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1999 Mercedes C230 Kompressor P0170 Code: Negative Fuel Trim Values & Diagnosis Tips

Model: 1999 C230 Kompressor Fault Code: P0170 Posted: 2015-03-08 21:07

Hello, I'm experiencing a persistent OBD2 fault code P0170 on my 1999 Mercedes-Benz C230 Kompressor, despite completing several common fixes. I've checked all vacuum hoses, ruled out oil leaks around wiring harnesses, replaced both oxygen sensors, cleaned the AIR lines, and installed a new MAF sensor. The engine now runs significantly better—almost like it's back to factory condition—and previously exhibited stalling and low power when cold. However, the P0170 code persists. My OBD2 readings show concerning negative fuel trim values: - SHRTFT1% = -1.6% - LONGFT1% = -31.1% Additional data from my scan tool: - DTC_CNT: 1 - FUELSYS1: CL (Controlled) - FUELSYS2: N/A - LOAD_PCT(5): 25.9% - ETC (F): 167°F - MAP (inHg): 9.5 - RPM (/MIN): 724 - VSS (mph): 0 - SPARKADV (' ): -15 - IAT (°F): 41 - MAF (LB/MIN): 0.624 - TP (%): 2.7% - AIR_STAT: OFF - O2S B1S1 --B2S-- - O2B1S1 (V): 0.805 - SHRTFTB1S1(5): 0.0 - O2B1S2(V): 0.480 I noticed that my MAF reading was initially higher than in other similar posts, and after replacing the sensor, values improved—this suggests a potential issue with the original sensor or calibration. The negative fuel trim percentages indicate that the engine control unit (ECU) is cutting fuel to compensate for perceived excess air. However, given that my throttle position shows only 2.7% opening while MAF reports just 0.624 lb/min, this suggests a mismatch between actual airflow and sensor input—possibly indicating a faulty or improperly calibrated MAF sensor. I'm wondering: Why are the fuel trim values negative? Could improper installation (e.g., reversed polarity) of oxygen sensors have caused this? The issue persists even after warming up the engine. I've also noticed that when I clear the code, it returns within 20–30 miles. Any guidance on how to perform a fuel pressure leak-down test would be greatly appreciated. Is a simple pressure gauge and fittings sufficient? And where is the best location to test this in the fuel line? Thank you for any insights or recommendations—this has been a frustrating issue to resolve.

Related fault codes
P0170
Comments (4)
Anonymous 2015-03-09 08:44

Negative trim values (like -31.1%) suggest the ECU is reducing fuel delivery, which may indicate an air intake or sensor issue. Check your ECT reading—167°F seems low for a warm engine; could this be affecting calculations? Also refer to official OBD2 resource: http://www.obd-codes.com/p0170

Anonymous 2015-03-09 15:26

Thanks for the reply. Before the MAF sensor replacement, the car ran poorly. Now it runs smoothly with no noticeable issues—though the P0170 code returns after about 20–30 miles. The negative trim values remain even when the engine is fully warmed up. Could reversed polarity during oxygen sensor installation have caused this? I didn’t warm up the engine thoroughly last night, which may explain the lower temperature reading. Still, the negative values persist—any idea what else might be causing this?

Anonymous 2015-11-27 17:26

Negative fuel trim values mean the ECU is cutting fuel to reduce mixture richness. In your case, with a throttle position of only 2.7% and MAF reading at 0.624 lb/min, there's a significant discrepancy—suggesting either a faulty MAF sensor or an issue in airflow measurement. The ECU sees less air than is actually flowing, leading to incorrect fuel adjustments.

Anonymous 2015-11-27 17:29

Avoid using generic aftermarket parts from stores like O'Reilly or AutoZone on German vehicles—these components often don't meet OEM specifications and may cause persistent issues such as P0170 or other sensor-related faults.