BULLETIN NO. 08-14 (DATE 9/30/2008)

TO: All ATP Technicians

FROM: James Strong, Customer/Technical Service Mgr.

RE: Check Engine Light On After Catalytic Converter Replacement


If a customer’s vehicle comes in to the shop for a failed catalytic converter replacement, just don’t replace it without diagnosing it first. On many of today’s cars and trucks, the catalytic converter may go the distance and outlast the vehicle itself. However, the converter can be fouled by contaminates causing premature wear and ultimately complete failure.

Catalytic converters are one of the most important emission control devices on a vehicle. Its catalyst contains precious metals that gives the converter its ability, through chemical conversion, to reduce and/or eliminate dangerous levels of HC (hydro-carbons), CO (carbon monoxide) and NOx (oxides of nitrogen).

Catalytic converter failures typically fall into one of four categories:

    1. Thermal failure (overheating)
    2. Plugged or partially plugged substrate
    3. Thermal shock
    4. Physical damage


Usually any of the four failures mentioned above requires catalytic converter replacement.

With OBDII compliant vehicles, the most obvious indicator of a bad catalytic converter (and also, the most accurate) is a check engine light followed by a catalytic converter related code. Usually a P0420 (Catalyst System Efficiency Below Threshold -Bank 1) or P0430 code (Catalyst System Efficiency Below Threshold-Bank 2).

Bank1 sensors are also known as upstream sensors which are usually located at the exhaust manifold or front pipe. Bank2 sensors are known as downstream sensors usually located on or behind the catalytic converter. In fact, some vehicles may contain numerous O2 sensors.

O2 sensors react to oxygen levels in the exhaust. The catalyst monitor compares the signal from the Bank2 O2 sensor(s) to that from the Bank1 O2 sensor(s).

If the catalytic converter was replaced because of an efficiency fault reading from the scan tool and the vehicle returned to the shop with the check engine light on, and with the same DTC code, more than likely it is not the catalytic converter unit.

In the July 2008 edition of the Brake and Front End magazine, it is mentioned that when an old converter was replaced with a brand new one, a low converter efficiency fault code that reappears would most likely be due to a lazy upstream (Bank1) O2 sensor(s).

The fix would not be replacing the catalytic converter a second time, but to replace the lazy upstream (Bank1) O2 sensor(s).

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