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P0170 Fuel Trim Issue on 1999 Mercedes-Benz C230: Diagnosis and Sensor Analysis

Model: 99 Benz C230 Fault Code: P0170 Posted: 2017-01-07 16:36

I'm experiencing a persistent P0170 fuel trim fault code on my 1999 Mercedes-Benz C230. The shop has already replaced the gas filter, checked for vacuum leaks, and cleaned both the MAF sensor and throttle body seal—none of which resolved the issue. Here are the relevant O2 and fuel trim readings: - Bank 1 Sensor 1 (O2): Voltage ranges from 0.11 to 0.86 volts - Bank 1 Sensor 2 (O2): After warm-up, voltage stabilizes at 0.49V; varies between 0.45 and 0.65V during startup - Short Term Fuel Trim (Bank 1): Fluctuates between -3% and +15% - Long Term Fuel Trim (Bank 1): Starts at 17.19% when cold, settles to a fixed value of 32.03% after warming up - Short Term Fuel Trim (Sensor 1, Bank 1): Ranges from -7% to +11% - Short Term Fuel Trim (Sensor 2, Bank 1): Stays consistently at 99.22%, regardless of temperature I'm now wondering whether the issue lies with Sensor 2 or the MAF sensor. I’ve attached a PDF with full diagnostic data for reference—please let me know if you need it. Thanks in advance for any insights or guidance on this persistent P0170 code.

Related fault codes

P0170

Comments (10)

Anonymous 2017-01-07 16:46

You mentioned that the Long Term Fuel Trim (Bank 1) starts at 17.19% when cold and settles to 32.03% after warming up—have you checked fuel pressure? This could be a key factor in diagnosing the P0170 code.

Anonymous 2017-01-07 16:49

No, I haven't tested fuel pressure yet. Can you guide me on how to check it and what the expected range should be for a 1999 Mercedes-Benz C230?

Anonymous 2017-01-07 16:51

Do you know how to upload a PDF file with vehicle diagnostic data in this forum? I’ve tried dragging it into the message box but couldn’t get it to attach.

Anonymous 2017-01-07 18:48

PDFs are allowed on forum posts (not private messages). The file size limit was previously 5MB, now increased to 10MB—please try uploading again.

Anonymous 2017-01-07 20:07

Here is the data collected after a few hours of driving. Note that by the end of this run, the Long Term Fuel Trim stabilized at 32.03%. Earlier in the day, after installing a new battery and starting on a cold morning, it began at 17.19%. Does this pattern indicate any specific issue? DataLog - 20170105 165818 formated 2.pdf (240.71 KiB) — Downloaded 250 times

Anonymous 2017-01-08 09:13

Check the fuel pressure—it may be a critical factor in resolving this P0170 issue.

Anonymous 2017-01-25 13:27

I finally obtained a fuel test kit and drove 200 miles to my daughter’s car. The rail pressure reads 47–48 psi with the PRV vacuum line connected. When I remove the vacuum from the PRV, pressure increases to 56 psi. Revving the engine does not increase pressure. I replaced the inline fuel filter—pressure remained unchanged. The fuel pump receives approximately 13 volts but produces no audible sound. When I remove the fuel pump fuse, fuel flow drops and the car stalls as if running low on fuel. Why does removing the PRV vacuum cause a pressure increase? What is the correct operating pressure? A Delphi replacement pump claims to deliver 160 psi (possibly at the pump), while other sources suggest 4 bar (~57 psi). The vehicle exhibits air-gulping behavior and fails to accelerate past 3000 rpm—appears to be seeking fuel. Any clarification on expected pressure or symptoms would be greatly appreciated.

Anonymous 2017-01-26 15:31

As of yesterday, I no longer have access to Mercedes-Benz service resources—unable to check TSBs or official fuel specifications. Tip: Have you cleaned the MAF sensor? Oil contamination can cause P0170. A reading of 48 psi is within acceptable range. Removing the vacuum line results in full pressure (55 psi)—this is normal under wide-open throttle conditions. There should be no fuel present in the vacuum line.

Anonymous 2017-01-26 17:56

Hi kev2—thank you for your input. I did remove and clean the MAF sensor last year using the recommended spray, but it didn’t resolve the issue. Should I re-clean it now? Looking at the data in the attached PDF, the MAF reading was taken at idle (not on highway). The rate is between 0.32 and 0.35 g/s over a 100-second period. Also, O2 Sensor 2 remains fixed at 0.49V, and Short Term Fuel Trim (Sensor 2) stays consistently at 99.22%. Does this provide any additional clues? Thanks again for your help.

Anonymous 2017-01-26 18:18

Note: The MAF reading was taken at idle—during the PDF data collection, not during highway driving. Last year I performed a road test and observed significant MAF variation as I accelerated. However, the car still showed P0170 with the MIL light on. During that test, the engine exhibited air-gulping behavior (gasping at idle), suggesting possible fuel starvation or intake issues. Could repeating this test help? The symptoms remain consistent.