“Now, connect the scan tool, start the engine and let it run until the tool shows that it’s in CLOSED LOOP. That means it’s no longer running off factory programming and is taking information from all the sensors and making its own calculations. Then, have a friend drive the vehicle while you monitor FUEL TRIM readings. Depending on your vehicle, you may be able to monitor both short and long-term fuel trim. Fuel trim is the amount of EXTRA fuel the PCM has to add to keep the engine running properly. In an ideal world with a brand new engine, the short term fuel trim should be 0. But in an older vehicle, a short-term fuel trim reading of 10% is acceptable. The maximum short term fuel trim is 25%. When the PCM exceeds that, it starts boosting long-term fuel trim, so check both. In other words, a low short term fuel trim with a higher long term fuel trim means you have an air/fuel problem.
What causes excessive fuel trim?
The number one villain is a vacuum leak after the MAF sensor has measured how much air is coming into the engine. That causes the upstream O2 sensor to see a continually lean exhaust and it adds fuel to compensate. Plugged fuel injectors, a bad fuel pressure regulator, and a bad fuel pump can also cause a lean condition.
If the fuel trims are in the normal range, next check the upstream O2 sensors. A healthy O2 sensor should switch between rich and lean at least 8-10 times in 10 seconds. Many times a scan tool won’t react that quickly and that may mislead you. So check the scan tool to see if it displays CROSS COUNTS. That metric actually does the counting for you. Professionals use a scope to check for cross counts. If you find that the upstream O2 sensor ISN”T switching that often, you have a “lazy” sensor. That causes the PCM to react too slowly, forcing the cat converter to see too much fuel, then nothing. That causes the downstream sensor to switch between rich and lean and cause a P0420 or P0430 code. Bottom line: if the upstream sensors are lazy, they’ll cause the downstream sensors to switch and make you think the cat converter is bad. Ignore the problem and the cat converter WILL go bad from all the extra fuel. So replace the lazy upstream sensors, clear the codes, and complete another drive cycle. Click
here to understand drive cycles.
CLUE: If the upstream O2 and downstream O2 are both switching rapidly, the cat converter isn’t doing its job. A properly operating system should show rapidly switching upstream sensor and a downstream sensor that rarely switches.
If the fuel trims are ok, and you’ve replaced the O2 sensors, confirmed that the fuel pressure is correct, that you have no excessive blow-by, worn valve stem seals, or coolant leaks, and you still have code P0420, you’ll have to replace the cat converter. Don’t buy a cheap one. It’ll set off the check engine light.”
© 2012 Rick Muscoplat