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BMW P0420: catalyst efficiency below threshold
The rear oxygen sensor shows the catalytic converter isn't cleaning the exhaust enough. Check for upstream causes before replacing the converter.
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Codes on this page
| Code | Definition | Source |
|---|---|---|
| P0420 | Catalyst system efficiency below threshold (bank 1) | Generic OBD-II |
| 121001 | Catalytic converter: efficiency too low (i3 REx) | SI B12 09 14 (PDF) |
P-codes are the generic OBD-II codes any reader shows. Six-character and four-character hex codes are BMW's own, read with a BMW-capable scanner; the wording is BMW's, from the source shown.
What it means
The DME compares the oxygen sensors before and after the catalytic converter. A healthy converter smooths out the rear signal; when the rear signal starts to look like the front one, it stores P0420 (bank 1). Engines with a second oxygen-sensor bank use P0430 for bank 2.
Can I keep driving?
Yes, but find the cause. Usually no change in how the car drives. A converter failing because of misfires or oil burning can get worse, so fix any misfire first.
Usual causes on BMWs
| Cause | Cars most affected | What you notice | Evidence | Repair prices (national average) |
|---|---|---|---|---|
| Aged or damaged catalytic converter | All | Often after a long period of misfires or oil consumption. | — | — |
| Oxygen sensor or exhaust leak | All | A slow rear sensor or a leak ahead of it can give a false reading. | — | — |
| Misfires or oil burning upstream | All; N63 oil consumption is well documented | Fix the cause or the new converter will fail too. | SI B01 21 18 (PDF) | N63 valve stem seals (oil burning / blue smoke) Indie $6,290–$8,208 Dealer $16,673–$21,989 |
| Manufacturing fluid in a new exhaust (i3 REx) | i3 REx | 121001 during the first range-extender run; BMW's bulletin covers it. | SI B12 09 14 (PDF) | — |
A bulletin isn't a recall and doesn't mean your car has the problem; open it for the exact models and build dates. Estimates are national averages built from BMW warranty labor time, regional shop rates and parts prices; open a repair for your ZIP. "Owners paid" is the middle half of sourced prices, shown where we have at least 3.
What to do
- Fix any misfire codes first.
- Ask for a check of the oxygen sensors and for exhaust leaks before replacing the converter.
- Emissions parts can have longer warranty coverage than the rest of the car, so ask a BMW center about your VIN.
Warnings it can bring up
Related codes
Other BMW bulletins that mention it
- SI B18 02 17 Diagnosing selective catalyst reduction (scr) efficiency faults E70, E90 · Dec 2017
- SI B04 23 10 Diagnostic hint: catalytic converter inspection after loss of crankshaft signal E60, E90, E92, E93, F01 and others · Jul 2019
3 of the BMW bulletins we hold mention it in their text; these are the closest matches.
Sources
Bulletins are cited in the tables above. Explanations are ours; where a source is an owner report rather than BMW, it is labelled as one.