O2 sens, Cat, fuel trim, PO420 codes great tech info and short read - Ferrari Life
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post #1 of 3 Old 07-14-2015, 08:04 PM Thread Starter
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O2 sens, Cat, fuel trim, PO420 codes great tech info and short read

Catalytic Converter Education Resources - Eastern Catalytic

I found some great info here that is explained in a short concise maner. I read the one on Fuel trim first LTFT and STFT and then started to click on some of the others. The PO420 one is loaded with great info if your check engine light comes on. I also never thought about breaking in a cat or not using silicone products on cars with O2 sensors.

These are all short one page three paragraph reads in laymans terms that anyone that has a ODBII scan tool should read.

I have a cheap ELM327 bluetooth device($10) and an app for my Android phone called Torq Pro that I use for scanning and check engine light codes(CEL).


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post #2 of 3 Old 07-18-2015, 07:18 PM Thread Starter
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Fuel trim can be a valuable diagnostic tool

By Jack George, Technical Rep. Eastern Catalytic
If you are trying to diagnose drivability issues but don’t have any trouble codes to chase, take a closer look at the vehicle’s “fuel trim”. When analyzed properly, it can be a valuable diagnostic tool, a window to the heart of the Fuel control system and how it is operating.
Understanding Fuel Trim
A vehicle’s computer system uses Fuel Trim to help maintain the ideal air/fuel ratio for complete combustion (stoichiometry) – 14.7 parts air to 1 part fuel. Three way Catalytic converters need the mixture to be constantly driven rich/lean around this ratio in order to work at maximum efficiency. Fuel trims can compensate for other vehicle issues. That’s why fuel trims are so useful. They can provide an overall picture of what is causing the problem such as an intake manifold vacuum leak (positive fuel trim – lean) or a stuck open fuel injector (negative fuel trim – rich).
Short Term Fuel Trim
Short Term Fuel Trim (STFT1 and STFT2) is the computer’s immediate response to adjust the air/fuel ratio. In positive corrections, fuel is added to adjust for a lean condition, while negative corrections respond to a rich condition. STFT corrections represent the current engine run cycle and react very quickly to O2 sensor input. If you were to create a large vacuum leak at Idle by disconnecting the PCV hose, the computer would immediately add positive fuel trim to balance the mixture. Short Term Fuel Trim is not stored in Keep Alive Memory (KAM) after shut down and automatically resets to 0 for the next start/run cycle.
Long Term Fuel Trim
Unlike STFT, Long Term Fuel Trim (LTFT1 and LTFT2) is learned over time while in closed loop operation. It is stored in the KAM and also used for open loop fuel calculations (like start up and wide
open throttle). LTFT is a coarser adjustment and also works to keep STFT within specification.
Diagnosing with Fuel Trims
Fuel trims can help you zero in on the problem, especially when there are no other trouble codes present. Knowing whether a vehicle is running too rich or too lean will help narrow down your diagnosis. Fuel trims that differ greatly from one cylinder bank to the other will also point you in the right direction. Always evaluate fuel trims at idle and at 2500 RPM.
Running too rich – High negative fuel trim corrections can be caused by MAF sensor problems, high fuel pressure, leaking fuel pressure regulator diaphragm, faulty evaporative emissions components, leaking injectors, defective O2 sensors, exhaust leaks/pinholes before the O2 sensor, coolant temp sensor problems, and base engine issues such as low compression and incorrect camshaft timing.
Running too lean – High positive fuel corrections can be traced to MAF and O2 sensor faults, vacuum leaks from intake gaskets/hoses, unmetered air (intake snorkel leak), clogged or dirty fuel injectors, fuel delivery issues, and exhaust restrictions such as a clogged catalytic converter.
Diagnostic Tip:
For a suspected vacuum leak, note the fuel trims at idle and increase engine speed to 2500 RPM and hold. If the STFT immediately decreases and moves to acceptable levels and the LTFT slowly starts to come back down, you have a vacuum leak. After the repair, reset the KAM and start the vehicle. Monitor the fuel trims to make sure they are within the normal ranges. It could take up to 10 miles of driving for an accurate LTFT reading.


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post #3 of 3 Old 07-18-2015, 07:20 PM Thread Starter
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Pesky PO420 Codes

How to Tame Those Pesky PO420 Codes

“Posted with permission from Motor Age magazine”
By: Charles Pantano, Eastern Catalytic
When it comes to service issues on today’s vehicles, the difficulty in correctly diagnosing and evaluating catalytic converter problems has to be at top of the list. One of the most annoying is the persistent PO420 diagnostic code (Catalyst System Efficiency Below Threshold (Bank 1), which not only shows its teeth when the catalytic converter is on the blink, but can also be generated by a variety of engine problems not directly related to the converter.
Possible PO420 code causes
The following engine related problems are known to generate the PO420 code:
  • Intake manifold air leaks
  • Fuel injector problems (leaks)
  • Incorrect spark plugs
  • Ignition timing
  • EGR problem
  • Defective catalytic converter
  • Oil or antifreeze entering exhaust
  • O2 sensor not operating correctly
  • Road damage to converter
  • Silicone contamination
Most, if not all, catalytic converter failures are caused by a problem or malfunction somewhere in the emission system ahead of the converter. So, it’s very important to determine what actually caused the converter to fail, so that the problem can be repaired and a recurrence can be prevented.
Here are some troubleshooting suggestions:
Testing the converter:
Method 1: Vacuum test
Connect a vacuum gauge to the vacuum on the intake manifold, carburetor, or throttle body. Note the reading at idle. Then raise and hold engine speed at 3,000 RPM. The needle will drop when you first open the throttle, but should then rise and level off. If the vacuum reading starts to drop, pressure may be backing up in the exhaust system indicating a blockage somewhere in the exhaust system.
Method 2: Backpressure test
Measure backpressure directly. If vehicle’s engine has air injection, disconnect the check valve from the distribution manifold and connect a low pressure gauge. Or, remove the oxygen sensor and take your reading at its port in the manifold or head pipe. A reading of more than 1.25 PSI at idle or more than 3 PSI at 2,000 RPM tells you there’s an exhaust restriction.
Method 3: Temperature test
In late model engines with fuel injection, the combustion is so efficient that the converter has little to process and the difference between the inlet and outlet temperatures may only be 50 F at 2,500 RPM. This is a lot less than the old rule of thumb that says a good converter should show at least a 100 F difference. At idle, the converter in many late-model vehicles may cool down so much that there’s almost no measurable difference between the front and back temperatures. So checking exhaust temperatures front and back of the converter at idle and 2,500 RPM may not be an accurate way to determine if the converter is working or not.
Watch out for RVT silicone contamination
Silicone-based products or Teflon sealants should not be used on any part of the exhaust system. They are not designed to operate at high exhaust temperatures and will out gas, causing damage to O2 sensors. Below are some examples of O2 sensors contamination caused by the use of RTV silicone on exhaust manifold flanges and other components as well as Teflon-type sealants used on O2 sensors


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