Catalytic converter codes

How its codes differ

• P0420 is the converter test on bank 1 (the side with cylinder 1, or the only bank on a four-cylinder engine). P0430 is the same test on bank 2 of a V-engine.
• One bank failing alone points toward that converter, its rear oxygen sensor or an exhaust leak on that side. Both banks failing together makes it worth ruling out causes they could share first, such as misfire, rich running or oil or coolant being burned.
• A converter code is a judgement drawn from oxygen sensor readings, not a direct measurement, so oxygen sensor codes and exhaust leaks near the sensors should be dealt with before the converter is condemned.
• Misfire codes (P0300 to P0306) are often the cause rather than a separate fault: unburned fuel overheats the converter, which is why the light flashes when misfire is bad enough to damage it.
• Downstream oxygen sensor codes (P0137, P0138, P0141, P0037) come from the sensor the converter test relies on, so fix them before trusting a converter code.

Codes

• P0420

  Catalytic converter efficiency below the computer's limit (Bank 1)

  Severity: Monitor

• P0430

  Catalytic converter efficiency below the computer's limit (Bank 2)

  Severity: Monitor

• P0300

  Random or multiple-cylinder misfire detected

  Severity: Inspect soon

• P0301

  Misfire detected in cylinder 1

  Severity: Inspect soon

• P0302

  Misfire detected in cylinder 2

  Severity: Inspect soon

• P0303

  Misfire detected in cylinder 3

  Severity: Inspect soon

• P0304

  Misfire detected in cylinder 4

  Severity: Inspect soon

• P0305

  Misfire detected in cylinder 5

  Severity: Inspect soon

• P0306

  Misfire detected in cylinder 6

  Severity: Inspect soon

• P0137

  The downstream oxygen sensor on bank 1 is reporting a voltage that stays too low

  Severity: Monitor

• P0138

  The downstream oxygen sensor on bank 1 is reporting a voltage that stays too high

  Severity: Monitor

• P0141

  The heater in the downstream oxygen sensor on bank 1 isn't working as the computer expects

  Severity: Monitor

• P0037

  The heater control circuit for the downstream oxygen sensor on bank 1 is reading too low

  Severity: Monitor

Signs it's failing

• Check engine light with P0420 and/or P0430
• Loss of power or poor acceleration when the converter is clogged
• Hard starting with a severely restricted converter
• Failed emissions inspection

How to test it

1. Fix any other stored codes first, especially misfire, fuel-trim and oxygen sensor codes; the converter test is only meaningful once they are cleared. (Low risk)

2. With a scan tool and the engine fully warm, compare the upstream and downstream oxygen sensor readings. The upstream sensor should switch rapidly; the downstream sensor should stay fairly steady if the converter is working. (Use caution)

   Warning: Keep hands, hair, clothing and tools clear of the fan and belts while the engine is running, and do not run the engine in a closed garage.

3. Look and listen for exhaust leaks ahead of or near the downstream oxygen sensor, which can make a good converter fail the test. (Use caution)

   Warning: Exhaust parts, the converter and nearby sensors get hot enough to cause serious burns and stay hot long after the engine is switched off. Work only on a cold engine, and never under a car held up only by a jack.

4. Testing for a restricted converter and replacing a converter are jobs for a shop. (Best left to a shop)

   Warning: Exhaust parts, the converter and nearby sensors get hot enough to cause serious burns and stay hot long after the engine is switched off. Work only on a cold engine, and never under a car held up only by a jack.

After the repair

• Confirm the root cause (misfire, rich running, oil or coolant use) has been fixed, or a new converter can fail the same way.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.
• Recheck the downstream oxygen sensor reading once warm: it should be fairly steady compared with the upstream sensor.