OBD-II trouble code
P0170: Fuel Trim Malfunction (Bank 1)
The PCM is reporting fuel trim on bank 1 is out of range, but unlike P0171 (lean) or P0172 (rich), this code doesn't commit to which direction. Most readers find this code after looking at P0171 or P0172 first — the diagnostic path here forces an extra step.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $30 – $1,500
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0170 mean?
P0170 is the bank 1 generic 'fuel trim malfunction' code — the mirror of P0173 on bank 2. It sets on platforms that report fuel trim trouble without committing to the lean (P0171) or rich (P0172) direction, or when the PCM detects trim values that swing erratically between extremes faster than the lean/rich thresholds would catch. Most modern vehicles trigger P0171 or P0172 first, but P0170 still appears regularly on older Ford and Chrysler V6/V8 platforms, certain Toyota and Lexus engines, and some European applications.
Bank 1 identification matters, and the shortcut most people reach for is wrong often enough to be expensive. Bank 1 is whichever bank contains cylinder 1 — nothing more — and which physical bank that is depends on how the manufacturer numbers cylinders, not on whether the engine sits transversely or longitudinally. There is no layout rule. On the GM LS and the Chrysler Hemi, cylinder 1 is on the driver's side, so bank 1 is the driver's side. On the Ford Modular V8 the cylinders are numbered down the passenger bank, so bank 1 is the passenger side — the opposite answer from its longitudinal V8 neighbours. On the Toyota 2GR-FE, cylinders 1, 3 and 5 run down the rear bank, so bank 1 is the firewall bank rather than the one facing the radiator. Verify cylinder numbering for your specific engine from service information before chasing a bank-specific code; grouping engines by layout is exactly how people end up working on the wrong side.
If you've read about P0173, almost everything you know applies here on the other side of the engine. The diagnostic challenge is identical: the code doesn't tell you whether the engine is running lean or rich on bank 1, so you have to read the live fuel trim data first. Short-term fuel trim (STFT) and long-term fuel trim (LTFT) positive numbers mean the PCM is adding fuel to compensate for lean. Negative numbers mean it's subtracting fuel to compensate for rich. Out-of-range in either direction can trigger P0170 on platforms that use this generic code.
If trim is positive (lean), the causes are the same as P0171: vacuum leak, MAF contamination, fuel delivery shortage, exhaust leak ahead of the upstream O2 sensor, or a failed upstream O2 sensor. If trim is negative (rich), the causes mirror P0172: failing fuel pressure regulator, leaky injector, restricted air filter, dirty MAF reading high, or contaminated O2 sensor. The split determines the entire diagnostic path. Don't replace parts before reading trim direction first.
One useful tactical note: if P0170 sets alongside P0173, the cause is almost certainly global rather than bank-specific. Both banks showing out-of-range trim points at fuel pressure, MAF, fuel pump, or a problem affecting both sides equally. P0170 alone (without P0173) points at bank-1-specific causes — intake gasket on the bank 1 side, bank 1 injector, or bank 1 upstream O2.
Common causes
- Vacuum leak on bank 1's side of the intake manifold (positive trim direction)
- Bank-specific exhaust leak ahead of bank 1 upstream O2 sensor (positive trim)
- Failed bank 1 upstream O2 sensor reporting falsely lean or rich
- Failing fuel pressure regulator affecting both banks equally (rich trim direction)
- Leaky injector on a bank 1 cylinder (rich trim direction)
- MAF sensor contamination or drift affecting global fuel calculation
- Restricted fuel injector on bank 1 (lean trim direction)
- Cracked or disconnected PCV hose routing to bank 1
- Bank 1 intake manifold gasket failure
- Fuel pump weakening, particularly under load
Symptoms
- Check Engine Light on, often illuminating after extended driving
- Reduced fuel economy
- Possible rough idle if the underlying issue is severe
- Mild surge or hesitation under light load
- May feel like nothing is wrong — fuel trim codes often set without obvious drivability symptoms
- Possible exhaust smell (rich) or hesitation under acceleration (lean)
Diagnostic steps
- 1.Identify bank 1 for your specific engine from its cylinder numbering before anything else, because every step below that names a bank depends on it.
- 2.Pull all codes and capture freeze frame data showing the conditions when P0170 set.
- 3.With a scan tool capable of live data, watch bank 1 STFT and LTFT at idle, at 2500 RPM no load, and during a road test. Note the direction (positive or negative) and the magnitude.
- 4.Compare bank 1 trim to bank 2 trim. Both out of range = global cause. Only bank 1 out of range = bank-specific cause.
- 5.If trim is positive (lean side), follow the lean diagnostic path: smoke-test the intake for vacuum leaks, check the PCV system, inspect the bank 1 exhaust ahead of the upstream O2, and look at fuel pressure under load.
- 6.If trim is negative (rich side), follow the rich diagnostic path: check fuel pressure for over-pressure, look for leaky injectors, inspect the air filter, clean the MAF, and check upstream O2 sensor health.
- 7.If fuel pressure has to be measured, use a gauge rated for that system, relieve residual pressure before opening any connection, keep ignition sources well away from the work, and have a passenger read the gauge if it must be watched under load — never read it yourself while driving.
- 8.Address the actual root cause rather than reflexively replacing the upstream O2 sensor. O2 sensors are often the symptom carrier, not the actual fault.
- 9.After repair, drive the vehicle through a full warm-up cycle and confirm bank 1 trims return to within ±10%.
Repair cost
$30 – $1,500
Vacuum leak repair is $100-700 depending on what's leaking. MAF cleaning is $10-50; MAF replacement is $150-450. Upstream O2 sensor replacement on bank 1 is usually $150-350; whether it is the cheaper or the dearer of the two banks depends on which physical bank holds cylinder 1 on that engine. Fuel pressure regulator replacement is $200-500. Leaky injector repair runs $300-800 per injector. Intake manifold gasket replacement is $400-1200 depending on engine. Diagnostic time up front is worth the investment to avoid wrong-part replacements.
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DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.