Aftermarket October 2022

I was recently tasked with looking at a poorly running Mazda3 1.6 petrol which was brought to us directly via the customer. This vehicle had been subject to multiple repair attempts elsewhere including the main agent to stop the MIL from coming on. The owner had spent over £500 on various sensors and actuators to not fix the vehicle. I began by carrying out a full system scan of all the ECUs on the vehicle. This showed that the engine ECU had a stored DTC of P0171 – System Too Lean Bank 1. Please refer to Fig.1.This DTC relates to the ECU’s ability to correct (Trim) a lean condition sensed by the upstream 02 sensor in the exhaust system. This DTC can be set for a multitude of issues varying from an air leak to low fuel pressure and everything between. Diagnosing this type of fault can be complex and time consuming, however I used a tried and tested diagnostic approach which I will be share with you. As with most DTCs, it is a good idea to look at the freeze frame data if it is available to you. Freeze frame data are pre-set parameters that have been captured by the ECU during the fault occurrence and saved giving you a snapshot of the operating conditions present when the DTC was set. This is where I initially started. The freeze frame data PIDs that I was particularly interested in were the following: fuel trim parameters, engine RPM, engine coolant temperature, loop status and manifold pressures. Fuel trim is the ECU’s reaction (correction value) to a fueling condition. As an example, if the engine ECU detects a lean fuel condition via feedback from the upstream 02 sensor, the ECU is designed to correct that condition by adding fuel (positive fuel trim) to return the air/fuel ratio to Lambda 1. If the ECU detected a rich condition, it would then compensate for that by reducing the amount of fuel being delivered (negative fuel trim). This is done by reducing the duty cycle of the fuel injector. Loop status (open or closed) is the ECU’s current 26 AFTERMARKET OCTOBER 2022 TECHNICAL www.aftermarketonline.net TOO RICH, TOO LEAN This issue, Ryan is offering up a case study that provides an example of how to diagnose an issue when there are lean and rich conditions BY Ryan Colley, Elite Automotive Diagnostics Fig.1 Fig.2