What is Maruti P0420?
P0420 means the ECM has detected that the catalytic converter is not converting exhaust gases efficiently enough. The ECM monitors the front (upstream) and rear (downstream) O2 sensors — if both sensors show similar switching patterns, the cat is not working. On Maruti petrol engines, P0420 is very common but is frequently misdiagnosed — a faulty rear O2 sensor mimics a failed catalytic converter. Always diagnose before replacing the catalytic converter (₹8,000–₹20,000) as the actual cause is often a ₹1,500 O2 sensor.
Symptoms of P0420 on Maruti
P0420 rarely causes drivability symptoms — engine usually runs normally.
Mild fuel economy reduction as ECM cannot fine-tune the mixture without catalyst feedback.
Vehicle will fail PUC/emission test — P0420 directly causes emission non-compliance.
Failed catalyst no longer converts H2S — causes distinctive sulphur smell from exhaust.
Common Causes of P0420
| Cause | Probability | Notes |
|---|---|---|
| Faulty rear O2 sensor | Very High | Rear sensor mimics front sensor output making ECM think the cat is dead — diagnose first |
| Failed catalytic converter | High | Substrate melted or poisoned — common if P0300 misfire was ignored for weeks |
| Exhaust leak near sensors | Medium | Exhaust leak between manifold and cat causes false lean reading at front O2 sensor |
| Engine burning oil | Medium | Oil ash poisons the catalytic substrate and coats the O2 sensor |
| Engine coolant in exhaust | Low | Head gasket failure contaminates the catalyst rapidly |
Diagnosis & Repair Steps
P0300 misfire or P0171 lean codes alongside P0420 mean the cat was damaged by those faults — fix them first.
Watch front and rear O2 sensor switching on scanner. Rear sensor should be steady (0.6–0.7V). If it is switching like the front, suspect faulty rear sensor.
Inspect manifold, flex pipe and all joints for leaks. An exhaust leak near the front O2 sensor causes false lean readings.
Check sensor heater resistance — should be 5–20 ohms. Open circuit means the sensor has failed.
Check for rattling (broken substrate) or physical damage. A rattle confirms internal failure.
Replace rear O2 sensor first (less expensive). If fault returns after O2 sensor replacement, the catalytic converter needs replacement.
Got P0420 on your Maruti?
HT ECM Solutions — Hyderabad's specialist for Maruti ECM diagnostics. Same-day service, written report, ₹499 onwards.