Trouble CodeFault LocationProbable Cause
P0421 Warm increase catalytic converter, financial institution 1 -efficiency listed below threshold Catalytic converter, wiring, HO2S 2

We introduce Torque Pro

What Does code P0421 Mean?

If friend are analysis this, i assume the your OBD-II equipped auto has save on computer a code P0421. This method that the powertrain regulate module (PCM) has actually detected a problem with the efficiency of the catalytic converter warmth up duty for engine financial institution one. Financial institution 1 shows that the breakdown has developed in the bank of the engine that consists of the number one cylinder.

You are watching: Warm up catalyst efficiency below threshold bank 1

The catalytic converter is supplied to reduce exhaust emissions indigenous diesel and also gasoline sustained engines. The is built using a filtration aspect of ceramic and precious steels enclosed in a steel housing and placed in the exhaust pipe. Harmful nitrogen oxides (N2O), carbon monoxide, and also unburned hydrocarbons space all convert to harmless ion of nitrogen, oxygen, carbon dioxide, and also water inside the catalytic converter. This is accomplished using the thick web the filtration and the too much heat that the engine exhaust. Catalytic converter temperatures need to reach at the very least 800-degrees Fahrenheit.

Catalytic converter efficiency is monitored utilizing upstream and also downstream oxygen (O2) sensors. The O2 sensors room zirconium dioxide sensing elements enclosed in vented stole housings, associated to cable leads in the O2 sensor wiring harnesses by platinum electrodes. O2 sensor wiring harnesses are associated to the PCM via the controller area network. The O2 sensors carry out the PCM with data concerning the percent of oxygen particles in the engine exhaust as contrasted to the oxygen contents of the ambient air.

Spent engine exhaust gases are driven through the exhaust manifold, right into the exhaust pipe, end the upstream O2 sensor, v the catalytic converter, and also they happen over the downstream O2 sensor. Exhaust gases circulation through vent holes in the steel real estate and throughout the sensing facet of the O2 sensors as outside air is attracted into a room in the center of the sensors v the wire lead cavities.


Inside the chambers, the air is heated, forcing the oxygen ions to develop (energy) voltage. Deviations in between the concentration that oxygen molecules in ambient waiting (drawn into the O2 sensor) and the concentration the oxygen ion in the exhaust reason voltage fluctuations. These deviations cause the oxygen ions inside the O2 sensor come bounce indigenous one platinum layer to the other, very rapidly and also repetitiously. Fluctuations in voltage occur as the surge oxygen ions bounce in between platinum layers. The PCM identify these alters in voltage together variations in exhaust oxygen concentration. These variations denote whether the engine is to run lean (too tiny fuel) or wealthy (too lot fuel). When the signal output from the O2 sensor is lower, more oxygen is current in the exhaust (lean condition). If the voltage signal output is higher, less oxygen is existing in the exhaust (rich condition). This data is supplied by the PCM come compute fuel shipment strategy and ignition timing, as well as other things.

If the upstream and also downstream O2 sensors reflect a similar exhaust oxygen concentration, a password P0421 will be stored and a breakdown indicator lamp might be illuminated.

What are the common causes of code P0421 ?

Faulty catalytic converterA faulty mass air circulation or manifold air pressure sensorDefective O2 sensor/sBurnt, chafed, broken, or disconnected wiring and/or connectorsEngine exhaust leak

What space the symptom of code P0421 ?

Diminished fuel efficiencyA absence of general engine performanceOther connected diagnostic trouble codes may additionally be storedService engine shortly lamp illumination

How execute you troubleshoot password P0421 ?

A diagnostic scanner, digital volt ohmmeter (DVOM), and a reliable vehicle information resource (such together All Data DIY) will certainly prove necessary when diagnosing a password P0421.

Misfire codes, throttle position sensor codes, manifold air push codes, and mass air circulation sensor codes must be addressed prior to attempting to diagnose a stored code P0421. The engine should be running efficiently prior to diagnosing this code.

With a P0421, ns would start with a visual inspection of device wiring harnesses and connectors. Meticulously check harnesses that space routed near hot exhaust pipes and also manifolds, and also those that room routed close to sharp edges prefer the ones found on exhaust shields.

Proceed by connecting the scanner come the car diagnostic port and also retrieving all problem codes and also freeze frame data. Create this info down because it might be useful if this proves to it is in an intermittent code. Intermittent codes deserve to be much harder to diagnose. Now, clear the codes and test journey the vehicle.

If the P0421 is automatically reset, begin the engine and allow it to reach regular operating temperature, then allow it come idle (with the infection in neutral or park). Use the scanner data present to watch O2 sensor input data. Narrowing the data present to incorporate only pertinent data will yield quicker data response. If the engine is running efficiently, the upstream O2 sensor will cycle on regular basis from one-millivolt (.100-volts) come nine-millivolts (.900-volts) and also downstream O2 sensor data have to reach a mid-line and settle there. If upstream and also downstream O2 sensor readings space too similar, ~ the PCM has reached closed loop operation, suspect a defective catalytic converter.

See more: 6 Trickiest Interview Questions And How Many Manhole Covers Are There In The Usa


Use the DVOM to check resistance that the O2 sensor/s in question, as well as voltage and also ground signals because that O2 sensor circuits. Use caution and disconnect connected controllers prior to attempting come test mechanism circuit resistance with the DVOM.