Coated undercarriage with fluid film, now got check engine light :(

2006 corolla. Coated undercarriage in black fluid film.

Now get this error:

P0420 - Powertrain
Catalyst system efficiency below threshold (bank 1).

From what I can guess, this could be from coating an O2 sensor in fluid film? Would rubbing it off these sensors (not even sure where they are or what they look like) fix it? Does “bank 1” indicate what sensor it is? Anyone know where this sensor is located? I’ll try to remove the fluid film from it if I can locate it.

The code indicates the cat is no longer working.

And to make sure this is true, a shop would connect a scanner to the vehicle and monitor the upstream/downstream O2 sensors.

If there’s fluid on the underside of the vehicle, it indicates something is leaking while the vehicle is being driven.

The fluid under the vehicle wouldn’t effect the O2 sensors…

If it did, you;d also have an O2 sensor code.


This is the guy with the junk corolla that he’s welding back together. He coated the undercarriage in lanolin for some odd reason.

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The only thing ive done between that error code is presser washing dirt / dust on the undercarriage, scraping off the rust, then putting a thick coat of fluid film. I have a hunch I put a fat glob of fluid film on the sensor which is causing this to do this, ill try and clean the sensor tomorrow and see if it fixes it.

Just for the smell

It won’t. The sensor portion is INside the exhaust. The electrical portion is sealed to water. No amount of lanolin will harm it.

Leave it alone for a week or two, the problem could go away. Oil on the oxygen sensors or sensor wires can cause slow or inactive oxygen sensor readings. Oxygen sensors require a clean air reference to function properly.

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“bank” refers to the exhaust bank. On v-configured engines (V6, V8) there’s two exhaust system banks. You have an inline engine probably, only has one exhaust bank. Since your car is 2006, you have OBD II technology, so there’s two o2 sensors, both on the same bank, one before the cat, one after. As mentioned above, o2 sensors (not sure if this applies to every type), but the basic versions must have access to outside air to work correctly. If that air path from the outside to the inside of the sensor gets coated w/oil, the sensor won’t be able to measure the o2 content of the exhaust gasses accurately.