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Toyota Service
Toyota Service, Toyota Expert
Category: Toyota
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I'm getting a P0303, P0171, P0300. The plugs and coils are

Customer Question

I'm getting a P0303, P0171, P0300. The plugs and coils are less than a year old. I tried a propane test around the air filter, hoses, and intake but couldn't get any reaction. Seems to idle ok when I drive it but my mother in law is concerned and reports sluggishness and roughness.
Submitted: 1 year ago.
Category: Toyota
Expert:  Toyota Service replied 1 year ago.

Hello. Welcome to Just Answer. Please allow me to assist you. Let's discuss the P0171 first. Since you have done the propane trick, we might be abls eo think that you have no air leaks in the induction system. A better and less dangerous was to check this is via carb cleaner. Spray the intake tube form the mass air flow sensor to the throttle body. Then spray the intake manifold area. Normally, a P0171 code is causes by microscopic debris building up on the metal elements on the Mass Air Flow sensor. The technique here is to disconnect the wire connector, remove the screws that hold the sensor in place and remove it. Using a good light, inspect the interior of the sensor. You will see 2 silve wires. This is what the sensor uses to measure the air flow. It there is dust or anything else on them, the air flow reading is compromised. The best way to clean it is spray brake cleaner. Spray the interior liberally and let it air dry. DO not use compressed air to dry the interior. You will ruin the sensor if you use compressed air.

Regarding the P0300 and the P0303, the issue gets a bit more tricky. We have had a host of issues in the shop with this issue, the carb cleaner spray did not detect the problem. Fortunatly, Toyota recognized this issue as a problem and took steps to rectify the issue by redesigning the intake manifold gasket. The recommendation here would be to replace the intake manifold gasket with a new one.

Additionally, when you see a p0300 and a p030x (cylinder) code, note that all 4 coils share the same ground. Cheaper, non-Toyota coils do not have the proper ground circuit isolation (via diodes) to keep the spark form leaking into the ground circuit. You could have a bad #2 coil, but get a cylinder #3 misfire code. ALways use genuine Toyota coils. And, always use genuine Toyota spark plugs. Not a lot of people know this, but non-Toyota plugs do NOT work well in these engines. The truth is that the combustion chambers are designed with a particular plug in mind, and that plug at a specific heat range. Aftermarket spark plugs (Bosch, AC, etc) do not have the rating needed for proper combustion.

I hope this helps you out.

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Customer: replied 1 year ago.
I replaced the intake gasket and cleaned the MAF wires. It seems to run ok so at least I didn't harm anything. How do I reset the codes to confirm?
Expert:  Toyota Service replied 1 year ago.

On this car, it has a memory circuit that is built into the computer. The memory is retained even if you disconnect the battery. The only way to clear the codes is to get a scan tool that is capable of interfacing with the car and use this tool to clear the codes. Most Auto parts stores do scans, perhaps one near you has a scanner that can clear your codes. Disconnecting the battery to 'clear the codes' or 'reset' the system is an internet myth. Any car with OBD-2 diagnostics (all cars after 1996) have this resident memory for codes. A scan tool is needed to clear the codes. The government made this decision, not the car manufacturers. This keeps people from clearing the codes before an emission inspection.

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