99 Tacoma check engine light code, P0171?? any ideas?

Hey,

I made it by Advance today when it came on and the code P0171 came up with something about lean in bank one, I think. The code was definitely P0171 and he said it could be the Maf sensor was bad or just dirty, so I bought some cleaner for that and it was dirty. So now I'll drive it and see if it comes back on.

Does anyone have a better interpretation of that code?

Thanks,

Scott..

Reply to
Scott
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If your truck has the 4-cyl., it only has Bank 1, and this makes sense that the MAF could be the trouble. Another thing that can make a lean is a vac leak and an errant O2 Sensor. Cleaning and looking for the leak are the proper approaches. Spending money on a sensor is the wrong approach. RayO has a test procedure to check the sensor output if it comes to the O2 Sensor as the fault.

If you have a V6, I'd expect the MAF to contribute to a lean in Bank 1 and Bank 2, which would discount it as part of your problem -- you did not report a lean condition in Bank 2.

Reply to
Jeff Strickland

It only showed bank one as being lean and yes it is a V6, forgot that part. This light comes on every once in a while and can be reset by pulling the efi fuse, after that I might not see it for a few days.

Thanks for the help,

Scott..

Reply to
Scott

Bank 1 is the bank containing the #1 cylinder. Since we are now talking about a V-engine instead of an inline, I'd start looking for cracked hoses and that sort of thing that are connected to the appropriate side of intake manifold.

A false report of a lean condition can arise from an O2 Sensor, so you can't discount that possibility yet.

The Scan Tool has a reset function ...

Reply to
Jeff Strickland

The first thing I would do is check the bank 1,sensor 1, O2 sensor for proper operation, especially if you have high mileage. It may just be a worn out O2 sensor.

Reply to
Mike

What type of checks can be done to ensure that it is or is not the O2 sensor?

Reply to
Scott

Easiest way would be to check it with a scan tool but you can also check it with a digital multimeter. Do you have access to either one ?

Reply to
Mike

Yes, I have a digital multimeter.

Thanks for the help,

Scott..

Reply to
Scott

My '99 kept throwing the same code every once in a while. I cleaned the MAF sensor 2 weeks ago and it has not thrown the code again. It also drives better- it did have a flat spot on acceleration and a little surge just after start up. Time will tell. (I also added some injector cleaner as I filled the tank.)

Reply to
Anyolmouse

I had the same code on my 2000 pre-runner v-6 last year. it was the mass air flow sensor. I paid $192 for a new sensor. Was easy to change. First failed part in 8 years ownership. Scott

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Reply to
zonie

For sensor 1 bank 1 the signal wire should be the white wire, you need to backprobe this wire in the connector with the red lead from your meter. Connect the black lead to a good ground. Set meter to DC voltage scale. Perform the following test.

Testing O2 sensors that are installed.

The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop operation. Attach the positive lead of a high impedance DC voltmeter to the Oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Check the specs for your meter to find out. Set your meter to look for 1 volt DC. Many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire.

When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check your connections. Next, check your leads to make sure you won't wrap up any wires in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes the sensor as hot and active once there are several cross counts.

You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through, your sensor is good. If not, is it steady high (> 0.45) near 0.45 or steady low (< 0.45). If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. If the reading is steady low, add richness by partially closing the choke or adding some propane through the air intake. Be very careful if you work with any extra gasoline, you can easily be burned or have an explosion. If the voltage now rises above 0.7 to 0.9, and you can change it at will by changing the extra fuel, the O2 sensor is usually good.

If the voltage is steady high, create a vacuum leak. Try pulling the PCV valve out of it's hose and letting air enter. You can also use the power brake vacuum supply hose. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good.

If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor.

Reply to
Mike

I have the same truck and solved the problem by cleaning the MAF and ran some fuel injector cleaner through the engine on a two hour trip. Engine light has been gone for 6 weeks. I also checked for an air leak (air induction system) between the MAF and engine including any lose or cracked vacuum lines (nothing found). The repair manual says to check the following (in order): P0171 System too Lean (Fuel Trim) Air induction system Fuel pressure Injector blockage Heated oxygen sensor (bank 1 sensor 1) Mass air flow meter Engine coolant temp. sensor

I didn't think there was a problem with the fuel pressure as high RPM power wasn't a problem. I went with the cheapest fixes first.

Paul.

Reply to
Paul.

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