OBD II Code P0170 - Fuel Trim Issues


What Code P0170 Means, And How To Fix It

With knowledge comes the power to tell when a mechanic tries to rip us off, so  with these posts, I am hoping to provide Carnity members with some basic knowledge of a particular fault code to help them understand what caused a particular generic OBD II code to be set, and how the system(s)/component(s) that relate to the code work. https://carnity.com/topic/9801-how-to-scan-trouble-codes/


Definition of code P0170

Code P0170 is defined as  “Fuel Trim Malfunction (Bank 1)”, with “Bank 1” referring to the side of the engine that contains cylinder #1 on engines that have two cylinder heads.  Codes that are related to fuel trim issues are rare on American and Korean cars, but common on imported European cars. Cars most affected are turbocharged petrol models in the Mercedes-Benz, Audi, BMW, and Volkswagen ranges.


What sets code PO170

This code is set when the PCM (Powertrain Control Module) cannot control or regulate the air/fuel mixture when required, or when it gets a request to adjust the fuel trim from post-catalytic converter oxygen sensors that exceeds its ability to adjust the fuel trim. Negative fuel trim means that the PCM is attempting to lean out the air/fuel mixture, and positive fuel trim means that the PCM is attempting enrich the air/fuel mixture.

Symptoms of code PO170

Symptoms of code P0170 vary widely between applications, and sometimes the only indication of a problem is the presence of a stored code, and an illuminated CHECK ENGINE light. Other possible symptoms could include-

·         Poor acceleration or power loss

·         Hesitation at start up

·         Misfiring at start up

·         Hesitation and/or misfiring upon acceleration

·         Visible black smoke from the tail pipe upon acceleration and /or start up

·         Increased fuel consumption

Note that some symptoms, such as  over fuelling (rich running) can damage the catalytic converter(s) if the fault is not fixed.

Causes of code PO170

The most common cause of code P0107 is oil leaking past turbocharger seals, which leads to  fouling of the mass air flow meter element. Other possible causes include-

·         Excessive oil in the inlet tract that causes short circuits in the mass airflow meter connector.

·         Cracked, split, or dislodged intake air ducting that allows unmetered air to enter the engine.

·         Oil-fouled oxygen sensors on turbocharged engines.

·         Poor quality, or mismatched after market mass airflow sensors.

·         Excessive fuel pressure caused by a defective fuel pressure regulator.

·         Low fuel pressure caused by defective a fuel pump or defective fuel pressure regulator.

·         Clogged, or dirty fuel  filter.

·         Vacuum system leaks, allowing unmetered air to enter the engine.
(Note that it is important to be sure that there are no vacuum leaks on the engine. It often happens that perfectly good MAF sensors and other components are replaced when the real problem was a vacuum leak.)

·         Damaged catalytic converters that fail to clean up the exhaust stream.

·         PCM failure is not altogether impossible, but this happens very rarely.

Basic troubleshooting steps for code P0170

NOTE #1: Diagnosing code P0107 requires that the engine be in perfect running condition. Any other codes present, and especially codes relating to crankshaft and camshaft position sensors, misfire, vacuum, catalytic converter, or fuel pressure related issues must be resolved before starting to diagnose code P0170.

NOTE #2: To diagnose code P0170, and assuming that there are no other codes present, you will need a repair manual, a code reader that can monitor live data streams, and a digital multi-meter. While it is possible to perform this diagnosis and repair on a DIY basis, it is recommended that the diagnosis and repair be performed by a competent mechanic. If you choose to do the repair yourself, proceed as follows-

NOTE #3: It is important to retest the system after each step in the repair procedure. It saves a lot of time, and it prevents the duplication of tests.



Record all stored fault codes and available freeze frame data for future reference. This may help you identify the cause(s) of possible intermittent faults later on. At this point, you need to determine if you have negative, or positive fuel trim condition, to save you time later on. Most code readers will indicate this.

Step 2

Since the most common cause of this code relates to the mass airflow meter, remove the meter from the inlet tract and inspect it for the presence of oil on the heated element wire. If oil is present, use an approved cleaner to remove the oil. Be sure to use only an approved cleaning agent to clean the element, since some cleaners will remove the special coating on some MAF sensor elements, rendering the MAF sensor useless.

NOTE #1: There is little point in replacing, or reinstalling  the  MAF (Mass Airflow Sensor) if there is excessive oil in the inlet tract. Most turbocharged engines have a light oily residue in the inlet tract, and this is normal, but liquid oil in the tract means there is a serious problem somewhere. The source of the excess oil must be found and the problem should be corrected before continuing the diagnostic/repair process.

NOTE #2: Excessive oil in the inlet tract will almost certainly have damaged the oxygen sensors and catalytic converter, which means that the code will persist even after replacing the MAF sensor with an OEM part. These components must be checked/tested for functionality before proceeding with diagnosing/repairing code P0107. Refer to the manual for the correct testing procedures for oxygen sensors.


Step 3

If there is no excess oil in the inlet tract, and you have cleaned the MAF sensor element, refit the MAF sensor, but before you retest the system, perform a visual inspection of all wiring, and connectors in the circuit. Refer to the manual to determine the location, function, and color-coding of each wire.

Look for damaged, burnt, or short circuited wiring, as well as damaged, or corroded connectors. Repair or replace wiring and/or connectors as required.


Step 4

If the wiring checks out OK, or if you have replaced or repaired wiring/connectors, you need to perform reference voltage, continuity, ground, and resistance checks on the control circuit wiring.

Consult the manual on the exact values. It is important to follow the instructions in the manual exactly, and bear in mind that obtained readings must either match stated values exactly, or fall within the specified range. However, make absolutely certain that you disconnect the system from the PCM before performing continuity checks to avoid damaging the PCM or other related controllers.

These checks will pinpoint electrical issues in the control circuit wiring that may not have been visible. Repair, or  replace wiring as required. Recheck the system after repairs to confirm that all values conform to the manufacturer’s specifications.

Step 5

If the wiring checks out OK, you need to test the MAF sensor. To do this, connect the code reader to the system, and start the engine, but set up the code reader in such a manner that only the live data from the MAF sensor is displayed. This gives more accurate data, but it also removes distractions that could be confusing.

To interpret the data obtained, you need to consult the manual to get minimum and maximum signal voltages for the application being worked on. As air passes through the MAF sensor, the airflow generates an electric current, which the PCM uses to calculate a suitable timing and fuel delivery strategy. Therefore, as the RPM’s change, the signal voltage generated by the MAF sensor should change as well.

Some code readers will present this information in the form of a graph, which is easy to interpret. One axis of the graph will represent the engine speed, and another axis will represent the signal voltage. To test the MAF sensor, have an assistant operate the throttle to change the engine speed - this will reflect as a changing voltage on the code reader display.

Step 6

However, the obtained voltage readings relative to the engine speed must match the values stated in the manual exactly; if they do not, the MAF sensor is likely defective, but do not assume it is. Test the oxygen sensors for functionality, and only replace the MAF sensor if the oxygen sensors are proven to be in good working order. Consult the manual on how to test the oxygen sensors. Replace any oxygen sensors that do not comply with the manufacturer’s specification, and retest the MAF sensor.

Step 7

If the wiring, oxygen sensors, and catalytic converter(s) are good, but the code persists, replace the MAF sensor with an OEM part, but only if you are absolutely certain that there are no vacuum leaks present.  After market MAF sensors are often the cause of code P0170 and related codes, so avoid using aftermarket components where possible.

Step 8

Test drive the vehicle with the scanner connected to be able to monitor the performance of the MAF sensor. At this point it is unlikely that the code will return, but if it does return, it is possible that there is an intermittent fault present. Some intermittent faults can be extremely challenging to trace and resolve, and you may have to a allow the fault to worsen before an accurate diagnosis and definitive repair can be made.

Codes related to P0170

·         P0171 -  System Too Lean (Bank 1)

·         P0172 -  System Too Rich (Bank 1)





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