust leak ahead of the oxygen sensor (s)
- Air leak after the airflow meter
- Cracked or broken air intake boot
- Defective or dirty airflow meter
- Crankcase or PCV ( positive crankcase ventilation ) leaks on breather side
PO171, PO174 Video Ford example
Of course there are times when the engine is actually running lean. This would be where air is entering the engine after the throttle body through a defective gasket or seal or because of a defective part not delivering fuel properly.
Here are some real reasons for a PO171, PO174 code to set;
- Intake manifold gasket leak
- Throttle body gasket leak
- PCV leaks on vacuum side
- Vacuum hose leaks
- Low fuel pressure
- Clogged fuel filter
- Restricted partially clogged fuel injector
- Leaking injector seal ( o-ring )
- PCM not injecting enough fuel because of incorrect sensor readings
PO171, PO174 Video Chevy example
As you can see there can be many different reasons for codes PO171,PO174 to set, either false or real, but fortunately for us we can get some clues as to why the code is setting by using our scan tool.
If your scan tool has freeze frame data, take a look at it to see what the engine was doing at the time of the code setting. It will tell you things like engine temperature and RPM, but more importantly it will tell you fuel trim readings, this number tells you if the PCM is adding fuel to compensate for a lean condition.
Fuel trim numbers like this +20 or +25 are telling you that the PCM thinks there is a lean condition and is adding fuel to compensate. Normal readings are around 0 to +5 or +6, anymore than this indicates a lean condition.
These fuel trim numbers can also help us after the repair to check our work. Just test drive your vehicle for a few miles and check the before and after readings, this number should start to come back into the normal range, if not, you may have more work to do.
Here are some common failures you may see in the field;
- Ford V-8 truck mid 90s to mid 2000s PCV hose soft and spongy and starts leaking ( rear top engine area ).
- Toyota any late 90s to mid 2000s car or truck, defective airflow meter, will not have airflow meter code.
- Ford mid to late 90s V-6 3.8L or 4.2L truck or van, leaking lower intake manifold bolt seals. The plastic lower intake manifold is held down by bolts that have a built in seal, these tend to leak when the engine is cold and then seal up when at normal operating temperature.
Well there you go, this information should help you repair you PO171, PO174 problem. Please tell us about your story in the comments area below.