Hi everyone, I have a 2002 Toyota 4runner SR5 with 117,000 miles. The check engine light came on a few days ago with p0420 and p0171. I recently fixed an exhaust leak and replaced the mass airflow sensor in hopes that that would solve the issues. Instead of solving the codes, 2 more came, p1133 and another p0420 in addition to the earlier issues. I definitely need help, so any suggestions would be amazing.

Thanks,
Charlie

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02 sensorrrrrrrr

Check the AFR and O2 sensors! The AFR(Air-Fuel Ratio) sensor is the one closer to the engine and the O2 sensor is nearer the exhaust. They can be very difficult to remove, especially if you're in the rust belt...

How would I check these sensors? Also, is the AFR sensor the one on the down tube by the pass side foot well?

Thanks for your help,
Charlie

You check the resistance on the sensor. You need a multi-meter for that.

Your code scanner may have some way of checking them out. You would need it to be able to live stream that data and then you need to be able to figure out what it is telling you. If your O2 sensor is bad it should be getting either no reading from it or some really weird data.

Frankly, it isn't easy. You would need either an OBDII reader than can plot the output of the sensor or an oscilloscope... and to know how to interpret that data.

Keep in mind, this is a complicated system. One thing will affect another which will affect another, and another etc... You may have more than one problem, you may have problems somewhere other than the sensors.

But, given that the AFR sensor has likely never been replaced, its a good idea to replace it anyways! You will see better fuel economy, and you'll be able to eliminate it from the equation.

My hunch is that your AFR sensor is not working properly, so your AFR is reading incorrectly. In turn, this could have damaged your catalytic converter(I hope not) OR its causing your O2 sensor to send data back to the computer that makes the computer "think" your cat isn't working properly.

Thanks so much for the in depth response. Sorry for the inexperienced question, but does the AFR sensor look like an o2 sensor on the down pipe? Also, how would a bad AFR sensor damage the catalytic converter?

Thanks again,
Charlie

My answers in BOLD

Thanks so much, I will replace the AFR sensor tomorrow and update with results.

So I replaced the AFR sensor, cleared the codes and drove about 20miles. I checked with my scanner and the original codes are gone but I saw two new ones; p0440 and p0446. What could this be? Thanks for all the help.

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Those codes are specific to the Fuel Evap system. It may be as simple as a charcoal canister replacement, or the entire evap unit may be bad. Not an inexpensive repair.

Try this first
Remove the battery cables and let the truck sit for 20 mins. While you are waiting check your fuel cap for proper vent function. Reconnect the battery cables and drive like normal. See if any codes come back.

Thank you. Wouldn't clearing the codes using the scan tool be just as effective? I will definitely check the gas cap.

Clearing codes does just that. Resetting the ECU forces it re-learn about the vehicle, take a fresh look so-to-speak, which will give you a more clear picture about the health of the vehicle and sensor network.

It may take up to 200 miles for all tests to complete/report.
Good luck

If it is the canister, you can usually source one from a junkyard (get a return policy and keep your receipt). If it's cracked, you can also seal it with FIPG/gasket maker.

I recently had these last 2 codes you have. I purchased a used vacuum purge valve off ebay and it didn't fix the problem. Took it off, tested it, and it was defective. GRR.. So, I bought a new one from advance auto for like $43.00, put it in, and a few hundred miles later, the CEL is still off. On our 2002 4runners, the vacuum valve is located over the rear axle area up by the purge cannister in the same grouping. You have to remove the spare tire to get to it but its not a difficult task. Hopefully your valve is good and you just have a small crack in the charcoal cannister you can patch with jbweld or maybe a hose has split.