What is Maruti Suzuki P0420?
P0420 means the catalytic converter is not efficiently converting harmful gases (HC, CO, NOx) into safe ones. The ECM monitors the upstream and downstream oxygen sensors — if the downstream sensor signal mirrors the upstream too closely, it means the cat is not working. On Maruti K-series and G-series petrol engines this is commonly caused by a failing cat after 80,000+ km, but can also be triggered by a faulty downstream O2 sensor.
Symptoms of P0420 on Maruti Suzuki
MIL illuminates — P0420 is a purely emissions-related code.
Vehicle will fail PUC/emission inspection with P0420 active.
Slight reduction in performance if catalytic converter is severely restricted.
Rotten egg smell from exhaust if cat is failing.
Common Causes of Maruti Suzuki P0420
How to Fix P0420 on Maruti Suzuki
Before replacing the catalytic converter (which is expensive), test both O2 sensors with a live data scanner. The upstream sensor should oscillate rapidly; the downstream should be stable. If downstream is oscillating like upstream, the cat is failing. If downstream signal is flat (dead sensor), replace O2 sensor first — this resolves P0420 in about 30% of cases.
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Maruti Suzuki Models That Get P0420
This fault code commonly affects: Swift, Baleno, Ertiga, Brezza, Alto, Dzire, S-Cross, Ciaz. If your Maruti is showing this code, bring it to HT ECM Solutions in Bachupally, Hyderabad for same-day professional diagnosis.