3rdGen2002
New member
P0420 Error Code
A few weeks ago I ran into the dreaded P0420 error code on my 3rd Gen. I did a lot of research related to the code and thought it might be useful for others to have the information I found. I can really only support the details mentioned below for the same vehicle year and engine, although this might extend to other engine designations and model years as well. Also as a disclaimer I am not a mechanic, the below assumptions and findings are only based on personal experience.
Vehicle Specs:
2002 4Runner SR5
Engine: 3.4L 5VZ-FE
Story:
I was driving down the road and the check engine light illuminated. On my truck, certain CELs will also trigger the VSC/Trac Off light as well. I checked in a dirt field and the traction control definitely turns off when the CEL comes on, so my assumption is VSC also is also non-functional in the CEL state. I can’t say I understand this design decision on the part of Toyota, but it is enough to make me resolve the CEL codes quickly.
I read the code and it was P0420. A few years ago my upstream air sensor (technically the Air/Fuel sensor) failed and I replaced it and so I assumed that the rear air sensor would need to be replaced to remedy the code. I brought the truck into the dealer to confirm and left with a $3500 estimate to replace the catalytic converter(s) on the Truck. I argued for a few minutes with the dealer about how jumping to the replacement of the cat converter seemed inappropriate, but they assured me they were 100% confident in the diagnosis. They stated a replacement of the rear air sensor would not solve the problem as the problem was the catalytic converters. To make matters worse my state inspection was two weeks away.
Some Background on the Catalytic Setup (cats) on this 4runner:
There are two cats on this model, they are in serial along the exhaust line. They are not in parallel like other V6s. On the engine side of the cats, there is an Air/Fuel Ratio sensor (Bank 1, sensor 1 on this model). On the muffler side of the cats there is an air/oxygen/O2 sensor (Bank 2, sensor 2 on this model). If the forward A/F sensor isn’t getting the right exhaust mixture, it isn’t happy and flags a code. If the rear O2 sensor isn’t getting the right reduction in exhaust particulates after the exhaust passes through the converters it isn’t happy and flags a code. If either of the sensors aren’t reporting back to the computer at all, the computer isn’t happy and flags a code. Some of the codes for each of these configurations are shown below, but note that there are many other things that can go wrong and flag other codes related to the sensors:
P0134 (front) O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)
P0140 (rear) O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
P0420 (rear) Catalytic Converter Inefficiency (Detected at Bank 1 Sensor 2)
P0420 was the code I was flagging. This didn’t make sense to me, as I do mostly highway driving, use high octane fuels, and generally didn’t think that catalytic converters were a high-failure part for these cars.
Repair Strategy:
First I wanted to confirm that $3500 was the true repair cost of a catalytic converter change. The theory the dealer had was that both should be replaced because they couldn’t be sure which converter was the problem. I called a few exhaust shops in the area and got quotes of $400-600 for both, which seemed a lot more reasonable to me. Further the exhaust shops would warranty the parts unlike Toyota.
My working theory for why P0240 was getting flagged was that the rear O2 sensor had some operating problems that were causing the sensor to show an incorrect waveform to the computer which was interpreted by the computer to be catalytic inefficiency, but the sensor wasn’t off enough to trigger a low-voltage condition. A potential cause in my mind was carbon deposition of the sensor over time.
Following this theory I determined that the first step should be to replace the rear O2 sensor. I rationalized that the sensor was 10 years old anyway, and time for replacing, so regardless of the P0240 influence the sensor was time to be changed. I was also afraid that if I paid for a new cat setup without changing the sensor, and the sensor was the problem, the code would eventually return after reset.
Next I looked into pricing and options for replacement of the rear O2 sensor. Quote from the dealer was around $325. This seemed to be a little pricey for something I wasn’t sure would work, so I looked into buying the part and doing the replacement myself or buying an aftermarket part. The part from Toyota was $225 and the PN is 89465-35580. Aftermarket sensors from Bosh are not reported to have good outcomes on this forum and other places, but Denso sensors are supposed to work well. After some research it appeared to me that Denso is the OEM supplier for Toyota for O2 sensors. Further, Denso makes an O2 sensor with the Toyota electrical connection hookup, and the cost on Amazon is very low ($50) Amazon.com: Denso 234-4261 Oxygen Sensor: Automotive
. After some examination I couldn’t find the difference between the OEM and Denso part, leading me to believe that they are identical, save for the Toyota stamp. Amazon comments for the part mentioned above reveal that many folks have resolved the P0240 code with this part swap. Additionally, after crawling under the car I found the O2 sensors to be positioned in a very accessible place (even without a crawler or lift)
Outcome:
I bought the sensor mentioned above and parts to replace it. Before I undertook the install I decided to clear the codes just to be sure that the code wasn’t a fluke. I pulled the battery terminals but apparently removing the ECU fuse works just as well. So I cleared the codes and when I turned the key the code was gone. Drove 25 miles-code was gone, drove 300 miles and the code was still gone, drove 5000 miles and the code was still gone. I never replaced the rear O2 sensor, but will do so as soon as the code comes back (if ever). I rationalized the P0240 code by thinking that it may have been possible that one of the cats became slightly clogged or the rear sensor had a temporary deviation due to build up, throwing the code once, but not consistently. Then after some driving the cause was burned off and after the code was cleared it was never flagged again.
Lessons Learned:
- Basic function of catalytic converter system on the car and associated codes
- A well-rated O2 sensor with good ratings for the 4runner is fairly inexpensive when compared to the ones sold by Toyota (and may even be the same as the OEM part)
- The O2 sensors and cats are pretty accessible
- Don’t take the dealer’s word on a fix (but you already knew that)
- Cat converter replacements from an aftermarket exhaust shop can be as low as $400 with labor
- You are well advised to clear codes before undertaking a repair for CEL issues
A few weeks ago I ran into the dreaded P0420 error code on my 3rd Gen. I did a lot of research related to the code and thought it might be useful for others to have the information I found. I can really only support the details mentioned below for the same vehicle year and engine, although this might extend to other engine designations and model years as well. Also as a disclaimer I am not a mechanic, the below assumptions and findings are only based on personal experience.
Vehicle Specs:
2002 4Runner SR5
Engine: 3.4L 5VZ-FE
Story:
I was driving down the road and the check engine light illuminated. On my truck, certain CELs will also trigger the VSC/Trac Off light as well. I checked in a dirt field and the traction control definitely turns off when the CEL comes on, so my assumption is VSC also is also non-functional in the CEL state. I can’t say I understand this design decision on the part of Toyota, but it is enough to make me resolve the CEL codes quickly.
I read the code and it was P0420. A few years ago my upstream air sensor (technically the Air/Fuel sensor) failed and I replaced it and so I assumed that the rear air sensor would need to be replaced to remedy the code. I brought the truck into the dealer to confirm and left with a $3500 estimate to replace the catalytic converter(s) on the Truck. I argued for a few minutes with the dealer about how jumping to the replacement of the cat converter seemed inappropriate, but they assured me they were 100% confident in the diagnosis. They stated a replacement of the rear air sensor would not solve the problem as the problem was the catalytic converters. To make matters worse my state inspection was two weeks away.
Some Background on the Catalytic Setup (cats) on this 4runner:
There are two cats on this model, they are in serial along the exhaust line. They are not in parallel like other V6s. On the engine side of the cats, there is an Air/Fuel Ratio sensor (Bank 1, sensor 1 on this model). On the muffler side of the cats there is an air/oxygen/O2 sensor (Bank 2, sensor 2 on this model). If the forward A/F sensor isn’t getting the right exhaust mixture, it isn’t happy and flags a code. If the rear O2 sensor isn’t getting the right reduction in exhaust particulates after the exhaust passes through the converters it isn’t happy and flags a code. If either of the sensors aren’t reporting back to the computer at all, the computer isn’t happy and flags a code. Some of the codes for each of these configurations are shown below, but note that there are many other things that can go wrong and flag other codes related to the sensors:
P0134 (front) O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)
P0140 (rear) O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
P0420 (rear) Catalytic Converter Inefficiency (Detected at Bank 1 Sensor 2)
P0420 was the code I was flagging. This didn’t make sense to me, as I do mostly highway driving, use high octane fuels, and generally didn’t think that catalytic converters were a high-failure part for these cars.
Repair Strategy:
First I wanted to confirm that $3500 was the true repair cost of a catalytic converter change. The theory the dealer had was that both should be replaced because they couldn’t be sure which converter was the problem. I called a few exhaust shops in the area and got quotes of $400-600 for both, which seemed a lot more reasonable to me. Further the exhaust shops would warranty the parts unlike Toyota.
My working theory for why P0240 was getting flagged was that the rear O2 sensor had some operating problems that were causing the sensor to show an incorrect waveform to the computer which was interpreted by the computer to be catalytic inefficiency, but the sensor wasn’t off enough to trigger a low-voltage condition. A potential cause in my mind was carbon deposition of the sensor over time.
Following this theory I determined that the first step should be to replace the rear O2 sensor. I rationalized that the sensor was 10 years old anyway, and time for replacing, so regardless of the P0240 influence the sensor was time to be changed. I was also afraid that if I paid for a new cat setup without changing the sensor, and the sensor was the problem, the code would eventually return after reset.
Next I looked into pricing and options for replacement of the rear O2 sensor. Quote from the dealer was around $325. This seemed to be a little pricey for something I wasn’t sure would work, so I looked into buying the part and doing the replacement myself or buying an aftermarket part. The part from Toyota was $225 and the PN is 89465-35580. Aftermarket sensors from Bosh are not reported to have good outcomes on this forum and other places, but Denso sensors are supposed to work well. After some research it appeared to me that Denso is the OEM supplier for Toyota for O2 sensors. Further, Denso makes an O2 sensor with the Toyota electrical connection hookup, and the cost on Amazon is very low ($50) Amazon.com: Denso 234-4261 Oxygen Sensor: Automotive
. After some examination I couldn’t find the difference between the OEM and Denso part, leading me to believe that they are identical, save for the Toyota stamp. Amazon comments for the part mentioned above reveal that many folks have resolved the P0240 code with this part swap. Additionally, after crawling under the car I found the O2 sensors to be positioned in a very accessible place (even without a crawler or lift)
Outcome:
I bought the sensor mentioned above and parts to replace it. Before I undertook the install I decided to clear the codes just to be sure that the code wasn’t a fluke. I pulled the battery terminals but apparently removing the ECU fuse works just as well. So I cleared the codes and when I turned the key the code was gone. Drove 25 miles-code was gone, drove 300 miles and the code was still gone, drove 5000 miles and the code was still gone. I never replaced the rear O2 sensor, but will do so as soon as the code comes back (if ever). I rationalized the P0240 code by thinking that it may have been possible that one of the cats became slightly clogged or the rear sensor had a temporary deviation due to build up, throwing the code once, but not consistently. Then after some driving the cause was burned off and after the code was cleared it was never flagged again.
Lessons Learned:
- Basic function of catalytic converter system on the car and associated codes
- A well-rated O2 sensor with good ratings for the 4runner is fairly inexpensive when compared to the ones sold by Toyota (and may even be the same as the OEM part)
- The O2 sensors and cats are pretty accessible
- Don’t take the dealer’s word on a fix (but you already knew that)
- Cat converter replacements from an aftermarket exhaust shop can be as low as $400 with labor
- You are well advised to clear codes before undertaking a repair for CEL issues
Last edited:
