[KnightFiyah]

So I've been troubleshooting the CEL my car's been throwing recently. It's the dreaded P0420 that everyone gets and I've done my research about replacing both sensors and have done exactly that.

However I bought an OBD II bluetooth scanner to check my O2 Data and see what the graph of the upstream sensor was showing versus the downstream sensor (to see if the CAT was really just going bad) and I only get data for the downstream one; it's like the upstream A/F sensor isn't even there. And it's definitely there lol, I just checked.

Going back through my car's previous maintenance history before I owned it, I noticed that the A/F ratio sensor was replaced at 173K. I replaced it at 183K.

I'm trying to figure out what I'm missing here.

Is it possible that my spark plugs or air filter just haven't been replaced in a while and are clogging up my sensor? I just checked my air filter and it seems fine but have yet to take a look at my spark plugs.

I'm buying a voltmeter tomorrow to test the resistance in my upstream sensor to make sure it's working and buying new spark plugs as well since they're probably due for a replacement anyways.

For reference: (hopefully I didn't buy the wrong parts)
I replaced the Air/Fuel Ratio Sensor @ 183K with Denso Part #234-9002.
I replaced the O2 Sensor @ 195K with Denso Part #234-4155.

Hoping you guys can give me some ideas as to what's going on. Thanks in advance.

[TheDurk]

Originally Posted by KnightFiyah

So I've been troubleshooting the CEL my car's been throwing recently. It's the dreaded P0420 that everyone gets and I've done my research about replacing both sensors and have done exactly that.

However I bought an OBD II bluetooth scanner to check my O2 Data and see what the graph of the upstream sensor was showing versus the downstream sensor (to see if the CAT was really just going bad) and I only get data for the downstream one; it's like the upstream A/F sensor isn't even there. And it's definitely there lol, I just checked.

Going back through my car's previous maintenance history before I owned it, I noticed that the A/F ratio sensor was replaced at 173K. I replaced it at 183K.

I'm trying to figure out what I'm missing here.

Is it possible that my spark plugs or air filter just haven't been replaced in a while and are clogging up my sensor? I just checked my air filter and it seems fine but have yet to take a look at my spark plugs.

I'm buying a voltmeter tomorrow to test the resistance in my upstream sensor to make sure it's working and buying new spark plugs as well since they're probably due for a replacement anyways.

For reference: (hopefully I didn't buy the wrong parts)
I replaced the Air/Fuel Ratio Sensor @ 183K with Denso Part #234-9002.
I replaced the O2 Sensor @ 195K with Denso Part #234-4155.

Hoping you guys can give me some ideas as to what's going on. Thanks in advance.

1. The A/F sensors use a different PID than the O2s, so it is possible you need to play with your scanner.

3. Voltage is useless on an A/F anyway, it is constant out of the EcU, then the sensor varies the current.

2. It is rare that the front sensor is the cause pf a P0420. Exhaust leak and rear O2 are more common.

3. resistance test is kind of useless--it only shows if the heater is OK, and the EcU will catch that anyway and set a code. The sensor itself can still degrade while heater is showing perfect resistance. You would not expect a heater issue to cause a standalone P0420.

check carefully for exhaust leaks. Very common on our trucks as metal gaskets have turned to dust.

If not, you may indeed have dead cats.

(Sorry, my cap c and cap r are broken)

[KnightFiyah]

Originally Posted by TheDurk

1. The A/F sensors use a different PID than the O2s, so it is possible you need to play with your scanner.

3. Voltage is useless on an A/F anyway, it is constant out of the EcU, then the sensor varies the current.

2. It is rare that the front sensor is the cause pf a P0420. Exhaust leak and rear O2 are more common.

3. resistance test is kind of useless--it only shows if the heater is OK, and the EcU will catch that anyway and set a code. The sensor itself can still degrade while heater is showing perfect resistance. You would not expect a heater issue to cause a standalone P0420.

check carefully for exhaust leaks. Very common on our trucks as metal gaskets have turned to dust.

If not, you may indeed have dead cats.

(Sorry, my cap c and cap r are broken)

Ahh, okay so maybe my scanner just isn't equipped for reading A/F sensors. That makes me feel a little bit better.

So if my rear O2 graph is reading what the upstream sensor should be showing (and it's not throwing any other codes) that must mean that both sensors are fine then right? That it has to be something with my cat or exhaust?

I've never had to check a car for exhaust leaks so if you could describe exactly what I should be looking for that would be great. Hoping there's a write-up somewhere about replacing the metal gaskets in the event I suspect an exhaust leak or that it's fairly straightforward.

Thanks again man.

EDIT***: Never mind, just watched a few videos on YouTube for how to go about checking for leaks. Are there certain parts of the exhaust that are notorious for leaking on 3rd gens? Figured it might help where I look first.

[TheDurk]

Originally Posted by KnightFiyah

Ahh, okay so maybe my scanner just isn't equipped for reading A/F sensors. That makes me feel a little bit better.

So if my rear O2 graph is reading what the upstream sensor should be showing (and it's not throwing any other codes) that must mean that both sensors are fine then right? That it has to be something with my cat or exhaust?

I've never had to check a car for exhaust leaks so if you could describe exactly what I should be looking for that would be great. Hoping there's a write-up somewhere about replacing the metal gaskets in the event I suspect an exhaust leak or that it's fairly straightforward.

Thanks again man.

EDIT***: Never mind, just watched a few videos on YouTube for how to go about checking for leaks. Are there certain parts of the exhaust that are notorious for leaking on 3rd gens? Figured it might help where I look first.

Uhhhh.....yeah, the joints where the gaskets are. Look only upstream of the downstream sensor.

[KnightFiyah]

Originally Posted by TheDurk

Uhhhh.....yeah, the joints where the gaskets are. Look only upstream of the downstream sensor.

Sweet. Thanks!

[speed metal]

I know this is an old thread but I have the same truck with the same problem, you ever figure it out? I replaced the downstream sensor and didn't get the 0420 code for four months, now its back. Upstream sensor is like $130, that one does not go bad as much so I'm hesitant to gamble on buying it.