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You checked the fuel pressure? What is it. P0171 says system lean, this can be a fuel pressure issue or lack of it, it can be a defective O2 sensor or wiring or an engine vacuum leak at the intake manifold or any vacuum hose.
In Toyotas is a common code when the customer use aftermarket air filters and particles of this filters get attached and melted in the hot wire of the Mass air flow meter, also it could be a vacuum leak for example when somebody check the air filter and do not connect the little vacuum lines that creates a vacuum leak, Bad fuel and low fuel pressure that also creates a p0171, if you tell me what kind of toyota you working on I can help you better.
Well an engine vacuum leak can cause the P0171 code to come on and having an oil cap missing is a vacuum leak. The first response for this code is to replace the O2 sensor because that is what the code is for...bad O2 sensor reading. Now too much air from a vacuum leak can cause a lean condition in the exhaust. If they are going to replace the O2 sensor for free then let them..If you trust them to replace it. It is possible that the lean condition could have damaged the O2 sensor. You may find that after a few days of driving with the cap back on the check engine light might go back off. I do not think anaything would be permanently damaged as long as the oil level did not get too low and nothing got into the engine thru the open hole in the valve cover. Does the car seem to be running ok? No overheating, miss, ect.
If a car has a check engine light on and has P0171 and P0174 codes stored, the most likely cause is a vacuum leak. These codes are set by the car’s computer when it sees too much oxygen in the exhaust. Excessive oxygen readings in the exhaust will set lean codes. This does not mean that the oxygen sensors are defective. It is a common mistake for people to replace oxygen sensors when lean codes are present. Just because the code is related to oxygen sensor readings, it doesn’t mean that the oxygen sensors are bad. If both of these codes are stored this means that both banks of the engine have excessive oxygen in the exhaust. If the oxygen sensors are replaced it’s most likely that the same codes will reappear. It is very uncommon for two oxygen sensors to fail at the same time.
Therefore the most likely cause of P0171 and P0174 codes is a vacuum leak. The source of a vacuum leak can be one of many things. The most common cause of a vacuum leak is a leaky hose. Vacuum hoses over time can become dry and brittle. The rubber deteriorates and can leak causing a vacuum loss. Many times a leaky vacuum hose can be found by listening for a hissing sound. A large vacuum leak can even make the car run rough at idle. In some cases it will not idle without feathering the gas pedal.
The P0420 code is a Catalytic converter efficiency code check the compression.Swap the #1 injector to a different location to see if the code follows.Test the coilpak. A cracked exhaust manifold is the most likely cause p0171 and 0174 Induction air leaks, vacuum or some kind fuel restriction, weak fuel pump, restricted fuel filter etc.You also could have an exhaust leak near the upstream o2 sensor. If youtake it in they will probably use smoke to find out where the leak is.
P0171 is system too lean bank 1, P0174 is too lean bank 2, and 302 is misfire bank 1. Look for vacuum leaks. If you don't find them, put a vacuum gage on it can check. The P0171 is almost always a vacuum leak. If not other possibilities are front O2 sensor is too old and should be replaced (if mileage is greater than 100k then replace). Try injector cleaner. Misfires are usually bad spark plugs, but could possibly be caused by insufficient fuel. I've never had a car of mine in bad enough shape to have that error.
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