3 ms·

Anything made after ‘96 has an o2 sensor downstream of the cat to verify the cat is working. If it stops working the ecu will toss a p0420 code and turn on the

Anything made after ‘96 has an o2 sensor downstream of the cat to verify the cat is working. If it stops working the ecu will toss a p0420 code and turn on the check engine light.

mindslight 5y ago
Is it physically downstream on another part though? It's been a while since I've been under a car but I remember seeing what I thought was the O2 sensor, and it was part of the whole catalytic converter assembly. If it comes as part of the same assembly, it would be easy enough for new ones to include a fake O2 sensor.
- mgarfias 5y ago
  It screws into either the cat, or the pipe right behind it. But it’s a separate part.
sokoloff 5y ago
Those downstream checks are easily defeated in most cars. They work fine as a signal that the converter isn’t working iff you’re like most people and not trying to defeat them. If you’re trying to defeat them, a simple RC or other timer circuit on the signal pins and enough current flow on the other pins to stand in for the heater (if equipped) will result in the ECU concluding everything is fine. Presumably someone trying to work around a catalytic converter issue would be more likely than average to fall into this category.
- asguy 5y ago
  That’s true for many early OBD-2 cars, but anything reasonably modern (e.g. 2006) correlates signals between upstream and downstream O2 sensors. Most workarounds you see on the market are patched ECU firmware to fib a scaled second O2 sensor value based on the first, so all of the diagnostic tests keep running. You’re seeing cars fail smog with these workarounds because states like California have started checking calibration IDs against the manufacturer’s listed values at the time of production.
- mgarfias 5y ago
  Oh sure. But if you’re trying to defeat them, just take a broom handle to the cat and smash the matrix out. Put it back in, and put the o2sims in (or turn off the p0420 code with some tuning software, which is what I did with my race car when I pulled the cats).
dylan604 5y ago
Does the p0420 code get triggered if you smoke a doobie in/around the car?
- mgarfias 5y ago
  No.

Anything made after ‘96 has an o2 sensor downstream of the cat to verify the cat is working. If it stops working the ecu will toss a p0420 code and turn on the check engine light.

mindslight 5y ago

Is it physically downstream on another part though? It's been a while since I've been under a car but I remember seeing what I thought was the O2 sensor, and it was part of the whole catalytic converter assembly. If it comes as part of the same assembly, it would be easy enough for new ones to include a fake O2 sensor.

mgarfias 5y ago
It screws into either the cat, or the pipe right behind it. But it’s a separate part.

sokoloff 5y ago

Those downstream checks are easily defeated in most cars. They work fine as a signal that the converter isn’t working iff you’re like most people and not trying to defeat them. If you’re trying to defeat them, a simple RC or other timer circuit on the signal pins and enough current flow on the other pins to stand in for the heater (if equipped) will result in the ECU concluding everything is fine. Presumably someone trying to work around a catalytic converter issue would be more likely than average to fall into this category.

asguy 5y ago
That’s true for many early OBD-2 cars, but anything reasonably modern (e.g. 2006) correlates signals between upstream and downstream O2 sensors. Most workarounds you see on the market are patched ECU firmware to fib a scaled second O2 sensor value based on the first, so all of the diagnostic tests keep running. You’re seeing cars fail smog with these workarounds because states like California have started checking calibration IDs against the manufacturer’s listed values at the time of production.
mgarfias 5y ago
Oh sure. But if you’re trying to defeat them, just take a broom handle to the cat and smash the matrix out. Put it back in, and put the o2sims in (or turn off the p0420 code with some tuning software, which is what I did with my race car when I pulled the cats).

dylan604 5y ago

Does the p0420 code get triggered if you smoke a doobie in/around the car?

mgarfias 5y ago
No.