2006 4 cylinder LE

About 6 weeks ago I began to get an intermittent CIL. The OBD scanner and it said it was a

"System too lean bank 1 code" It has gone on and off sometimes with it remaining off for days. When I had my oil changed I had my mechanic take a quick look at it and he saw no obvious damage to hoses and the connections were tight. It was only a casual inspection since for a more thorough inspection I would have to make an appointment for that since he was booked that day.

Since its now getting warmer for me to be out my my garage I wanted to try and resolve the issue myself before escalating it to the mechanic. What I am looking for is the proper sequence with which I should approach diagnosing the source of the issue.


I recorded the live data yesterday from a cold start and running through the steps I found recommended elsewhere. The variations in percentages seemed greatest at the first 20 or so minutes of running the car while parked but at the end of the test drive I took after that the swings in numbers had settled down. And after 30 min of sitting off while I ran my errand the numbers remained more consistent.

What is the best order of troubleshooting to figure out the cause of this. I have very limited experience so I'd appreciate any advice on how to approach this so I don't make it worse.
Thanks

 

What is the capacity of your scanning tool? Many of the listed issues that can cause this code can be monitored with the right scanner.

At cold start the engine operation is in open loop as the exhaust A/F sensors are not in play. Once they are then the system goes into closed loop. This open and closes loop can show up in fuel ratios etc. as in less wide variations.

Is this code causing the check engine light to come on and if so when does this happen?

P0171 is the fuel/air ratio is too lean. Here is the list.

Engine air induction system. Check for damage allowing outside air to bypass the MAF sensor.

Injector blockage. How does the engine run? Need to pull the injector to test them for flow.

MAF sensor. Can try cleaning it. Does your scanner output MAF signals, if so can check this.

Engine Coolant Temperature (ECT) sensor. Sends coolant temp signals to ECU. Does your scanner output ECT signals if so.check if coolant temps appear normla and the sensor is sending correct signal.

Fuel pressure. The pump has a pressure regulator. One needs to hook up a pressure gauge to the fuel line.

Exhaust gas leakage from exhaust system. Inspect exhaust when engine running for leakage.

Open or short in A/F sensor (sensor 1) circuit. Check wiring to sensor.

A/F sensor (sensor 1). The sensor has its own code. A scanner can monitor if the sensor is working OK.

A/F sensor heater (sensor 1). This has its own code. This is the heater. A scanner can show if it’s working.

Engine room J/B (EFI relay). A relay.

A/F sensor heater and EFI relay circuits. A relay.

PCV system. Check hoses for damage and hose connections

ECM. Seldom fails.

Might check any emissions hoses going to the engine air intake system. These are small hoses.

Any link between braking and this code. The brake booster would have a vacuum hose to engine and if bad could cause outside air to enter the engine.

Check for other codes in addition to P0171.

Could be an air leak, bad MAF, lazy A/F sensor.

The Toyota service manual has scanner spec for many of the above issues.

See private message.